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PACS Observer`s Manual - Herschel

1. Figure 6 3 Spatial footprint layout of the chop nod observing mode two chopper positions are shown for nod A and B The zoom in the on source position reveals the field rotation between the two chop posi tions Note In case some disturbing sky features would fall in within the chopper throw radius around the tar get the observer has to consider to setup a chopper avoidance angle constraint The angle can be specified in equatorial coordinates counterclockwise with respect the celestial north The avoidance angle range can be specified up to 360 degrees with a minimum range of 15 degrees to avoid too much restrictions on scheduling Setting up a chopper avoidance angle requires an additional con straint on mission planning therefore this parameter should have to be used only for observations where it is absolutely necessary amp The sequence of line scans is repeated at two nod positions of the telescope In the second nod posi tion nod B the source is located in the off chopping position of the first nod nod A With this technique one can eliminate the telescope background what happens to be different at the optical angle of the two chop positions F observed NodA on off NodB on off 2 T2 F source T1 T1 F source T2 2 F source where T1 and T2 are the telescope back ground fluxes at the two chopper positions In this term we assume the sky background is negli gible compared to
2. B3A 50 70 micrometer 10 B2B 70 100 micrometer 10 R1 100 220 micrometer 20 4 10 2 3 Relative flux calibration accuracy between spaxels The currently available PACS spectroscopy pipeline uses a fixed nominal response value for every pixel are based on flux calibration standard measurements of sources placed on the central spaxel The absolute calibration of the surrounding spaxels is tied to the central spaxel via a flatfield de termined on the telescope background These flatfields reproduce very well When we divide the master flatfield by the individual flatfields measured we see a standard deviation of 2 3 The Ta ble 4 5 gives the relative accuracy peak peak when comparing line or continuum fluxes in differ ent spaxels The main uncertainty in interpreting line flux differences in different spaxels is the knowledge about the source structure and the beam probed by the different spaxels Table 4 5 Relative flux calibration accuracies between spaxels 41 PACS spectrometer scientific capabilities Spectral band Relative line and continuum flux accuracy between spaxels B2A 50 70 micrometer 10 B3A 50 70 micrometer 10 B2B 70 100 micrometer 10 R1 100 220 micrometer 10 4 10 2 4 Flux calibration accuracy of unchopped spectroscopy modes 4 11 The absolute flux calibration of the PACS spectrometer is based on observations of flux calibration standards usin
3. Setting the map size The centre of the map is at the coordinates given by the target position The map size along a raster line can be expressed as the number of raster points per line times the raster point step In perpendicular direction the map size is given by the number of raster lines times the raster line step Mapping parameter ranges are defined as e Raster point step in 2 480 arcseconds 97 Observing with PACS spectrometer Parameter name Signification and comments e Raster line step in 2 480 arcseconds e Number of raster points per line in 2 100 e Number of raster lines in 1 100 Please consult Section 6 1 6 4 Section 6 1 6 5 and Section 6 1 6 6 for in structions how to set up optimal step sizes Setting the map orientation angle Selecting the chop nod mode the map raster line orientation is defined by the chopping direction the observer has no direct access to the map orienta tion parameter disabled field The sky reference frame can be selected only in unchopped grating scan mode Please note in unchopped grating scan mode the HSpot default option is sky reference but we highly advise to switch to instrument mode if suitable for the science case In unchopped grating scan mode if an AOR raster covers an elongated area e g a nearby edge on galaxy then the observer might have no other option then using sky reference frame and turn the raster to the right direction
4. 4 5 Spectrometer field of view and spatial resolution The spectrometer and in particular its image slicer is used over a large wavelength range The pho tometer pixel size of 9 4x9 4 arcseconds is a compromise between resolution at short wavelengths and observing efficiency mapped area at long wavelengths The principle of integral field spectro scopy is illustrated in Figure 2 6 Full spatial sampling requires a fine raster with the satellite for spectral line maps with full spatial resolution For the sensitivity calculation this is neglected as the line flux will always be collected with the filled detector array It was confirmed during the Performance Verification Phase that no flux is lost in the integral field unit i e point source flux can be fully recovered from a single pointing if the source photocenter is on the central spaxel Although the observed noise level is varying over the 5x5 field of view the flux can be recovered by integrating over the neighbouring spaxels The spatial calibration of the PACS spectrometer section consists of the detailed characterization of the relative locations on the sky of the 5x5 spatial pixels spaxels in the blue and red sections and for all operational chopper positions 3 1 5 0 5 0 Detailed extended rasters on point sources HIP21479 and Neptune have been carried out during the Performance Verification Phase at a few wavelengths and the resulting spaxel geometries are
5. 45 40 first order uJ ul w o 60 80 100 120 140 160 180 200 wavelength microns Blue band Filter Filter A 38 B 2 ord Redband 17 Order Nominal and parallel ranges 60 80 100 oot 1 E 1 6 34 d wR yj l l l l 120 140 160 180 200 Figure 6 14 Wavelength as a function of spectrometer grating position incident angle of light beam Colours represent the three grating orders in use the chart in the bottom shows nominal and parallel ranges Figure 6 14 shows the parallel ranges covered for a primary defined wavelength range Note that this information is provided directly in HSpot in the time estimation report as well as in the sensitiv ity plots Sensitivity plots can be generated on line with HSpot including for the parallel range s in the other spectral orders covered simultaneously and for free The parallel ranges can also been es timated quickly with Figure 6 14 As a general rule for every scan range defined in the 2nd or 3rd order there is a parallel scan covered in the 1st or 91 Observing with PACS spectrometer der e fora scan defined in the 1st order there might 1 or 2 parallel ranges in the 2nd and 3rd orders Warning e Grating step sizes are always determined for the primary nominal range In practice this means the same wavelength range could be observed with different step sizes depending in which order the range was defined see Table 6 6 For ins
6. Note cee The Herschel Observers Manual provides further information about Herschel pertinent to using the i observatory from the perspective of an observer 1 2 Background The Herschel Space Observatory is an ESA cornerstone mission for high spatial resolution observa tions in the FIR and sub millimeter regime launched on the 14th of May 2009 aboard an Ariane 5 rocket together with Planck It was placed on a Lissajous 700 000 km diameter orbit 1 5 million kilometers away from Earth at the second Lagrange point of the Earth Sun system The mission is named after Sir William Herschel who discovered the infrared radiation in 1800 It is the first space observatory to cover the full far infrared and submillimetre waveband It per forms photometry and spectroscopy in the 55 670 um range with its 3 5m diameter radiatively cooled telescope with three science instruments housed inside a superfluid helium cryostat Herschel is designed to observe the cool universe The main scientific objectives of the mission are e to study the formation of galaxies in the early universe and their subsequent evolution toinvestigate the formation of stars and their interaction with the interstellar medium to observe the chemical composition of the atmospheres and surfaces of comets asteroids plan ets and satellites e to examine the molecular chemistry of the universe Herschel is operated as an observatory facility for three years of routine
7. e Option 1 70 220 microns 2nd Ist orders default option e Option 2 51 73 and 103 220 microns 3rd Ist orders As a result of the selection the redshifted wavelengths of line centres have to be specified in the PACS Line Editor table either within the 70 220 microns band or within the 51 73 and 103 220 microns band Wavelength microns Mandatory parameter The rest wavelength of the line centre If no redshift is specified the PACS grating will perform an up and down scan centred around the line s central wavelength Redshifted wavelength microns Parameter calculated by it appears only in the PACS Line Editor and in dicates the redshifted wavelength of the line centre The bottom pull down menu allows the reference frame for the redshift to be input Line flux unit This menu gives the flexibility to switch between physical input units sup ported by the PACS AOT logic Line flux Optional parameter User supplied line flux estimate in units specified by the Line Flux Units option Line flux input is used for signal to noise es timation as well as for the optimization of the dynamic range Leaving the parameter as the default 0 0 value means the PACS Time Estimator will not perform signal to noise estimation sensitivity estimates are still provided and default integration capacitor may be used providing the smallest dy namic range Continuum flux density mJy Optional p
8. If the target is at higher ecliptic latitudes then you may select instrument reference frame and put a time constraint on the AOR The appropriate time window can be identified in HSpot Overlays AORs on images option by changing the tentative epoch of observation This way the array can be rotated to the de sired angle by the time dependent array position angle 98 Chapter 7 Pipeline processing and data products This chapter describes the standard processing steps pipeline for the different photometry and spectroscopy observa tion modes of the PACS instrument The pipeline steps are coded as java jython tasks using well defined interfaces To be able to use the most recent calibration information version con trolled calibration files FITS format are loaded during the pipeline processing the different intermediate formats of the PACS data throughout the reduction For a more de tailed description of the pipeline refer to the PACS Data Processing User s Manual 7 1 PACS photometer standard data pro cessing 7 1 1 Scan map pipeline processing issues The current automatic pipeline processing provides reductions up to PACS photometer level 1 detector readouts calibrated and converted to physical units and level 2 products fits maps At this point in the Herschel mission the goal of the pipeline is to deliver useable but not necessarily optimal products Instead the automatic pipelines are optimize
9. The selection of the correct gain LOW or HIGH is driven by source flux estimates given by the observer The switch to low gain is required for the flux limits given in Table 5 3 Each observa tion chop nod or scan map can be repeated several times driven by the observer specified repe tition factor to increase the depth of the observation the sensitivity scaling with the inverse of the on target integration time number of readouts Table 5 3 PACS bolometer readout saturation levels high gain setting 64 Observing with the PACS photometer Filter Point source Jy Blue 220 Green 510 Red 1125 This standard ADC gain of the bolometers allows photometry on a large flux density dynamical range from the mJy level up to about 200 1000 Jy before the brightest pixel saturates ADC satura tion and not the detector Hence this standard gain shall be appropriate for almost all types of sci entific observations However for very bright sources such as planets or stars in star forming re gions a low bias gain could be needed Note LER The low gain setting increases the flux dynamic range by a factor less than 2 at the expense of los P ing sensitivity at low flux levels as the noise is not properly sampled anymore with the low gain due to the coarser digitalization s Important Y The low gain shall be used with caution and under exceptional circumstances only If the low gain is select
10. are quite small Suitable corrections factors for a wide sample of SED shapes can be found in Poglitsch et al 2010 3 3 Photometer flux calibration The absolute flux calibration of the photometer is based on models of standard stars in the 0 6 15 Jy flux range in the three filters amp Boo Arcturus o Tau D And Ceti amp y Dra see Dehaes et al 2010 and on thermophysical models for a set of more than 10 asteroids M ller amp Lagerros 1998 2002 building up on similar approaches for ISOPHOT Schulz et al 2002 and Akari FIS Shirahata et al 2009 Together they cover a flux range from below 100 mJy up to 300 Jy Both types of sources agree very well in all 3 PACS bands and the established absolute flux calibration is consistent within 596 Neptune and Uranus with flux levels of several hundred Jansky are already close to the saturation limits but have been used for flux validation purposes At those flux levels a reduction in response of up to 1096 has been observed For comparison the latest FIR flux model of Neptune is considered to be accurate to better than 5 Raphael Moreno priv comm amp Fletcher at al 2010 For the 5 primary PACS standard stars the measured absolute flux accuracy is within 3 of pre 17 PACS photometer scientific capabilities dicted values in the blue and green filters and within 5 of the predicted values in the red filter See PACS Photometer Point Source Flux Calibration M
11. sssssse eme 68 6 1 2 Spectral leakage regions escort peteret oto dee dO EOS deat oe ties IE RU DR dhs ce 0H 68 6 1 3 Observing spectral lines in the 51 55 micron range see 69 6 1 4 Pointed mode 5 cre eee EEE ded ESETE RESE OSTOS EREE SPTO RU Seance 70 6 1 5 Pointed with dither mode sss eme 70 6 1 6 Mapping mode ei t be e re PEDE EE soe ORT he e PORE TEC 70 6 1 7 Standard chopping nodding mode sssssss ee 72 6 1 8 Bright lines chopping nodding mode sse 78 6 1 9 Unchopped grating scan mode sssss eme 79 6 1 10 Wavelength switching mode 0 eee cece eee cence ence eeceeeceeeeaeeeu esau sean eeaneeas 84 6 2 Range Spectroscopy AOT syes ie oerte yy nea gus Ferner etx DIE re oS UPe EE ER dosh esee A E EEEE 87 6 2 1 Flux estimates and dynamic range ssssssss ee 88 6 22 Spectral leakage TePlOnS incierto Gu ied E etit Leo dee eios eedem eine ta 89 6 2 3 Pomted mode o eae RU ee DU dii UE 89 6 2 4 Pomted with dither mode uei ete ette et tru sesh ed easy ors eo OPE Eo wean seo 89 6 2 5 Mapping mode eee eet doy d rcs Sas tree priore t Ine dels wee se eter yeso e erect 89 6 2 6 Range scan modes 5 etre ed cte aree TREE EXT E SEE UE EUR EO RETE PUO SE een 89 6 2 7 SED Mode c ansehen iue S E EEEE S EEE A aE ETES 95 7 Pipeline processing and data products issi iiiroreersoss i otot esses ceca tena
12. 3 and computing T on source integration time as follows T scan leg length 20 number of legs map repetition factor 5 3 Chop nod point source photometry mode Even though the chop nod point source mode is still available in HSpot and perfectly calibrated we discourage to use this mode for science observations as it is less sensitive than scan mapping for the same AOR execution time We recommend to use the mini scan map technique in all science cases related to point sources compact sources and also in cases of faint extended emission around point sources For more information we refer to the technical note PACS Photometer Point Compact Source Ob servations Mini Scan Maps and Chop Nod 61 Observing with the PACS photometer The point source photometry observing mode makes use of a chop nod technique and shall be used for sources that are significantly smaller than a single matrix 50 arcsec x 50 arcsec i e mostly point sources The PACS photometer chop nod point source mode uses the PACS chopper to move the source along the Y spacecraft axis by about 50 arcsec corresponding to the size of about 1 blue green bolo meter matrix or the size of about half a red matrix with a chopper frequency of 1 25 Hz The nod ding is performed by a satellite movement of the same amplitude but perpendicular to the chopping direction i e along the Z spacecraft axis to compensate for the different optical paths as illustrated i
13. Figure 4 35 unchopped line sensitivities are shown for the default faint and bright line sub modes The results show that within the uncertainty of the measurements the two modes yield the same rms for a single repetition when normalized to the same integration time In both modes the measured rms is somewhat larger than the HSpot estimates We note that the fact that the bright line and standard unchopped mode rms noise estimates are quite similar should not be surprising Al though the bright mode scans over a narrower region of the spectrum compared with the standard unchopped mode the region of the spectrum centered on the line is sampled just as fully However it should be noted that if the central wavelength of the line is not well known e g uncertain redshift for galaxy for example the line may be shifted to a region of the spectrum that is much less well sampled than in the case of the standard unchopped mode m Note Ce The 1st generation AOT mode for crowded field spectroscopy wavelength switching had a per al formance very close to the chopped mode however did not reach the same level of sensitivity RMS continuum PACS full sampling range scan 0 9 0 8 0 6 0 5 RMS Jy 0 4 0 3 0 2 0 1 0 0 60 80 100 120 140 160 180 200 Wavelength um Figure 4 29 Spectrometer point source continuum sensitivity in high sampling density mode for both 43 PACS spectrometer scientific capabilities line r
14. Offset drift correction Offset drift correction is performed by applying a simple high pass fil ter on the data This is adequate for point source fields but not for structured fileds and may res ult in dark halo artefacts Increasing the filter window so that it covers a scan leg length already improves the map in the case of structured objects Map reconstruction Map reconstruction with photProject is a simple projection of the data cube on the map grid considering the intersection areas of the native pixels with the map pixels For this purpose the ra dec coordinates of the four corners of each detector pixel in the cube are computed on the fly in photProject An alternative optimal map maker MADmap Cantalupo et al 2009 is provided in the data processing environment This is a java implementation of the original MADmap C code MADmap uses optimal map making techniques to produce the best fit solution for the final reconstructed maps The primary advantage of MADmap is that it does 99 Pipeline processing and data products not require the use of high pass filter to mitigate 1 f noise and thus preserves spatial structures up to the size of final maps Currently MADmap is only available as an interactive tool and re quires pre processing to remove correlated signal drifts prior to optimal map reconstruction MADmap additionally requires apriori knowledge of the detector noise properties as the inverse of the time time noise cor
15. The user is therefore advised to build a square raster map to be position angle independent in other words to define a map with the same number of raster points and step sizes on the raster X and Y axis Note Cer Raster lines are performed along the Z axis in contrast to photometer raster map where raster lines i are along the Y axis i e perpendicular to the chopping axis as can be visualized in HSpot with the AOR overlay functionality In unchopped grating scan mode the map can be defined in instrument as well as sky coordinates with a maximum size of 2 degrees and sparsely sampled maps are possible 70 Observing with PACS spectrometer take into account internal redundancy The sensitivity shall increase roughly with the square root of Warning e In mapping mode the sensitivity given by HSpot refers to each single raster point and does not the redundancy factor number of times a sky pixel is seen by a spectrometer spatial pixel 6 1 6 1 Map orientation reference frame The recommended raster step line size settings see below have been optimised only for zero de gree map orientation in instrument coordinates In case of applying sky reference frame then optimal spatial sampling cannot be guaranteed because the PACS footprint rotation with respect to the raster line orientation depends on the position angle of the detectors footprint determined by the day of the observation The sky reference frame can be selected onl
16. contours indicate 10 50 and 90 of the peak response 28 PACS spectrometer scientific capabilities 16 14 12 7 y 5 10 99 g y 8 V FWHM 2 z E FWHM x2 z FWHM y2 LL 4 2 0 40 60 80 100 120 140 160 180 200 wavelength um Figure 4 12 Width of the PACS spectrometer beams as a function of wavelength We show the FWHM in two directions of the assymetric 2D Gaussian fit blue squares red diamonds and the mean of the two yellow triangles Note that the beams are not Gaussian these numbers are a rough indication of the beam size only 4 7 Spectrometer spectral resolution and in strumental profile 4 7 1 Spectrometer spectral resolution The spectrometer effective resolution for the three orders is plotted in Figure 4 13 and Figure 4 14 The effective resolution is the quadratic sum of the grating resolution and the spectral pixel resolu tion The achieved resolution is in the range c6A A 55 320 km s or A 5A 940 5500 The instant aneous 16 pixel spectral coverage varies from 600 to 2900 km s corresponding to 0 15 1 0 um wavelength coverage Note Cee The main thrust of the PACS spectrometer resides in its high spectral resolution The spectrometer i is aimed at the study of emission absorption lines rather than continuum sources although a SED mode in the Range Scan spectroscopy AOT is available too 29 PACS spectrometer scientific capabilities Spectrometer effective spectra
17. eens em eee mee eher 99 7 1 PACS photometer standard data processing ssssssesse eee 99 7 1 1 Scan map pipeline processing issues ssssss ce ee ce eeca tenn cena eeaeeneeennees 99 7 1 2 Level 2 pipeline products for scan maps generated with HCSS in the HSA archive 100 7 2 Spectroscopy processing levels and data products sseeee 100 8 Change record ss oss eerie t tete ente deren tee ese a E ae eet E deuu cae su esed egeo aee ad eet der ee dogs 102 References 5 oe oed e RE I Be E det bue EL SEP RETO 104 Chapter 1 Introduction 1 1 Purpose of document The PACS Observer s manual is intended to support astronomers in the definition of their observa tions with the PACS instrument The purpose of this document is to provide relevant information about the PACS instrument on board Herschel Space Observatory to plan PACS observations in HSpot The information is mainly targeted to be a general overview of the instrument and its per formance in order to help the astronomer to plan prepare and execute scientific observations with PACS The structure of this observer s manual is as follows we first describe the instrument Chapter 2 and its scientific capabilities of the photometer and spectrometer Chapter 3 amp 4 the use of astro nomical observation templates AOTs to enter PACS observations in HSpot Chapter 5 amp 6 and the manual ends with a description of the pipeline Chapter 7
18. expands the beam to an elliptical cross section to illuminate the grating over a length required to reach the desired spectral resolution The grating is actuated by a cryogenic motor with arcsec precision which allows spectral scanning stepping for improved spectral flat fielding and for coverage of extended wavelength ranges The settling time for typical motions used in the PACS AOTS is sufficiently short to allow for grating scans at various sampling densities The light from the first diffraction order is then separated from the light of the two other orders by a dichroic beamsplitter and passed into two optical trains feeding the respective detector arrays stressed unstressed for the wavelength ranges 102 220um and 51 105um Anamorphic re imaging optics is employed to independently match the spatial and spectral resolution of the system to the square pixels of the detector arrays The filter wheel in the short wavelength path selects the second or third grating order It is possible to operate both spectrometer detector arrays simultaneously For wide scans full spec tra can so be obtained in both selected grating orders In a spectral line mode in the grating order of interest the other array yields narrow band continuum data or in suitably line rich sources serendipitous lines Image slicer The image slicer s main function is to transform the 5x5 pixel image at its focal plane into a linear 1x25 pixel entrance slit for the grating sp
19. filters show a small gain in trans mission at cryogenic temperatures but since not all of the actual filters could be measured we as sume their ambient temperature performance as a good and somewhat conservative estimate The filter transmission curves for the three photometer bands are plotted in Figure 3 5 The photometer transmission curves are available in HCSS as PCalPhotometer_FilterTransmission_FM_v1 fits and the bolometer response in the PCalPhotometer Absorption FM v2 fits 16 PACS photometer scientific capabilities PACS photometer system transmission transmission 60 80 100 120 140 160 180 200 220 Wavelength um Figure 3 5 Filter transmissions of the PACS filter chains The graph represents the overall transmission of the combined filters with the dichroic and the detector relative response in each of the three bands of the photometer The dashed vertical lines mark the original intended design values of the band edges The reference wavelengths chosen for the 3 photometer filters are 70 100 and 160 um These rounded values are close to the wavelengths that minimize the colour correction terms with the flight model filters No indications of any near or mid infrared filter leakage could be identified Required colour corrections for the photometric PACS reference wavelengths 70 100 160 um which have been de termined from the photometer filter trasnmission curves and bolometer responses See Figure 3 5
20. in 70 105 and 102 220 microns 2nd 1st orders Range scan in 51 73 and 102 220 microns 3rd Ist orders Range scan in 51 73 and 102 146 microns 2nd Ist orders Up to ten wavelength ranges low and high wavelength pairs can be entered either in the 70 105 and 102 220 um interval 2nd and 1st orders or in the 51 73 and 102 220 um interval 3rd and 1st orders In the third option the 51 73 um range is covered in the second diffraction order the corres ponding range in the red channel is 102 146 um This option provides higher continuum sensitivity but lower spectral resolution in the 51 73 um range see Section 4 11 therefore its use is only re commended for observing broad spectral features very strong lines or when the primary scientific interest is the determination of the continuum level over long spectral ranges Note Cee Band B2A is also superior to B3A in terms of sensitivity in cases where the line is resolved Le if d the purpose is to detect a broad line e g in ULIRGs or AGN and measure a flux rather than have a high resolution line profile B2A is more sensitive i e faster The mode provides two different grating sampling densities of the up down scans either high sampling density the same step sizes as in line spectroscopy Table 6 4 but here for customized ranges corresponding an objective of more than 3 samples per FWHM of an un resolved line in each pixel at all wavelengths orthe
21. in the first 102 220 um and second order 71 105 um combination or first and third order 51 73 um can be observed within a single AOR to avoid filter wheel movements If lines of second and third grating order are to be observed on the same target at the same time two AORs shall be concatenated This AOT is mainly intended to cover rather limited wavelength ranges up to a few microns in high sampling mode see below to study broad lines larger than a few hundred km s which wings would not be covered sufficiently in Line Spectroscopy AOT or a set of closed lines But in the second case the relative depth of the line cannot be adjusted as in the Line Spectroscopy case Note Cer Contrary to the Line spectroscopy AOT there is no way to adjust the range with a redshift for a s broad line The user has to compute the redshifted range to cover in the observation The Range Spectroscopy AOT is also intended to cover larger wavelength ranges up to the entire bandwidth of PACS in SED mode in low sampling mode this time otherwise integration times get quickly prohibitive But one should remember that the power of the PACS spectrometer is its high spectral resolution rather than continuum sensitivity Unlike in Line Spectroscopy depending on the requested wavelength range grating order the observer may consider the parallel channel data for an efficient coverage of the required wavelength range this aspect becomes more important for long rang
22. in which the point sources contributes to the flux strictly speaking the profile will be not 100 Gaussian but it is very close see below As the five by five PACS spaxel arrangement on the sky is fed into the image slicer it is re arranged into a 1 by 25 entrance slit for the grating When the light from this slit falls on the grating and the point source is centred in the middle of the spaxel the grating will output a Gaussian profile However if the point source is offset in the slit direction a skewness is introduced in the line profile as happens with standard slit spectroscopy This shifts the central wavelength and changes the line width and it also reduces the line fluxes as some of the flux now falls on neighbouring spaxels This dependence is depicted in Figure 4 17 Line profiles with well characterised photocentre position on the slit of the central spaxel are shown in Figure 4 18 the skewness of line profiles the shift of the peak wavelength as well as loss of line flux as a function of photocentre offset can be well seen 32 PACS spectrometer scientific capabilities 63 18 micron 2 5 2 0 1 5 1 0 Skewness e e 10 8 6 4 2 0 2 4 6 8 Offset arcsec Figure 4 17 Line profile skewness varies with offset from the slit centre Consider module 12 spaxel 2 2 If the point source is nicely centred the offset and skew are zero As the point source moves off centre in one direction for example towards
23. input parameters for scan map mode Parameter name Signification and comments which of the two filters from the blue channel to use In case observations in the two blue filter bands are required to be performed consecutively two AORs shall be concatenated Orientation reference frame The reference frame for the scan map orientation array or array with sky constraint for instrument reference frame sky or sky with array con straint for sky coordinates scans 57 Observing with the PACS photometer Parameter name Signification and comments Orientation angle Array to map angle if scan in instrument reference frame see Figure 5 2 or map orientation angle if scan in sky coordinates see Figure 5 3 in de grees Orientation constraint Map orientation angle range if scan in instrument reference frame see Fig ure 5 2 or array to map angle range if scan in sky coordinates see Fig ure 5 3 Scan speed Scan leg length Slew speed of the spacecraft either high 60 arcsec s or standard 20 arc sec s Length of a line scan leg the maximum length is 20 degrees homogeneous coverage If selected Yes HSpot computes the exact cross scan distance in order to perform a homogeneous coverage i e a scan map where the time spent on each sky pixel of the map is approximatively the same discarding gaps between matrices This choice is available only when the scan ma
24. level of the peak signal ripples start to shape the extended line wings at the longest PACS wavelengths around 200 microns The full profile model matches the line wings on a few percent accuracy although wings show an increasing level of asymmetry in the blue half of the 1st diffrac tion order from 102 to about 160 microns which is not modeled at present Data obtained on line wings follow very closely the same trend during grating up and down scans This confirms that wing shapes are predominantly related to grating dispersion instead of memory effects of the strongly illuminated detectors In the process of optimal spectrum extraction line flux is usually determined from Gaussian or skewed Gaussian fitted profiles The power conserved in line wings has to be corrected by applying a profile correction factor for point sources Fitting of the line wings after the first minima would very difficult for faint lines especially at wavelength shorter than 150 microns therefore the profile correction factor is advised to use Profiles of various narrow lines typically observed in the PACS wavelength range are shown in Figure 4 19 The Gaussian fitted model provides reliable flux estimation over the entire wavelength range Figures were made by standard data reduction pipeline plotted spectra are created on a wavelength grid corresponding to the Nyquist sampling of the pectral resolution at the line peak wavelength respectively Fitted profile
25. module 7 the skew increases as the offset increases If the point source moves in the other direction towards module 17 the skew becomes more negative as the offset in creases CRI 1 Li Lursilssiili Li 5 63 10 63 15 63 20 63 25 63 30 63 35 Signal V s Landbiaabahaihiabiulaudhassdunl 1 05 63 00 63 05 63 10 63 15 6320 6325 6330 c 25 1 fi 1 f 1 i 0 5 63 00 63 05 63 10 63 15 6320 63 25 6330 6335 Figure 4 18 Line profile skewness varies with offset from the slit centre Consider module 12 spaxel 2 2 profiles are plotted for a point source Skewness and the wavelength offset of the line peak increases as 33 PACS spectrometer scientific capabilities the point source moves off centre perpendicular the slit direction Red lines are fitted skewed Gaussians The instrumental profile is defined as the instrument response when scanning over a line which is intrinsically much narrower i e unresolved than the profile itself The IP characterization on ground has been done by fitting parametric models to the measured intensity profile of laser lines In the first order approximation Gaussian profiles are fitted These fitting parameters obtained at laser wavelengths have been used as input for phase matching of a numerically calculated profile derived from the sinc square function The peak of the profile closely follows a Gaussian model at about 10
26. name The product names start with HPS which stands for Herschel Pacs Spectroscopy Level 0 data Level 0 products are complete sets of data as a starting point for scientific data re duction Level 0 products may reach a data volume of 200 MB hour For all standard observing modes the scientific information is contained in the structures called HPSFITB R objects of the Frames class which constitute the starting point for the pipeline These contain all measured 100 Pipeline processing and data products slopes described in section 2 4 5 i e the 18 x 25 x time line slopes for one camera Correct in terpretation of the signal requires knowledge of the instrument status at every time That can be found in sub structures of these Frames but levelO products also contain many useful additional information like spacecraft pointing time correlation and selected spacecraft housekeeping in formation raw data i e un fitted ramps for some selected pixels etc Finally level 0 contain the calibration data needed for data analysis The fit ramps products HPSFITB R are of dimen sions 18 x 25 x time line the averaged ramps products HPSAVGB R are of dimensions 18 x 25 x time line x 4 A fit ramps HPSFITB R product contains for each pixel 18x25 one value for each ramp taken over the course of the observation these values are the slopes that have been fit to each and every ramp The averaged ramps product contains for each pixel
27. performs well compared with expectations within 15 46 PACS spectrometer scientific capabilities 1c line sensitivity 450s observing time HSpot 20 T T T T T T 1 1 T T T T T e Chop Nod re Unchopped Standard Unchopped Bright line 15 H m N L s E n I UA o 1 OF e l rc I j I N F IVV S a L e V 5 6 N X a predictions for Fr Standard Unchopped e a p x T le L e 6 L 4 1 i LL 1 al D f L 4 L n 100 150 200 Wavelength um Figure 4 35 A Comparison of the line sensitivities in chop nod unchopped standard faint and bright line mode The circles show results from in flight measurements overplotted on HSpot predictions 4 12 Spectrometer saturation limits The PACS spectrometer gives access to a large dynamic range in flux densities by selecting 4 differ ent integrating capacitances The uplink logic automatically selects the integrating capacitance based on estimated continuum and line fluxes Figure 4 36 shows the saturation limit in Jansky for the de fault integration capacitance This is the limit for continuum and peak line flux together Figure 4 37 shows the default capacitance saturation limits for unresolved lines on a zero continuum Both fig ures should allow to judge if the observation can be executed with the default integration capacit ance If
28. pointed observation since a single line takes rough 10 minutes to execute three to six separate lines could be observed within the AOR At the end of this period an off observation will be ex ecuted If a small map is executed the user should balance the efficiency of the map making with the need for a frequent off It may be more efficient to allow the map to execute to the end of a small map before going to the off This would be achieved by setting the Repeat off position after nth rater position variable an appropriate value To give an example If a single line is ob served on a 2 x 2 raster setting the repeat off position after nth raster to 4 will ensure that the off is taken at the end of the sequence resulting in an AOR of duration 23 minutes which would be ideal If two lines were observed for the same map the same parameters would lead to an off be ing taken at the end of the sequence which in this case would be after approximately 40 minutes which is still acceptable Observations with the off taken up to 2 hrs from the on still seem to produce results that show only a small degradation in S N ratio although we recommend keeping the interval as short as possible In this mode off positions of up to 2 degrees from the tar get may be specified Fs Note Ce The user can specify an off position either by offset or by RA and Dec specification Note that al by default these offsets are set
29. provided in forthcoming HIPE versions Alternatively the full flux or flux density of a source can be recovered by co adding the spectra obtained from several spaxels aperture photometry also described in Section 4 10 1 52 Chapter 5 Observing with the PACS photometer A typical PACS Observation Day OD contains predominantly either photometer or spectrometer observations to optimise the observing efficiency within a photometer cooler cycle After each cool er recycling procedure which takes about 2 5 h there are about 2 5 ODs of PACS photometer prime or parallel mode observations possible Mixed days with both sub instruments e g to ob serve the same target in photometry and spectroscopy close in time are only scheduled in exception al cases Three observing modes or Astronomical Observing Templates AOT are validated on the PACS photometer side e Pointsource photometry mode in chopping nodding technique Scan map technique for point sources small and large fields e Scan map technique within the PACS SPIRE parallel mode The originally foreseen small source mode and large raster mode in chopping nodding tech nique are replaced by the scanmap technique for better performance and sensitivity reasons We refer to the SPIRE PACS Parallel Mode Observers Manual for more information on the use of the parallel mode and describe in this chapter the the scan mapping mode including the particular case of
30. range scans the different scans will be per formed with a small offset so that one spectral resolution element is seen by as many pixels as pos sible In case more than two SED scans are required then it is advised to combine range repetitions with observing cycles Note Cee As the wavelength template is hard coded for SED observations HSpot does not create any range in the Range Editor Table once you switch to SED mode However it is strongly recommended you click on Add range button and fill up the free parameters necessary for flux estimation This is the only way one can make sure the observation will be executed with properly selected integra tion capacitance i e the dynamic range is optimised for the specified flux level and detectors will not saturate The optimization mechanism of dynamic range is described in Chapter 6 6 2 7 2 Comparison of chopped and unchopped SED implement ation Taking the example of an SED B2B scan in both chopped and unchopped observing modes the grating visits 262 steps with step size 2400 units Such a scan up and down takes 1048 seconds in the chopped mode for both ON and OFF fields without overheads 262 1 8 16 for 2 nod positions gives 1048 In unchopped mode a single repetition scan takes 524 seconds ON or OFF source two repetitions integrates 1048 seconds and so on A concatenated ON OFF pair of SED B2B unchopped AORs with range repetition 2 takes 1048 ON 1048 OFF s
31. scan direction in the first leg counted positive counterclockwise in the sky This configuration corresponds to refer ence frame array in HSpot and is illustrated in Figure 5 2 PACS does not have a fixed magic angle like SPIRE it it left as a free parameter to the user It is however advised not to use 0 or 90 degrees as gaps between matrices would then stay in the final map if a sky position is visited only by one scan line leg An array to map angle of 45 degrees al lows to get the same depth in two scan maps with orthogonal mapping directions a array to map angle p map orientation angle PA array position angle B a PA Line scan direction Array with sky constraint Figure 5 2 Scan maps in instrument reference frame The array to map angle o is defined by the user This effectively defines the map orientation angle in the sky D as the array position angle is not a free parameter it is function of target coordinates and observation time However a constraint on the map orientation angle can be put in HSpot In this configuration if the homogeneous coverage parameter is selected HSpot computes the ap propriate distance between scan legs cross scan step to achieve an homogeneous coverage which is a function of the array to map angle selected above Hm Note Cee al In the case of a square scan map in instrument reference frame the orthogonal coverage to cover the same area is achieved by simply a
32. sequence of the unchopped grating scan mode is sketched in Figure 6 9 In the on source block the observer defined line repetitions are internally multiplied by two i e a single repetition is made of two fast scans The on source integration time in this mode is half the time PACS spends on source in the chopped mode including both nod positions In pointed mode following an on source block the spacecraft slews to an off position within two de grees radius with respect to the target coordinates The off position has to be carefully selected for an efficient subtraction of the telescope background Ideally the background sky emission towards the target and towards the off position is at the same level respectively and the background field is free of confusing structures i e the 25 spaxels see a similar level of sky emission The off position is always executed with a single repetition irrespective what the observer has defined for line repetition factor in the PACS Line Editor This means the duration and signal to noise in the off position blocks will be lower than the on target block if line repetition factor has been set larger than one jm Note Cee The repetition cycle has to be used to increase the exposure the entire on off block is repeated as is al in the chopped mode For instance in two cycles the spacecraft follows an ON OFF OFF ON pointing sequence between the two positions For deep observations if necessary the off p
33. since they require different magnification Directly in front of their baffle enclosures the blue detectors have filter wheel mechanisms which contain the band pass fil ters for short wavelength photometry and the order selection band passes for 2nd and 3rd order op eration of the grating spectrometer respectively Telescope Entrance Optics chopper calibration optics Bolometer Figure 2 2 Functional block diagram of PACS overall optics The focal plane sharing of the instrument channels is shown in Figure 2 3 The photometric bands which can be observed simultaneously cover the same field of view The field of view of the spec trometer is offset from the photometer field see Figure 2 3 However this has no effect on the ob serving efficiency The focal plane unit provides photometric and spectroscopic capabilities through five functional units common input optics with the chopper calibration sources and a focal plane splitter e a photometer optical train with a dichroic beam splitter and separate re imaging optics for the two short wavelength bands 60 85 um 85 125 um selectable via a filter wheel and the long wavelength band 125 210 um respectively two bolometer arrays with cryogenic buffers multiplexers and a common 0 3 K sorption cooler e a spectrometer optical train with an image slicer unit for integral field spectroscopy an ana morphic collimator a movable diffraction grating in Littrow mount anamo
34. stored as calibration files within the data processing environment HIPE Figure 4 6 shows as an example the result for chopper posi tion zero in relative spacecraft units with respect to the virtual aperture of the PACS spectrometer which is defined as the central pixel of the blue field of view Asymmetrical optical distortions between chopper on and off positions cause unavoidable slight misalignment smaller than 2 for individual spaxels between spacecraft nod A and B within the double differential data acquisition scheme As shown on Figure 4 7 the apparent field rotation becomes larger with increasing chopper throw 25 PACS spectrometer scientific capabilities Spectrometer FOV fingerprint 23 EE xde ax ae 99 eo a E eo e y O eo 9 e 3 a i e of e J o 8 H E i e gt 8 amp FL e e o E E j 20F i d lt se 30u lissi NENNEN Lrbrrrrrirar Tycasiceg caer acean i 30 20 10 o 10 20 30 Y spacecraft coordinates of fitted PSF Figure 4 6 Spectrometer field of view for blue circles and red squares spaxels in spacecraft Y and Z coordinates for chopper position zero chopper 0 position bec position chopper chopper position Figure 4 7 Apparent field rotation for the two chopper positions in a symmetric chopper pattern 4 6 Spectrometer Point Spread Function PSF 4 6 1 Measured vs model PSF A further res
35. target the observer may consider setting up a chopper avoidance angle constraint The angle is specified in Equatorial coordinates anticlockwise with respect the celestial North East of North The avoid ance angle range can be specified up to 345 degrees Setting the map size The centre of the map is at the coordinates given by the target position The map size along a raster line can be expressed as the number of raster points per line times the raster point step In perpendicular direction the map size is given by the number of raster lines times the raster line step Mapping parameter ranges are defined as e Raster point step in 2 480 arcseconds e Raster line step in 2 480 arcseconds e Number of raster points per line in 2 100 e Number of raster lines in 1 100 Please consult Section 6 1 6 4 Section 6 1 6 5 and Section 6 1 6 6 for in structions how to set up optimal step sizes Setting the map orientation angle Selecting the chop nod mode the map raster line orientation is defined by the chopping direction the observer has no direct access to the map orienta tion parameter disabled field The sky reference frame can be selected only in unchopped grating scan mode Please note in unchopped grating scan mode the HSpot default option is sky reference but we highly advise to switch to instrument mode if suitable for the science case In unchopped grating scan mode if an AOR raster covers an elonga
36. the continuum they scan over the line profile only This could make difficult a proper response cor rection as no flat continuum level can be determined for all the pixels Note LE Since the bright line mode scans about 1 4th of the wavelength range scanned in faint line mode for broadened lines this might limit the wavelength extent of the baseline measured Unchopped grating scan mode The unchopped grating scan is an alternative to the chopping nodding mode if by chopping to a maximum of 6 the off position field of view cannot be on an emission free area for instance in crowded fields or for spectral line mapping of extended objects with diameter larger than 5 respect ively The direct way of acquiring data raises limitations on the applicable flux regime with respect to the standard chop nod mode This mode is not recommended for very faint lines target lines needs to be above typically 1 Jy peak to continuum and the continuum level can be recovered in a reliable way only for bright sources i e at a minimum continuum level of 20 30 Jy The con tinuum level can be determined by off position subtraction which could efficiently eliminate the telescope background for bright objects Unchopped faint line mode In this default version of the unchopped grating scan mode the line is scanned with the same grating step as in chopped line spectroscopy i e every spectral pixel samples at least every 1 3 of a resolu tion element The numbe
37. the telescope background The nod sequence can be repeated within one obser vation to increase the depth of the observation Nod cycles are repeated in a way that A B slew times are minimised for instance in case of two repetitions the spacecraft follows the pattern A B B A In chop nod mode in total one half of the science time is spent on source The principle of line spectroscopy is illustrated in the diagram of subsequent instrument and spacecraft observing blocks in Figure 6 5 i Note LEF For low number of repetitions up to 5 6 it is recommended to repeat line scans increase line re petition factor while for deeper observations line repetitions should be combined with a number of nod cycles 74 Observing with PACS spectrometer 2000 7I I TUECCUEURODPEIGgUEN MESI 3E i 486000 EN Grating step 1 960 n 484000 1280 g 1800 PBB BBB e a7 1000 d 1700 acca a 482000 5 1020 amp I 6 1040 dd 1600 cee x 4 480000 e h l o 1060 D o AA A 478000 8c 1080 V o t i 1100 6 1400 ol 476000 1120 1300 1140 474000 1200 1160 un Tm B els gp Readouts Chopper position Grating position X Signal Figure 6 4 At each grating position in an up down scan 16 ontegration ramps are taken plus 1 for syn chronisation at the begnning The ABBA sequence represents a chopper on off off on cycle which is repeated two times The duration of such a grating pl
38. to a very narrow range of values modulo 180 degrees as the Z axis is always pointing towards the sun 1 degree in the ecliptic plane Therefore the map orientation angle cannot be too different from the array to map angle 90 degrees modulo 180 This shall be checked with the overlay AOR facility in HSpot culation so that the Herschel visibility windows are not affected by that constraint The user is thus Warning c When a map orientation angle is set the constraint is not yet fed back in HSpot to the visibility cal invited to assess himself the impact on the visibility of the constraint 5 1 2 Scan maps in sky coordinates a array to map angle p map orientation angle PA array position angle B a PA Line scan direction Sky with array constraint Figure 5 3 Scan maps in sky coordinates The map orientation angle in the sky D is fixed by the observ er therefore there is no control on the array to map angle which depends on the target coordinates and exact observation time However a constraint on the array to map angle can be put in HSpot Another way to define rectangular areas in the sky with scan mapping is to select mapping in sky coordinates with the option sky in HSpot In this configuration the map orientation angle is defined by the observer i e the angle from the equatorial celestial north to the line scan direction in the first leg However in this case there is no direct control o
39. to zero and it is important that the user specify a non zero value away from the target The specification of an off position is provided in a special part of the HSpot window for both the Pointed and Mapping component of the Set Observing Mode op tion 83 Observing with PACS spectrometer 6 1 10 Wavelength switching mode The wavelength switching technique mode is an alternative to the chopping nodding mode if by chopping to a maximum of 6 arcminutes the OFF position field of view cannot be on an emission free area for instance in crowded areas Warning Eg The wavelength switching mode has been deprecated in HSpot v5 0 and later versions For crowded field spectroscopy and in more general for science cases where chopping is not possible it is advised to use the unchopped range scan mode instead Observers with AORs in wavelength switching mode could still read these observations in HSpot but time estimation is not possible any further In wavelength switching mode the line is scanned with the same grating step as in chopped line spectroscopy i e every spectral pixel samples at least every 1 3 of a resolution element In wavelength switching we refer to this step as a dither step At every dither step the signal is modu lated by moving the line over about half of the FWHM This allows one to measure a differential line profile canceling out the background Figure 6 12 The modulation on every scan
40. used to apply to adjust the absolute sensitivity and finally blue in the mapping mode indicates that overlapping foot prints in a small raster produce deeper coverage towards the centre of the map 6 2 1 Flux estimates and dynamic range For bright sources where the default integrating capacitance will result in saturation you need to enter the expected continuum flux line flux and line width at a reference wavelength in the selected spectral channel this can be either the nominal or parallel channel We advise to select the reference wavelength at the highest risk of saturation see section on saturation limits in Section 4 12 You may check parallel ranges in Figure 6 14 or alternatively the parallel coverage can be plotted in the HSpot Range sensitivity plot as well as printed in the PACS time estimator message The provided continuum flux estimates are used to scale a Rayleigh Jeans law SED Then the RJ law SED is evaluated at the wavelength of the peak response in the red and in the blue range and in case a non zero line flux is provided then the peak flux falling on a single resolution element will be ad ded to the continuum after the RJ law extrapolation These flux estimates are used to select the op timal integration capacitor If an observation contains nominal and parallel ranges that fall in differ ent flux regimes the largest capacitance will be chosen for the entire observation If ranges in the same observation fall i
41. 0 um and 95 110 um do overlap respectively leakage regions on measured spectra are shown in Figure 4 20 Figure 4 21 and Figure 4 22 Continuum shapes and flux densities in these border ranges are therefore less reliable than in band centres Band B2A is not affected by spectral leakage N Flux density Jy oren w e aaou vooon 120 140 160 180 240 Wavelength u Figure 4 20 Spectral leakage in band R1 the spectrum between 190 220 um has an unreliable line flux calibration and shows superimposed spectral features from order 2 95 110 um Flux density Jy BP NN WW PP uuo cO uc ununcucuocouso 80 85 90 Wavelength u m Figure 4 21 Spectral Spectral leakage in band B2B beyond 98 um the response is very low and spectral 35 PACS spectrometer scientific capabilities features from order 3 63 70 um are superimposed on the spectrum Flux density Jy 10 U x p TIT TTTT TTTTTTTTTTT S M MAC o gI N m e NENNNRNENARNRRRRNNNRRNNRNERNERENRNRNENENS 3 54 56 58 60 62 64 66 68 70 72 74 Wavelength um Figure 4 22 Spectral leakage in band B3A beyond 70 um the order 4 52 5 54 5 um spectrum is added to the 70 73 um order 3 spectrum The 51 52 um order 3 spectrum also shows the 76 78 um order 2 spec trum At 69 um the leakage is of the order of 2 This can still be important in the few cases where the OMI line at 51 8 um is extremely bright as in the example displayed 4 9
42. 1 In Line Spectroscopy AOT the highest sensitivity range uniform coverage is at least 4x larger than the FWHM of an unresolved line In case of Range Spectroscopy AOT the calculation of suffi cient margins on both sides of a broad line remains a task for the observer In order to make sure the entire line profile is fully covered by the uniformly sampled part of the range you need to apply the PACS instantaneous coverage as a margin on both blue and red edge of the requested range in case of high sampling density and especially for faint lines The wavelength margin is provided in column Instantaneous spectral coverage 16 pixels of Table 4 1 i Note Cer The calculation of highest sensitivity range and adjustment of range borders to the homogeneously al sampled part might be a necessary step only for short ranges i e observing broad lines and only in case of faint detections 6 2 6 1 Standard chopping nodding mode The principles of chopping nodding in range spectroscopy is very similar to its implementation in line spectroscopy as described in Section 6 1 7 In range spectroscopy chopping nodding can be combined either with high sampling density scan or with the coarser and shallower Nyquist sampling option In order to increase the depth of high sampling density range scans even for relatively short ranges it is advised to increase both range repetition and nodding cycles rather than only the range repeti tion factor As t
43. 1 0 107 5 0 mA 0 0 60 80 100 120 140 160 180 200 Wavelength um Figure 4 32 Spectrometer point source line sensitivity in SED mode range spectroscopy AOT for both range repetition and nodding repetition factors equal to one Blue third grating order filter A B3A green second order filter B B2B red first order R1 RMS Jy RMS W m x107 7 60 80 100 120 140 160 180 200 Wavelength um 45 PACS spectrometer scientific capabilities Figure 4 33 One sigma continuum sensitivity upper plot and line sensitivity lower plot for a number of faint line detections in comparison to the HSpot predictions for a single Nod and single up down scan by the grating with a total execution time of 400 440 s depending on wavelength The different col ours represent the different spectral PACS bands and grating orders Nyquist binning two bins per FWHM has been used to derive the measured line detection sensitivity in each bin while the HSPOT prediction refers to total line flux Thus the actual sensitivity values shown here are very conservative RMS continuum PACS full sampling range scan 9 Unchopped e Chop nod RMS Jy 0 0 A aaa aca La aca aac aac La caca aaa 60 80 100 120 140 160 180 200 Wavelength um Figure 4 34 The continuum rms uncertainty for chop nod and unchopped observations compared with HSpot predictions for an equivalent 450s total on source integration The unchopped mode
44. 220 240 Wavelength u m Figure 4 41 Saturation limit for unresolved lines on a zero continuum using the third 0 46 pF integra tion capacitance including 80 safety margin 50 PACS spectrometer scientific capabilities PACS spectro saturation limit Capacitance 12 telescope background included 10 10 Saturation limit Jy 10 1 40 60 80 100 120 140 160 180 200 220 240 Wavelength u m Ri B2B B3A B2A Figure 4 42 Saturation limit for the largest 1 15 pF integrating capacitance PACS spectro saturation limit Capacitance 12 telescope included eo Saturation limit W m 10 40 60 80 100 120 140 160 180 200 220 240 Wavelength u m RI B2B B2A B3A Figure 4 43 Saturation limit for unresolved lines on a zero continuum for the largest 1 15 pF integrat ing capacitance 51 PACS spectrometer scientific capabilities 4 13 Astrometric accuracy The absolute pointing error APE for Herschel is specified and measured to be 2 arc seconds at the 1 sigma level on pointed observations However if the roll angle of the telescope does not change significantly between two consecutive observation typically the case for concatenated spectro scopy AORs then such AORs could have a systematic offset to a given direction as the guide star distribution within the star tracker field of view remains pretty much unch
45. 3 Chopper Differential measurements are required to extract faint signals from celestial sources from the dom inant thermal background radiation of the warm 80K Herschel mirror For this purpose a small tilting mirror the chopper flips alternately on the astronomical source and on a nearby sky position with a variable throw up to 6 arcmin on the sky for the spectrometer and 3 5 arcmin for the photo meter This allows full separation of an object field and a reference field The chopper is also used to alternatively look at the two internal calibration sources ICS which are located at the left and right side of the instrument FOV see Figure 2 3 for frequent calibration measurements The chopper is capable of following staircase waveforms with a resolution of 1 and delivers a duty cycle of 90 at chop frequency of 5 Hz The chopper axis is stabilized in its central position by flexular pivots and rotated by a linear motor The chopper design allows a low heat load in the PACS FPU 2 3 Photometer After the intermediate focus provided by the entrance optics the light is split into the long wavelength and short wavelength channels by a dichroic beam splitter with a transition wavelength of 125 um and is re imaged with different magnification onto the respective Si bolometer arrays The blue channel offering two filters 60 85 um and 85 125 um has a 32x64 pixels arrays while the red channel with a 125 210 um filter has a 16x32 pixe
46. 4 values per ramp taken over the course of the observation these being the set of 4 averages taken for each ramp The pipeline starts on the HPSFIT products Level 0 5 data Processing until this level is AOT independent Additional information like pro cessing flags and masks saturation damaged pixel signals affected by chopper and grating transitions is added block selections generated basic unit conversions applied digital readouts to Volts s and for the spectrometer the wavelength calibration inclusive velocity correction is done Also the center of field coordinates are computed for every frame and sky coordinates are assigned for every pixel The only products here are the fit ramps the HPSFITB R of dimen sions of 18 x 25 x nb of slopes Level 1 data The automatic data generation of level 1 products is partly AOT dependent De tector readouts ar flux calibrated and converted to physical units in principle instrument and ob servatory independent Level 1 processing includes the flux calibration and adds further status information to the product e g chopper angle masks etc These are the biggest products in the processing chain and may reach 2GB h in the case of spectroscopy The spectroscopy Level 1 product PacsCube HPS3DB R contains fully calibrated 5 x 5 x nb of slopes cubes per pointing spectral range Level 2 data Further processed level 1 data to such a level that scientific analysis can be per formed Processi
47. 6 e For pointed observations other lines out of the 51 55 micron range should remain in PACS Line Spectroscopy AORs You can concatenate the newly defined range with the original AOR For mapping observations it might be more efficient to also define other lines as a small range In such a configuration the spacecraft and instrument overheads would be minimised with no compromise on data quality The most efficient setup is simply the shortest duration set of AORs which contain all requested spectral lines If you planned to observe broad lines originally in PACS range Spectroscopy AOT then you only need to make sure lines in the 51 55 micron range are observed in band B2A Pointed mode The default mode for point source spectroscopy a single pointing on the source The integral field concept allows simultaneous spectral and spatial multiplexing for the most efficient detection of weak individual spectral lines with sufficient baseline coverage and high tolerance to pointing errors without compromising spatial resolution The PACS spectrometer arrays have 5 by 5 spatial pixels covering a 47 by 47 arcseconds field of view respectively both channels viewing almost identical positions on the sky The line flux from a point source object will always be collected with the filled detector array with most the source flux falling on the central pixel Therefore for the plain detec tion of a line source one pointing is sufficient for a point or
48. 7 5 5 10 5 lhour mJy To a first order the sensitivity in all mode scales with the inverse of the square root of the on source observation time This scaling is used for the sensitivities and S N ratios reported by HSpot 19 PACS photometer scientific capabilities The on array chopping technique is only used in point source photometry mode the sensitivity reached is worse than in min scan mapping mode as the 1 f noise cannot be filtered out by chopping at 0 8Hz as efficiently as in the spatial modulation of the scan mapping See chapter 4 for more in formation on the observing modes 3 6 Astrometric accuracy The absolute pointing accuracy for Herschel is measured to be 2 arc seconds at the 1 sigma level on pointed observations However some larger deviations as large as 5 to 8 arc seconds solid shift in some rare cases have been reported for some scan map observations Deviations of the actual tele scope pointing from the scan legs great circles in the sky that is reported in the pointing product cause a smearing of the PACS PSF in the reconstructed scanmap in particular in the blue green bands The jittering along scan legs is estimated to be usually around 1 arcsec level therefore the ef fect is rather small even in the blue band The scan speed profile exhibited sometimes some significant bumps above the average speed before OD 320 April 2010 due to warm pixels in the star tracker CCD This caused al
49. 7E E E 4 gt SE 34300 3E d 300 g 5 E ilL Or ilez x sE 3H250 gt x 2 4 ae 3 E 2 A 44200 E E E 14200 i 16 3 fz SE d 150 oF 3150 SE 34100 1E H 100 1E 4 E E oF 4150 2E 11 50 PLATE ETE ETEEEEEEEEIA TERETE RS phe a baa d asia o 156 0 156 5 157 0 157 5 158 0 158 5 159 0 159 5 160 0 156 5 157 0 157 5 158 0 158 5 159 0 159 5 Wavelength u m Wavelength u m Figure 6 11 A comparison between standard unchopped faint and bright line mode for on and off ob servations The upper figures show the on and off observations blue and green and the lower figures show the difference spectra The green line shows the spectral coverage frequency which is a measure of the number of samples averaged in each wavelength bin Notice how the broader coverage of the faint line mode produces a flatter baseline than the bright line mode 6 1 9 3 The choice of off block frequency The choice of off frequency is a user defined quantity and will depend on the science goals If you are primarily interested in line properties then obtaining an off at a convenient point in your obser vational sequence roughly every 30 60 minutes would be ideal For unchopped line scan the mode is set up so that the observer gets an off at the end of the ob serving sequence To ensure that the off is observed every 30 60 minutes it is desirable to arrange that the main observing block does not last more than 30 60 minutes So for example for a single
50. Absolute flux calibration accuracies 40 PACS spectrometer scientific capabilities Spectral band RMS Peak to peak accuracy B2A 50 70 micrometer 11 30 B3A 50 70 micrometer 11 30 B2B 70 100 micrometer 12 30 R1 100 220 micrometer 12 30 4 10 2 2 Relative flux calibration accuracy within a band and de tection limit for broad features Broad spectral features a few micrometer and continuum shape difference can be introduced by transient effects and pointing offsets Corrections for these effects are under study In the mean time such features should not be interpreted blindly Note that due to the origin of these effect they will be seen differently in every observation so dividing two PACS spectra will not eliminate these in strumental effects The Table 4 4 summarises the resulting accuracy to assume when comparing rel ative line fluxes within a spectral band When comparing line fluxes across spectral bands the abso lute flux accuracies in Section 4 10 2 1 apply This is also the current limit on detection of broad spectral features solid state features dust continuum shape These numbers apply to the wavelength regions not affected by spectral leakage see Section 4 8 Table 4 4 Relative flux calibration accuracies within a band Spectral band Broad spectral feature detection limit and relative line flux accuracy within a band B2A 50 70 micrometer 10
51. Green 60 6 89 x 9 74 62 3 60 parallel mode 6 98 x 12 7 63 0 10 10 46 x 12 06 7 6 20 10 65 x 12 13 9 3 Red 60 11 31 x 13 32 40 9 60 parallel mode 11 64 x 15 65 53 4 Further details of the PACS PSF can be found in the technical note PICC ME TN 033 version 2 0 Apr 04 2012 The aperture correction factors for the are listed in the table in section Figure 3 4 PACS photometer scientific capabilities Encircled Energy Fraction 1 1 a ad OC T rie p he Lp a ew 3 34 EEF 0 1 peg pee a a 444 0 5 10 15 20 25 30 Radius in arcsec Figure 3 2 Encircled energy fraction as a function of circular aperture radius for the three bands De rived from slow scan OD160 Vesta data The EEF fraction shown is normalized to the signal in aperture radius 60arcsec with background subtraction done in an annulus between radius 61 and 70 arcsec This information is now known to be obsolete the values tabulated in HIPE should be used instead Warning Ed The Encircled Energy Fraction data given here have subsequently been made obsolete by later ob servations The correct values are given in HIPE and only these should be used to derive the aper ture correction until a full update of the figure and table here can be made S N for increasing aperture LEEN AET EXDPES4 tis kt ery erry ti e A N e o 0 08 0 06 1 153954 4 3 rd 3 l x Ia 3 3 4T 0 04 EEF Radius S N 0 02 TT L
52. HERSCHEL DEaSERVATORY PACS Observer s Manual HERSCHEL HSC DOC 0832 Version 2 5 1 09 July 2013 PACS Observer s Manual Published version 1 0 01 February 2007 Published version 1 1 14 March 2007 Published version 1 2 04 June 2007 Published version 1 3 04 July 2007 Published version 1 4 08 October 2007 Published version 1 5 17 October 2007 Published version 2 0 18 May 2010 Published version 2 1 01 June 2010 Published version 2 2 07 April 2011 Published version 2 3 for OT2 call 08 June 2011 Published version 2 4 for OT2 call Phase 2 22 December 2011 Published version 2 5 small update for post Operations 03 July 2013 Published version 2 5 1 further small update for post Operations 09 July 2013 bc To IU Figure 1 RCW 120 HII emission nebula PACS100 160 um SPIRE250um colour composite image Zavagno A et al Star formation triggered by the Galactic HII region RCW 120 A amp A 2010 Table of Contents Mis LC A m 1 T 1 PUrpOse OF document c erre rre votes dae PRX er Suen PEENE EPIS REEF PRENNE E PORSE ewe named E PIO 1 1 2 Background M E 1 1 3 Acknowledgements 2 2 5 rrt tmt t ssa E RR EORR RESET RE TEE E EARS 2 T 4 A CrODYIIS a io pee rer oreet pe cesta soba desks our eU ego e epa e ve RU E Erde eere NER ts 2 2 The PACS instrument mec o ce rere viv ed bind Seas cht OPNS em rng 3 2 1 Overview instrument Concept icis epu detect inet lee
53. II 157 7 122 R1 100 220 micrometer NII 205 3 178 4 10 Spectrometer flux calibration 4 10 1 Recovering full beam line fluxes and flux dens ities for point sources The fraction of the Herschel PACS PSF seen in one spaxel varies with wavelength In order to re cover full beam line fluxes or flux densities for point sources a wavelength dependent correction factor needs to be applied to the line fluxes or flux densities as measured in the central spaxel only see Section 4 4 The wavelength dependent fraction between the point source flux seen in the cent ral spaxel and the full beam flux has been modelled and scaled to actual measurements of the PACS spectrometer PSF maps measured on Neptune The correction curve is depicted in Figure 4 5 This correction curve assumes a point source perfectly centred on the central spaxel This curve is applied in the interactive pipeline scripts provided for point source data reduction and can be found in the calibration table calTree spectrometer pointSourceLoss Alternatively the full flux or flux density of a source can be recovered by co adding the spectra ob tained from several spaxels aperture photometry In this case of course no correction factor needs to be applied However if accurate line shapes are to be preserved at the level of the instru mental resolution or below it is not recommended to co add the spectra obtained in different spaxels As with all slit spectr
54. Second pass spectral ghost A second pass in the optics of the PACS spectrometer can cause a ghost image on some spaxels Figure 4 23 shows the spaxels where a second pass ghost might appear and the location of the cor responding spaxels where the originating real emission is located The ghost appears shifted in wavelength If a source in one of the originating spaxels shows a strong spectral line typically an atomic fine structure line a weak broadened line can be seen at an offset wavelength in the corres ponding spaxel affected by 2nd pass ghosts The peak flux of this line is typically 5 of the line peak of the originating line The integrated line flux can be up to 14 of the integrated line flux of the originating line An example is shown in Figure 4 24 The wavelength offset between the origin ating line and the ghost line depends on the spectral order of the band and varies with wavelength A few examples for the strongest fine structure lines in the PACS wavelength range are given in Ta ble 4 2 A more comprehensive list of frequently observed lines is provided in the PACS Spectro meter Calibration Document Appendix B Before interpreting broad spectral lines in spaxels potentially affected by the 2nd pass ghosts ob servers should use these tables and if available the spectra observed in the corresponding ghost source spaxel for the presence of a strong line at the originating wavelength Point source observa tions well cente
55. Section 6 1 updates in unchopped line spectroscopy AOT usage Section 6 2 updates in unchopped range spectroscopy AOT usage Version 2 5 3 July 2013 The information about Enclosed Energy Function EEF in Section 3 1 is now obsolete As a temporary measure pending a full update Figure 3 2 and Table 3 2 have been marked as Obsolete and the appropriate warnings added to the text along with a comment that until updated here the information should be extracted from HIPE Various small formatting errors have been corrected Figures have been converted to JPEG in the source file as PNG is no longer supported in this version of Docbook Version 2 5 1 9 July 2013 Reference to a pre launch version of the Technical Note on the photometer PSF updated to the latest version and a link added 102 Change record Various additional small formatting errors have been corrected 103 References RD1 Herschel Observer s Manual RD Herschel Observers Manual HERSCHEL HSC DOC 0676 PACS Calibration Document RD2 PACS Calibration Document PACS MA GS 001 January 27 2009 is sue 1 0 Ulrich Klaas et al Cantalupo et al 2009 Cantalupo C M Borrill J D Jaffe A H Kisner T S Stompor R 2009 ApJS 187 212 MADmap A Massively Parallel Maximum Likelihood Cosmic Microwave Background Map maker Dehaes et al 2010 arX1v 0905 1240 Dehaes S Bauwens E Decin L et al 2010 A a
56. a narrower wavelength range than the standard faint line implementation right 75 grating steps Colours represent the coverage of individual 16 spectral pixels in a module while the grating scans through a spectral line ms Note Cer The shorter wavelength coverage on both sides of the line profile may result a higher uncertainty in a the continuum level reconstruction HSpot sensitivities therefore report 30 increase in S N if the bright line mode is used You can see a comparison of bright and faint line mode observations on Figure 6 11 Because the bright line mode is faster than the standard unchopped mode and because we recom mend only using the mode with 1 repetition then automatically the off position will be observed quite soon after the target is observed This may be a small advantage compared with the standard unchopped mode since in principal detector transients may be better handled At the time releasing this document do not have sufficient data to determine if this is a significant gain It should be noted that the unchoppped bright line mode does not sample the spectrum as uniformly nor as comprehensively as the standard unchopped mode 50 instead of 75 grating steps Thus there is much less redundancy in the measured sampling of the edges of the band in the bright line case This has two effects Firstly the number of points that can be binned in a given spectral resolution element is less at the spectrum edges le
57. achieved either chopping nodding spectroscopy Section 6 1 7 for single lines or larger wavelength ranges on point or small diamter less than 5 sources with a clean background within 6 and the unchopped grating scan Section 6 1 9 for single lines or larger ranges on point or ex tended sources without clean background for chopping Note Cer As a general rule it is recommended to define any individual PACS Spectroscopy observation i e ai AOR no longer than 5 6 hours This is rather a safety requirement than a strict restriction on in strument use In case of a contingency an observation may not be recoverable i e declared lost for science but the next observation could be executed again in clean conditions Note Cer The wavelength switching mode Section 6 1 10 has been decommissioned and replaced by the ial 2nd generation unchopped grating scan mode the optimized solution for crowded field spectro scopy versions but time estimation has been disabled and submission to HSC is not possible These wavelength switching observing requests can be updated to unchopped grating scan mode by Warning e HSpot v5 0 and later versions allow reading of wavelength switching AORs created by older HSpot switching the mode selector button in HSpot see below for more details The two observing modes can be used in a single pointing or repeated in a raster pattern on the sky There are two sets of recommended raster patterns for mappin
58. ading to a larger rms away from the center of the observed spectrum Secondly systematic uncertainties in the RSRF which are partly averaged out when many spectral pixels contribute to a given resolution element are subject to systematic effects at the edge of the band This can lead in some cases to slopes in the baseline of the spectrum near the spectrum edges Such slopes are much less noticeable in the standard unchopped mode However if the line is strong enough these effects will be relatively insignificant when compared to the strength of the line even in the bright line mode However if the line is too faint this could become a seri ous problem and should be avoided 82 Observing with PACS spectrometer OBSID 1342215700 OBSID 1342215701 Faint mode Bright mode 198 TT PTT TT 192 E173 at 196 ON BLUE 4 291 3 ON BLUE OFF GREEN j 190 E OFF GREEN 194 d ai1s gt BE 2 1925 2 188 Ea j Ew E J E 186E E 168 J oo 1a5E EECIS J Bim E ssl i 183E iz imn E E BEL E J 182 n 181 igo Ecclesia d aaa daas ada aa laua 180 Ea Lia baa aaa aaa laa 156 0 156 5 157 0 157 5 158 0 1585 1590 15 15655 157 0 157 5 1580 1585 159 0 159 5 Wavelength u m Wavelength u m OBSID 1342215700 157 74 p m line fit OBSID 1342215701 157 74 pm line fit Faint mode Bright mode iF mor E i 10 E Data 450 6E Data xl oF Model 3 400 5E Mode 8E Coverage 350 AE Coverage 3 350
59. alibrators corrected with the response de rived from the calibration block divided by model predictions band B3A 39 PACS spectrometer scientific capabilities Module 12 RD B2B LZUITETStLTTEXYIGCULITETIUCO O TT TT 1 15 J 1 10 i J wm i M i 3 w 1 05 Ph H X j e 2x d 1 00 j N j d 0 95 E E 0 90F J S j 0 85 d H al 0 80 gl zl OozsLEu Ty a a OE 13 T E PS 4 esl 160 180 200 220 240 260 280 300 320 OD XX Ceres Key X AlpTau Key O Ceres SED x lt GamDra Key Neptune RSRF O Neptune SED O Pallas SED Thisbe Key D Uranus RSRF O Uranus SED Vesta Key Figure 4 27 Average observed PACS spectrum of sky flux calibrators corrected with the response de rived from the calibration block divided by model predictions band B2B Module 12 RD R1 1 20 AACE se 1 15 h 1 10 1 05 4 i 1 00 EE 0 95 0 90 0 85 Normalized ratios 0 80 t 0 75 8 iil i ea ee ee op Ci ie a RE 233 y EPS3 a E P rs 160 180 200 220 240 260 280 300 OD XX Ceres Key 0 X AlpTau Key O Ceres SED x lt GamDra Key Neptune RSRF O Neptune SED O Pallas SED Thisbe Key D Uranus RSRE CO Uranus SED Vesta Key Figure 4 28 Average observed PACS spectrum of sky flux calibrators corrected with the response de rived from the calibration block divided by model predictions band R1 Table 4 3
60. ampling duration duration duration um high dens Nyquist factor factor sec high sec sec ity SED high dens Nyquist density Nyquist chopped ity SED un chopped SED B3A 51 73 168 2220 41 1 3 1 31512 2384 2728 682 B2A 51 73 188 2300 36 7 3 0 28160 2304 1032 258 B2B 71 105 188 2400 36 2 2 8 26728 2096 2096 524 R1 102 220 240 2500 27 9 2 7 34264 3288 parallel range In high sampling density mode integration times can be very long for instance a full up down scan in the first order takes more than 5 5 hours The time scale of the detector drifts does not allow such a long scan as the time spend on one nod position will be too long In order to improve data quality for deep Nyquist sampled observations a spectral dithering scheme has been implemented for repeated ranges the subsequent scans are performed with a small offset so that one spectral resolution element is seen by as many pixels as possible You can take advant age of this dithering for range repetition factor equal or larger than 2 The Nyquist sampling shall therefore be the default for large wavelength range coverages as it al lows obviously faster scans than the high sampling density option but at the expense of sensitivity 90 Observing with PACS spectrometer Grating angle wavelength relation in Littow configuration angle grating normal light beam 70 Diffraction orders and 60 spectral bands 55 ul o
61. an be specified in the table For instance in the case that 10 ranges are selected the Range repetition factor has to be 1 for each range if 3 ranges are selected then the total of the 3 repetition factors has to be less or equal to 10 e g 4 5 1 or 24 343 If the sum of repetitions exceeds 10 then you must either remove spectral range s or reduce the scan repetition factor s Nodding unchopped grating scan or mapping cycles The absolute sensitivity of the observation can be controlled by entering an integer number between 1 and 100 In chop mode the on source time is in creased by repeating the nodding pattern the number of times that is entered For each of the nod positions the sequence of line scans is repeated with the relative depth specified in the PACS Range Editor Chopper throw The chopper throw and chopper avoidance angle can be selected The choice of Small Medium and Large refer to 1 5 3 0 and 6 0 ar cminutes chopper throws respectively on the sky The chop direction is de termined by the date of observation the observer has no direct influence on this parameter If an emission source would fall in within the chopper throw radius around the target the observer may consider setting up a chopper avoidance angle constraint The angle is specified in Equatorial coordinates anticlockwise with respect the celestial North East of North The avoid ance angle range can be specified up to 345 degrees
62. ange repetition and nodding repetition factors equal to one in the line spectroscopy or range spec troscopy AOTs Solid blue line third grating order filter A B3A dotted blue line second order with filter A B2A green second order with filter B B2B red first order R1 line RMS PACS full sampling range scan TOA ho e 17 1 8 10 l5 10 17 1 4 107 12 tz for 8 0 10 RMS W m2 6 0 10 4 0 10 2 0 107 0 0 60 80 100 120 140 160 180 200 Wavelength um Figure 4 30 Spectrometer point source line sensitivity in high sampling density mode for both line range repetition and nodding repetition factors equal to one in the line spectroscopy or range spectro scopy AOTs Blue third grating order filter A B3A green second order filter B B2B red first order R1 RMS continuum PACS SED range scan RMS Jy 60 80 100 120 140 160 180 200 Wavelength um 44 PACS spectrometer scientific capabilities Figure 4 31 Spectrometer point source continuum sensitivity in SED mode range spectroscopy AOT for both range repetition and nodding repetition factors equal to one Solid blue line third grating order filter A B3A dotted blue line second order with filter A B2A green second order with filter B B2B red first order R1 line RMS PACS SED range scan Se 10 i EE EEEE E E 4 5 107 4 0 1077 3 5 1077 3 0 1077 2 5 10 2 0 1077 RMS W m2 LS 9
63. anged Such a systematic offset is meant to be the main error component contributing to the APE therefore positions should be still within 2 arc seconds at the 1 sigma level to the commanded position Nodding observations require a move of the telescope boresight as many times nodding cycles are defined Therefore this observing mode requires spacecraft attitude change re pointing even for pointed observations The pointing uncertainty for such small displacements is defined by the space craft relative pointing error SRPE what is typically found around 1 2 arc second even for the largest 6 arcminutes chopper throw and the equal length nod slew This means the observer could safely use nodding cycles without compromising data quality more than what is defined by the abso lute flux calibration error Observations what suffer from a larger than l sigma pointing error are not considered being severely compromised although in case of a point source measurement the source photocentre might be significantly off from the central spaxel s geometric center position In such a case flux can be recovered two ways using a beam profile map available for the closest wavelength Fig ure 4 11 the source offset from the reference aperture is established After this offset correction the full beam line fluxes and flux densities can be recovered as described in Section 4 10 1 An offset correction task is currently being developed in the PACS ICC and will be
64. arameter Continuum flux density estimate at the line redshifted wavelength The value of this parameter is interpreted by the PACS Time Estimator as flux density for a spectrometer resolution element Leaving the parameter as the default 0 0 value means the PACS Time Estimator will not perform signal to noise estimation sensitivity estimates are still provided and default integration capacitor may be used providing the smallest dy namic range Line width unit This menu gives the flexibility to switch between physical input units sup ported by the PACS AOT logic Line width FWHM Optional parameter The spectral line full width at half maximum value in units specified by the Line Width Units pull down menu Line width in put is used only for checking purposes It helps the observer to ensure the specified line width fits within the predefined wavelength range hard coded in the PACS Line Spectroscopy AOT logic 85 Observing with PACS spectrometer Parameter name Signification and comments Line repetition Mandatory parameter The relative line strength fraction of on source time per line is taken into account by specifying the grating scan repetition factor for each line A maximum of 10 repetitions in total can be specified in the table For instance in the case that 10 lines are selected the Line re petition factor has to be 1 for each line if 3 lines are selected then the total of the 3 rep
65. ateau is 8 integrations x 1 8 sec integration time x 2 ABBA cycles 2 seconds START observation On target slew Nodding A position Nodding B position Line1 x1 192 sec Line2 x 1 192 sec Pointing layout example of a nodding 3x2 raster observation Point step START Line step Figure 6 5 Instrument observing blocks are shown for a typical chop nod measurement combining two lines with single repetition in B3A and a third line with two repetitions in band R1 3rd 1st orders set ting Line repetitions adjust the relative depth required for spectral lines while the number of observing cycle is used to repeat the entire nodding cycle In the bottom the spatial layout of the chop nod scheme is sketched Note the raster patterns has been distorted for better visibility in real observations the nod slew is significantly larger than the raster step size and nodding direction is enforced along the raster columns vertical direction 75 Observing with PACS spectrometer The full wavelength range covered by the scan and the range covered to the highest sensitivity i e the wavelength seen by all 16 spectral pixels are shown in Table 6 4 and compared with respective FWHM of the spectrometer at these wavelengths for an unresolved line The spectral coverage in line spectroscopy modes is shown in Figure 6 6 The reference for the wavelength range as specified in HSpot is spec
66. ch wavelength sees a slightly different spatial position even for spectra within a single spaxel Level 3 data These are the publishable science products where level 2 data products are used as input These products are not only from the specific instrument but are usually combined with theoretical models other observations laboratory data catalogues etc Their formats should be VO compatible and these data products should be suitable for VO access 101 Chapter 8 Change record Version 2 0 20 May 2010 New issue based on in flight experience Performance Verification phase and Science Demonstration phase all sections re written Version 2 1 28 May 2010 Various minor corrections and typo formatting corrections Analytic formula to compute sensitivity in mini scan map mode given in Section 5 2 Warning added in Section 5 1 3 that the column central area point source sensitivity is only applicable in the mini scan map mode Version 2 2 7 April 2011 Version 2 3 7 June 2011 Various minor updates on the use of the scan mapping and its performances Section 4 4 updated point source correction factors Section 4 6 observed beam maps and beam efficiency figures added Section 4 9 on second pass spectral ghosts introduced e Section 4 10 Spectrometer flux calibration accuracies updated Section 4 11 unchopped sensitivity plots added Section 4 13 new section on pointing accuracy and impact on data e
67. chopper cannot rotate this ef fectively defines an avoidance angle for the satellite orientation Hence it is a scheduling constraint The range of position angles that will be available for a given target can be visualized with the AOR footprint overlay functionality for different observing dates in the visibility windows The exact angle values can be determined with the Herschel Focal Plane overlay functionality 63 Observing with the PACS photometer Note The position angle returned by HSpot in the AOR overlays is the angle from the north to the space craft Z axis counterclockwise perpendicular to the chopping direction Therefore the chopper avoidance angle can be derived from the position angle by adding 90 degrees modulo 180 de grees Warning For pointings close to the ecliptic plane the position angle is constrained to a very narrow range of values the inclination of the ecliptic plane and the chopping direction is perpendicular to the ec liptic plane For such targets the chopping avoidance angle is at best unnecessary and at worse renders the observation impossible For observations at higher ecliptic latitudes the user shall check that the range of chopping avoidance angles is compatible with the position angles in the vis ibility windows Warning When a chopping avoidance angle is set the constraint is not yet fed back in HSpot to the visibility calculation so that the Herschel visibility windows are no
68. coarser Nyquist sampling considering the 16 spectral pixels of the instantaneous PACS coverage with grating step size of 6 25 spectral pixels 89 Observing with PACS spectrometer The reason of applying the faster Nyquist sampling is twofold broad ranges can be covered within a reasonable amount of time and deeper observations can be achieved by applying a higher number of up down scans what provides sufficient redundancy and robustness against response changes on longer time scales typically longer than half an hour Scan parameters of range scan mode and full scan durations are summarized in Table 6 6 Note Cet The reference for the wavelength range as specified in HSpot is spectral pixel 8 i e the specified 5 range is not covered by all spectral pixels and the actual range in the data set is a bit larger than specified with S N going up at the edges Table 6 6 Scan parameters in range scan modes Grating settings are shown for four grating orders and for high sampling density and Nyquist sampling options separately the duration of atomic observing blocks are for a single grating up and down scan without overheads on a full range the oversampling factor gives the number of times a given wavelength is seen by multiple pixels in the homogeneously sampled part of the observed spectrum band wavelengt grating grating over over 1 full scan 1 full scan 1 full scan hrange step size stepsize sampling s
69. compact source This mode allows the observer to set up a pointed observation in combination with chopping nod ding or unchopped grating scan techniques Pointed with dither mode Warning CJ HSpot v5 0 and later versions allow the reading of AORs in Pointed with dither mode but time es timation has been disabled and submission to HSC is not possible This mode has been decomm sissionned It has been proven during the Performance Verification Phase that flux reconstruction from a single pointed observation is as good as in dithering mode therefore dithering option is not recommended anymore For sources with a well confined photocenter point or compact sources the pointing mode can be changed from Pointed with dither to Pointed To maintain the observation integration time nod repetition and or scan repetitions should be increased until the original observing time is reached Nod or range repetition x 3 should be the appropriate change for most observations Obser vations requiring spatial oversampling should use a minimum 2x2 size raster with recommended step sizes Mapping mode This mode allows the observer to set up a raster map observation in combination with chopping nod ding or unchopped grating scan techniques In chop nod mode the map can only be defined in instrument coordinates and the map size shall be restricted to 6 by 6 to obtain clean offset positions with the large chopper throw for each raster pos ition
70. continuum and expected line fluxes are higher than the saturation limits for the default capa citance it is mandatory to enter the expected continuum and line flux for every range at the time the observation is designed and filled in HSpot Figures Figure 4 38 to Figure 4 41 show the flux limits at which a larger integrating capacitance is selected If an observation contains lines that fall in dif ferent flux regimes the largest capacitance will be chosen for the entire observation If lines in the same observation fall in different flux regimes it is recommended to split the observation into seperate observations per flux regime Figure Figure 4 42 and Figure 4 43 show the saturation limits using the largest integrating capacitance If the expected line flux and continuum flux are higher than these limits then detectors may run into saturation and flux calibration becomes unreliable 47 PACS spectrometer scientific capabilities PACS spectro saturation limit Capacitance 0 telescope background included 10 Saturation limit Jy S 40 60 80 100 120 140 160 180 200 220 240 Wavelength u m RI B2B B3A B2A Figure 4 36 Saturation limit with the default smallest 0 14 pF integrating capacitance including 80 safety margin PACS spectro saturation limit Capacitance 0 telescope included Saturation limit W m 40 60 80 100 120 140 160 180 200 220 240 Wavelength u m Fig
71. d are also different by 10 between two separate spectra taken in separate observations This con tinuum uncertainty for faint targets has been discussed previously observers should not use this mode to measure the continuum 79 Observing with PACS spectrometer od 4 1 8 iy 186 1 5 fF 1 4 1 3 f LAs Flux lt chop nod gt ti 1 0 M E t ca ook cs Low 156 0 156 5 157 0 1575 1580 1585 1590 159 5 Wavelength 1 z US after OFF US before OFF Chop Nod Figure 6 8 A comparison of same faint red emission line source observed with unchopped mode and a chop nod observation The turquoise and blue spectra are the two unchopped spectrum taken in differ ent observations and show a variation between each other in the continuum of the order of 10 The Green spectrum shows the same source observed in chop nod mode where the continuum is much more accurately measured and is lower than the unchopped mode by approximately 30 Note that the line itself is well reproduced in both flux and line shape The systematically larger amplitude of the line sig nal seen in the unchopped mode compared with Chop Nod roughly 10 is a known artifact of the way the line is calibrated in this example using methods designed for chop nod calibration Future calibra tion tables designed for the unchopped Mode will remove this scaling difference The example of an observing block
72. d be avoided without consulting a PACS expert i e contact Helpdesk 6 1 3 Observing spectral lines in the 51 55 micron range Noise performance investigations revealed that continuum and line sensitivities provided in HSpot v6 0 and earlier were too optimistic shortward of 55 mu i e in the shortest edge of the PACS wavelength range In HSpot v6 1 0 a factor 2 at 55 micron and factor 3 5 at 52 and 50 micron has been introduced in band B2A resulting the sensitivities in Table 6 2 Table 6 2 Updated continuum and unresolved line seinsitivities in the 50 60 micron range in spectral band B2A for an observation with single repetition These sensitivities are available in HSpot versions v6 1 0 and higher Wavelength um Continuum RMS Jy Line RMS W m 2 51 0 38 47 1 75E 16 52 0 3 02 1 13E 16 55 0 0 43 1 60E 17 60 0 0 22 8 01E 18 Sensitivity numbers have been obtained from observations of sources covering the 30 670 Jy con tinuum flux range at 51 mu Uranus Neptune NGC5315 HD44179 NGC1068 NGC4418 and Arp220 The PACS Instrument Centre no longer recommends observing lines in the 51 55 mu range in band B3A as this spectral band has an incorrect flux calibration in this range and its sensitivity estimates are still unreliable in HSpot v6 1 0 Note LER Shortward of 52 5 mu in band B3A the continuum is not reliable because the detected light origin zi ates from a mix of second and third diffraction
73. d be equal to the duration of an on scan The reason is in the AOT logic by every range repetition of a Nyquist sampled range PACS is introducing a small wavelength offset to improve spectral sampling In case off position data are not available for precisely the same grating positions as the on scan data then it raise a limitation on which off subtraction technique can be used in data pro cessing After all it could lead to degradation of the final off subtracted spectrum It would be possible to use the same off for more than one set of on AORs if the on observa tions were relatively short duration range scans This is a very efficient use of the mode for in stance in a crowded field of clustered targets more than one on target scan could share the same off position observation if the time gap between any on off pair does not exceed the recommended limit of 60 minutes The most simplistic example of such a sequence is ON_target1 OFF ON_target2 i Note LE It is recommended to concatenate AORs of an ON OFF sequence Concatenation is allowed up e to 2 degrees angular separation between 2 neighboring positions in a chain i Note LEF For mapping raster observations the off AOR does not have to be a raster The requirement is a similar to a pointed case in a sense that the total duration of off scans should be equal to the dura tion of an on scan obtained at any raster position In practice this means first you create th
74. d for stability speed and delivering browse quality data The pipelines are expected to mature as both the instrument effects and the un derstanding on how to mitigate said effects evolves The major data processing modules of the pipeline are discussed below Cosmic rays removal The default deglitching algorithm for PACS photometer is the multi resolution median transform MMT which uses wavelet scales Starck et al 1998 PASP 110 1935 to differentiate between a cosmic ray hit and signal The method produces reasonable res ults except for bright point like sources e g compact galactic nuclei etc and fast 60 sec scan speeds Under such conditions MMT incorrectly masks point source cores Users are ad vised to check the coverage map to look for any gaps in exposure depth at the location of point sources The optional module 2nd order deglitching can be used in the interactive data pro cessing as a replacement for MMT deglitching 2nd order deglitching uses spatial redundancy and sigma clipping to reject cosmic ray hits Cross talk correction The red bolometer array shows cross talk between column 0 and 16 That is when a source is present in column 16 its flux is also observed in column 0 Investiga tions on proper removal of cross talks are underway Users are currently advised to mask all sig nal in columns 0 and 16 to avoid any artifacts from cross talk in the final maps when observing very bright point sources e
75. dding 90 degrees to the array to map angle and recomputing the cross scan distance If the array to map angle is 45 degrees the cross scan distance if even the same i Note A small array to map angle for instance 10 degrees modulo 90 degrees allows to get rid of the effect of gaps between matrices but also to get a homogeneous exposure map toward the edges for small scan maps hence minimizing the the science time Scan maps defined in instrument reference frame should in principle be used to cover square areas 55 Observing with the PACS photometer as the orientation of the scan map on the sky can not be known in advance it depends on the array position angle which itself depends on the exact observation day However in order to cover specific rectangular areas in the sky a constraint on the orientation of the scan map in the sky can be introduced by selecting a range for the map position angle i e the angle from the celestial equatorial north to the scan line direction counted positively east of north This corresponds to the option array with sky constraint in HSpot shown in Figure 5 2 Warning e Introducing a sky constraint puts a constraint on the scheduling and therefore shall be used only if necessary Moreover certain combinations of array to map angle and ranges of map position angle might not be feasible For instance for pointing close to the ecliptic plane the array position angle gets constrained
76. ding the fixed overhead of 3 min for the initial slew to target 62 Observing with the PACS photometer 5 3 1 Figure 5 8 Exposure map of a point source AOR in HSpot An example of an exposure map as generated by the HSpot exposure map tool for the blue channel is shown in Figure 5 8 The chop nod point source mode works fine for bright sources and empty fields But the achieved sensitivities are worse by a factor 1 5 2 compared to the pre flight prediction and the mini scan map mode see Section 5 2 because the chopping frequency is not high enough to remove the 1 f noise at 3 Hz Despite the degraded sensitivity this mode has advantages for intermediately bright sources in the range 50 mJy to about 50 Jy a small relative pointing error RPE of 0 3 arcsec and high photomet ric reliability and reproducibility Chopper avoidance angle in point source mode In the point source photometry mode the properly imaged field i e with chopping and nodding is rectangular about 52 arcsec x 2 5 arcmin see Figure 5 8 The user might therefore want to exclude some position angles of the chopping direction to avoid chopping into a bright close by infrared source For this purpose an interval of chopper avoidance angles can be entered in HSpot The chopper avoidance angle is counted positive east of north i e counterclockwise in the sky from the north to the direction of the object to avoid i e the Y spacecraft axis As the
77. divided by model predictions The Table 4 3 shows a summary of results the RMS of PACS observation predicted model flux and peak to peak scatter around the expected flux densities This is the absolute flux calibration accuracy to assume for a single PACS spectro scopy observation Module 12 RD B2B L20rgc T4 UtrT4ESI SUGITZEYYEIUUHGITEATUS 1 15 o 1 10 h i l k 1 05 ty 1 00 0 95 0 90 f Normalized ratios 0 85 0 80 j POY spe EypudagqppedyTIEP Ag qa gu 160 180 200 220 240 260 280 300 320 OD X AlpTau Key X Ceres Key O Ceres SED x GambDra Key Neptune RSRF O Neptune SED O Pallas SED Thisbe Key D Uranus RSRF CO Uranus SED Vesta Key tix laxa Ea error E od Tasa Esas Fo asa Figure 4 25 Average observed PACS spectrum of sky flux calibrators corrected with the response de rived from the calibration block divided by model predictions band B2A Module 12 RD B3A L2G ToL so nL eta a we Lhe on oo a 1 10 j 0 95 1 156 0 90 0 85 ii 0 80 i 0 75 7g eh te a e a en ae 5815 160 180 200 220 240 260 280 300 OD X AlpTau Key Ceres Key O Ceres SED lt GamDra Key Neptune RSRF O Neptune SED O Pallas SED Thisbe Key D Uranus RSRF CO Uranus SED Vesta Key Normalized ratios eee ceed borra borra Dona borra Eon Eon bor birra w N 20 Figure 4 26 Average observed PACS spectrum of sky flux c
78. dow The sensitivity in the central area can be significantly better for small scan maps where the cover age map is highly peaked towards the centre for instance in the mini scan map mode Warning e HSpot also returns a central area point source sensitivity but this works only in the case of the mini scan map mode Alternatively the sensitiviity can be assessed with the exposure map tool Overlays gt Show expos 58 Observing with the PACS photometer ure map on current image which returns the integration on the sky in seconds from the coverage map The point source sensitivity can be derived by scaling the 16 1 second sensitivity values given in Table 3 2 with the square root of the integration time given by the exposure map tool as follows Sensivity 16 S 16 1s V T with S 16 1s the sensitivity values given in Table 3 2 and T on sky integration time in seconds Figure 5 5 Example of a depth of coverage map for a small scan map as given by HSpot together with AOR display overlaid 5 2 Mini scan map mode The mini scan map mode is a particular case of the scan mapping mode where the the scanning is done along the 2 diagonal of the bolometer detector We advocate to use this mode for point sources in a so called mini scan map configuration with short 3 arcmin scan legs The advant ages for the mini scan map mode over the original chop nod point source mode are e better point source s
79. e and wavelength Order sorting Filters The PACS order sorting filters enable the spectral purity of the selected band by suppressing contri butions by other orders the detector is sensitive to There are in total 3 bands in the PACS spectro meter 55 72 um 72 102 um and 102 210 um The filter transmission is shown in Figure 4 3 The filter train of both the photometer and spectrometer channels is illustrated in Figure 2 4 Photoconductor arrays The spectrometer employs two Ge Ga photoconductors arrays low and high stressed with 16x25 pixels on which the 16 spectral elements of the 25 spatial pixels are imaged The Ge Ga photoconductor arrays have a modular design they are made of 25 linear modules of 16 pixels each are stacked together to form a 2 dimensional array Ge Ga photoconductors are sensitive in the wavelength range 40 110 120 um without any stress A stress is therefore applied to improve the long wavelength sensitivity The stressing mechanisms ensures homogeneous stress on each pixel along the entire pile of 16 spectral elements The low stressed blue detectors has a mechanical stress on the pixels which is reduced to about 1046 of the level needed for the long wavelength re sponse of the red detectors Light cones in front of the actual detector block provide an area filling light collection in the focal plane and feed the light into the individual integrating cavities around each individual mechanically stressed detector cry
80. e on source raster then copy this AOR into an off scan by changing the observing mode from raster to pointed of course do not forget to change the target coordinates from on to off position In brief the off position needs to be observed for two purposes see also Figure 6 15 e 1 Continuum recovery For bright sources gt 20Jy differencing the on and off can provide 93 Observing with PACS spectrometer a source continuum within uncertainties comparable with chop nod observations However ex periments have shown that responsivity drifts on long timescales make continuum levels uncer tain at 4 5 level of the telescope background typically 200 Jy what makes continuum recov ery unreliable for faint sources For such faint targets the continuum level could be adjusted us ing photometer observations Note the in band shape error is much smaller typically 1 of the telescope background This means the continuum shape even for targets below 20Jy could be recovered in a fairly reliable way e 2 Suppression of RSRF noise Observations taken in the off position can allow small uncer tainties i e wriggles in the relative spectral response function to be corrected in the on obser vation This is especially valuable for range scan observations where the subtraction of the off can significantly improve signal to noise of the on spectrum Normalized continuum spectra Normalized continuum
81. e option Nyquist sampling are shown in figures Figure 4 31 and Figure 4 32 for con tinuum and line detection respectively The in orbit performance verification results indicate that with optimized detector bias settings and 42 PACS spectrometer scientific capabilities modulation schemes chopping spectral scanning NEPs measured in laboratory can actually be achieved The line and continuum sensitivities as a function of observing time have been verified in orbit and are consistent with pre launch predictions Figure 4 33 shows the comparison of pre launch sensitivity predictions with in orbit line scan observations s Note Ce I Sensitivity plots can be easily produced in HSpot go to the Range Spectroscopy AOT define a full e wavelength range in Nyquist or high sampling density and adjust the repetition factors By click ing on Observation estimation and then Range sensitivity plots a pop up window will show both line and continuum sebsitivities as a function of wavelength for the integration time defined in the AOR The best 50 1 hour sensitivity in the first order corresponds to about 100 mJy for the continuum 2x10 5 Wm for the line sensitivity and a factor 2 5 worse roughly in the 2nd and 3rd order Figure 4 34 shows a comparison between the measured rms range uncertainty in chop nod and un chopped observations scaled to a 450s integration time Results agree favorably with respect to HSpot predictions On
82. e spectroscopy Note LER HSpot provides information about sensitivity and wavelength coverage in the nominal as well as 7 in the parallel range s For long range spectroscopy it is imperative to verify the actual wavelength coverage in both channels in order to optimize observing strategy Similar to Line Spectroscopy background subtraction is achieved either through standard chopping nodding for faint compact sources or through unchopped grating scan techniques bright point or extended sources of the grating mechanism The observer can select either chopping nodding or un chopped grating scan in combination with one of two observing mode settings pointed and map ping As in line spectroscopy three chopper throws are available Small 1 5 arcmin Medium 3 ar cmin and Large 6 arcmin except in the mapping mode where only the large chopper throw is 87 Observing with PACS spectrometer allowed in order to chop out of the map In this case the map size is also limited to 4 arcminutes eon PACS Range Spectroscopy Figure 6 13 The PACS Range Spectroscopy AOT front end is shown in HSpot v5 0 left and the para meter input fields for pointed mode up right and for mapping mode bottom right The coloured circles indicate ways to adjust the depth of line observations red shows line repetitions used to adjust the relative depth of various lines in the Range Editor Table green shows the observing cycles
83. econds to execute therefore this configuration has exactly the same sky time 2096 s as a chopped SED B2B observation of a single repetition Note for very bright sources 95 Observing with PACS spectrometer applying a single range repetition the unchopped mode integrates half the time including ON OFF comparing to the shortest chopped version However we strongly discourage users from adopting this method of observing for purely efficiency reasons if chopping is an option Table 6 7 User input parameters for Range Spectroscopy AOT Parameter name Signification and comments Wavelength ranges Which of the torder combinations to use or SED templates e Option 1 Range scan in 70 105 and 102 220 microns 2nd 1st orders default option e Option 2 Range scan in 51 73 and 102 220 microns 3rd Ist or ders e Option 3 Range scan in 51 73 and 102 146 microns 2nd Ist or ders e Option 4 This mode has been deprecated SED Red 71 210 mi crons 2nd Ist orders Option 5 This mode has been deprecated SED Blue 55 73 mi crons 3rd order Option 6 This mode has been deprecated SED Blue high sensitiv ity 60 73 microns extended 2nd order e Option 7 SED B2B long R1 70 105 um 140 220 um e Option 8 SED B2A short R1 51 73 um 102 146 um Option 9 SED B3A long RI 47 73 um 140 219 um As a result of the selection the wav
84. ectrometer The slicer assembly consists of 3 set of mir rors e The slicer stack 5 identical spherical field mirrors individually tilted which forms separate pu pil images for each slice on the set of 5 capture mirrors e The capture mirrors re combine the separate beams into the desired linear image on the set of 5 spherical mirrors at the exit of the slicer assembly The PACS instrument The field mirrors at the exit re combine the pupils separated in the slicer into a common virtual pupil The collimators of the spectrometer will later form an anamorphic image of this virtual pupil onto the grating At the same time the field mirror apertures serve as the entrance slit of the grating spectrometer a 9 4 x 9 4 E 5x 5 pixels E HEN spectrograph w slit x B nd Ga ui spatial dimension PANAN 16 x 25 pixel detector array 16 x 25 pixel detector array spectral dimension spectral dimension Figure 2 6 Integral field spectrometer concept projection of the focal plane onto the detector arrays in spectroscopy mode The image slicer re arranges the 2D field along the entrance slit of the grating spec trograph such that for all spatial elements in the field spectra are observed simultaneously Note the blank space left between the slices to reduce crosstalk between left and rightmost pixels of adjacent slices see also Figure 2 9 On the righ hand side the spatial slicing sch
85. ed it applies to both the red and the blue channel 65 Chapter 6 Observing with PACS spectrometer Two observation schemes are offered for the PACS spectrometer Line Spectroscopy and Range Spectroscopy Astronomical Observing Templates AOTs Line Spectroscopy AOT A limited number of relatively narrow emission absorption lines can be observed for either a single spectroscopic FOV 47 x 47 or for a larger raster map The fixed angular range scanned by the grating mechanism is optimized for a given diffraction order and ensures the detection of the full profile of an unresolved line with sufficient continuum cov erage symmetric to the line centre Faint and bright line options are available for bright lines the redundancy in the spectral domain has been compromized in terms of the angular range scanned by the grating Range Spectroscopy AOT This is a more flexible and extended version of the line spectro scopy mode where a freely defined wavelength range is scanned by stepping through the relev ant angles of the grating with selectable grating sampling density high sampling density and spectral Nyquist sampling mode Both arrays are used at a time spectra can be obtained from the combination of nominal and parallel ranges This AOT has to be used for long range spec troscopy as well as for full coverage of broad spectral lines There are two PACS spectrometer observing modes used by both AOTs background subtraction is
86. ee ccee eee cee e m e m mH mH I eI ener 38 4 10 1 Recovering full beam line fluxes and flux densities for point sources 38 4 10 2 Flux calibration accuracies ssssssesseseeee e emm mH emere 38 4 11 Spectrometer sensitiVity c serer ni neern e rr b Ee MEO Ra ses EO Da Fee P tav PO E Eee 42 4 12 Spectrometer saturation limits cece cece cece cece e eme mH mee men nennen 47 4 13 Astrometric Accuracy k iiis eir cime reote Ee EEKEREN OSEE SEERE RETES 52 5 Observing with the PACS photometer sssssssssse eene enm e eee enne 53 5 1 Scan mapping mode d rt ette rte re POR ERO Ro Eae Patto eee 53 5 1 1 Scan maps in instrument reference frame ssssssesesee e 55 5 1 2 Scan maps in sky coordinates sss emere 56 5 13 Sca maps Sensitivity 545 ciment th erui a o IRE SURE deat ee Doe e E 58 5 2 Mini scan Map mode cse es aE pus ere ote DAN ete esc eee ds 59 5 3 Chop nod point source photometry mode 2 0 0 0 eee cece ence ence eeeeeece ee 61 PACS Observer s Manual 5 3 1 Chopper avoidance angle in point source mode sese 63 3 4 Gain setting for brigbt SOUtCes eco ese t tertiis seo tee ER etre neben 64 6 Observing with PACS spectrometer vssait erie re Ee III ehe hen ent emen nre rennen 66 6 1 eine Spectroscopy AQT iioc ote n tete a Peer ese se iet Ufo Et Der decodes e oed 67 6 1 1 Flux estimates and dynamic range
87. eee oeko ae ds eer odores 3 2 2 COMMON OPES 2 5er ee NER RF es eU dese Ud 3 2 2 Entrance Optie Serenassa e oe Orfeo Rer eo erp use se trt eher tenore eoe Pe E uae sense 5 2 272 Calibration SOUrCes c eec emet e rre oe ental seagate cdsdane ete oed oes 5 PAP PSPE GHI HEC 6 2 3 Photometer oie oe epp EEE EEEE EEE ENEE EEES E E EE 6 2 3 1 Filters 55e er rete T 6 2 3 2 Bolometer arrays eee tee tdeo eerie udo reete dieto oos e ese edere ge conve 7 PESE 7 pA ETII M 8 2 4 Instrument design 2 3 scenes sce cesses votes terr ert E E EE PRIE ret ERE PEE 8 2 42 Image SUCEL eee eet testet err iot geste teure des heey hess sonia o e eden t o 8 23 Gratng osea oet e eds Gas eats ata e eo d t ee Qui eee UEM d 9 2A Ap Order sor ng Filters eey nep ertet eee sented oe bawuanasng Ete Per e dob e EE UPo pete Dre po 10 2 4 5 Photoconductor arrays sese eme mH He me Ihe mee rentrer 10 3 PACS photometer scientific capabilities esses emere 13 331 Port spread Function gy ee Ue eee Reve esee esso UE URS 13 3 2 Photometer filters ire rr ap t er reor EE RP D PRESE PEDES TISERE vine Sa Eni 16 3 3 Photometer flux calibration 2 0 0 0 cece cece cece cece eem HH HH He e meer rennen 17 3 4 Photometer bad pixels sess ett rrt t tem pa Pee Pe E Ero ORE Eee POUR FUE PESE RE 18 3 5 Photometer sensitlVity 1 eere teneretur p peer eR EEE AE EEEE RETEN Te pegas 19 3 6 As
88. eeper a ee Tan ae yD 9 99 Cerri tiiriitiviirtdirriritiiiit iii i 0 5 10 15 20 25 3 Radius in arcsec Figure 3 3 The signal to noise curve under the assumption that noise scales linearly with aperture radi 15 PACS photometer scientific capabilities us Note that this assumption is not met for scanmaps with 1 f noise Radius arcsec EEF blue EEF green EEF red i mm ip 2 0069 3 0 402 0 146 5 0 642 0 341 6 0 701 0 438 T 0 750 0 524 8 0 597 9 0 656 10 0 700 12 0 759 i i 18 0 871 20 0 897 25 0 934 30 0 953 40 0 982 50 60 1 0 Figure 3 4 Encircled energy fraction as a function of circular aperture radius for the three bands De rived from slow scan OD160 Vesta data in the three photometer bands The EEF fraction shown is nor malized to the signal in aperture radius 60 arcsec with background subtraction done in an annulus between radius 61 and 70 arcsec This information is now known to be obsolete the values tabulated in HIPE should be used instead 3 2 Photometer filters The transmission of the filter chain in each of the instrument channels has been calculated from measurements of the individual filters All filters have been measured at room temperature some filters or samples taken from the same filter sheet as used for the flight filter have also been meas ured in a contact gas cryostat near Helium temperature Generally
89. elengths ranges to be specified in the PACS Range Editor table have to be either within the 70 220 microns band or within the 51 73 and 103 220 microns band ON OFF Selector Optional parameter Default value is Undefined if unchopped grating scan mode is enabled then either ON or OFF can be selected depending on the purpose of the observation Blue edge microns Mandatory parameter The starting wavelength of the range to be observed The PACS grating will perform an up and down scan starting at this wavelength The Blue Edge must have a shorter wavelength than the Red Edge the minimum permitted separation is 1 micron Note sampling by the 16 pixels is not homogeneous at the border of ranges i e the S N in creases at the two extremities Observers need to broaden ranges if homo geneous S N is an issue for the observation Red edge microns Mandatory parameter The longest red wavelength of the range to be ob served The PACS grating will perform an up and down scan terminating at this wavelength Line flux unit This menu gives the flexibility to switch between physical input units sup ported by the PACS AOT logic 96 Observing with PACS spectrometer Parameter name Signification and comments Line flux Optional parameter User supplied line flux estimate in units specified by the Line Flux Units option Line flux input is used for signal to noise es timation as wel
90. eme is illustrated by the image of an extended structure 2 4 3 Grating The grating assembly consists of a Littrow mounted grating a mounting bracket that interfaces with the FPU structure the actuator with redundant coils that provides positioning of the grating the re dundant position sensors a launch lock mechanism with redundant coils for the launch lock actuat or the redundant temperature sensors and the duplicated cryo harness see Figure 2 7 The PACS instrument 2 4 4 2 4 5 Figure 2 7 Flight model grating unit A torquer motor is used to actuate the grating angle which is meas ured with sub arcsecond precision by an inductosyn angular resolver The grating blank has a length of 320mm with a groove period of 8 5 0 05 grooves mm with a total of approximatively 2720 grooves The reflection grating is operated in the first 102 210 um the second 72 102 um and the third diffraction order 51 72 um Grating deflections from 25 de grees to 70 degrees are possible to cover the full wavelength range of each order A graphical correl ation of the grating angle of incidence versus order and wavelength is given in Figure 2 8 Grating angle wavelength relation in Littow configuration 70 maaa a aaa aa 65 third order 60 55 50 45 40 first order angle grating normal light beam 60 80 100 120 140 160 180 200 wavelength microns Figure 2 8 Relation between grating angl
91. ensitivity in all bands as a high pass filter can be used to remove 1 f noise up to higher frequencies it provides a better characterisation of the close vicinity of the target and larger scale structures in the background also targets with positional uncertainties of 10 arcsec or more are still perfectly covered the final map has a much larger area of homogeneous coverage about 50 arcsec in diameter de pending on observation configuration 59 Observing with the PACS photometer more pixels see the target the impact of noisy variable and dead pixels is less problematic no negative beam in final map In case of using the scan map mode for point sources and very small fields we propose the follow ing configuration medium scan speed 20 arcsec s scan angle in array coordinates along the two array diagonal directions 110 and 70 degrees scan length 3 0 the array diagonal has about 4 arcmin The source is on array during satellite constant speed parts if the observer follows the mini scan map recommendations but during satellite turn arounds and acceleration and deceleration phases the source is outside the array In order to have the source always on array during all phases one should select a scan leg length of 2 5 arcmin Note that scan legs have to be multiples of 5 arcsec small and even number of scans 4 6 8 10 for minimisation of satellite movements and a match to the array diagonal small leg s
92. eparation 2 5 arcsec with the smaller separation for a larger number of scan legs and vice versa to have the source on array in all legs Examples 8 scan legs of 3 arcmin length and 4 arcsec separation This map would then match a sky region with the width of about 3x FWHM in the red band with very high coverage repetition factor as needed to reach the required sensitivity cross scan maps it allows to apply all kinds of map making techniques and not just the high pass filtering The cross scans are also useful to obtain higher photometric accuracy for faint sources and better spatial characterisation in the near source vicinity It is recommended to group concatenate the 2 cross scan AORs to minimise slew overheads Each AOR will have its own 30 s calibration block Warning Do not use the homogeneous coverage in mini scan map mode 60 Observing with the PACS photometer Figure 5 6 Coverage map for 2 mini scan maps at array angles of 70 and 110 degrees The homogen eous high coverage area circle is about 50 arcsec in diameter The 1 hour point source sensitivities that can be achieved with this mode are reported Table 3 2 Note ce The sensitivity in miniscan map mode is returned in the column central area point source sensitiv ity in the instrument performance summary window Otherwise the sensitivity in the central area of the mini scan map mode can also be directly estim ated using the formula in Section 5 1
93. etition factors has to be less or equal to 10 e g 4 541 or 24343 If the sum of repetitions exceeds 10 then you must either remove spec tral line s or reduce the scan repetition factor s Redshift Selection The redshift selection menu allows the observer to make adjustments to the observing wavelength The input can be specified either in known radial ve locities or redshifts Once any area in the PACS Line Editor is clicked on the Redshifted Wavelength field turns to show the wavelength to be ob served Nodding unchopped grating scan or mapping cycles The absolute sensitivity of the observation can be controlled by entering an integer number between 1 and 100 In chop mode the on source time is in creased by repeating the nodding pattern the number of times that is entered For each of the nod positions the sequence of line scans is repeated with the relative depth specified in the PACS Line Editor In case of un chopped grating scan mode this cycle is used to repeat the ON OFF blocks the number of times that is entered Chopper throw The chopper throw and chopper avoidance angle can be selected The choice of Small Medium and Large refer to 1 5 3 0 and 6 0 ar cminutes chopper throws respectively on the sky The chop direction is de termined by the date of observation the observer has no direct influence on this parameter If an emission source would fall in within the chopper throw radius around the
94. f the homogeneity of the map coverage as the cross scan distance to achieve this purpose depends on the array position angle which itself depends 56 Observing with the PACS photometer on the exact observation day The user shall be very careful in selecting a cross scan distance when in sky coordinates Values above 105 arcsec may lead to non overlapping legs depending on the ar ray to map angle In order to allow a minimum overlap between consecutive legs the user is ad vised not to select a cross scan distance above 105 arcsec to be immune against all possible values of the array to map angle os Note cr A cross scan distance of 51 arcsec i e the size of single blue array matrix gives relatively flat ex M posure maps for scan map in sky coordinates whatever the array to map orientation angle Conversely the array to map angle of scan maps in sky coordinates can be constrained with the op tion sky with array constraint in HSpot as shown in Figure 5 3 Again certain combinations of map orientation angle and constraints on array to map angle might be impossible this shall be checked by the user with the overlay AOR functionality of HSpot Table 5 1 lists the user input parameters required in HSpot in scan map mode A decision tree to choose the most appropriate orientation reference frame a scan map is given in Figure 5 4 Figure 5 4 Decision tree for scan maps orientation reference frame Table 5 1 User
95. factor has to be 1 for each line if 3 lines are selected then 67 Observing with PACS spectrometer 6 1 1 6 1 2 the total of the 3 repetition factors has to be less or equal to 10 e g 4451 or 24343 Background subtraction is achieved either through standard chopping nodding for faint compact sources or through unchopped grating scan techniques for line measurements especially of bright point or extended sources of the grating mechanism The observer can select either chopping nod ding or unchopped grating scan in combination with one of two observing mode settings pointed and mapping eo PACS Line Spectroscopy PACS Line Spectroscopy AOR example 70 220 microns 2nd 1st orders Waveleng Redshifte Line Flux Line Flu Continuu Line Width Line Wi Line Repe Add Line Manually_ Add Line From Database_ Modify Line JIK Delete Line_ a m Add Comments JU AOR Visibility Figure 6 1 The PACS Line Spectroscopy AOT front end is shown in HSpot v5 0 left and the paramet er input fields for pointed mode up right and for mapping mode bottom right The coloured circles indicate ways to adjust the depth of line observations red shows line repetitions used to adjust the relat ive depth of various lines in the Line Editor Table green shows the observing cycles used to apply to ad just the absolute sensitivity and finally blue in the mapping mode indicates that o
96. frequency of 0 625 Hz between the two PACS internal calibration sources 19 chopper cycles are executed each chopper plateau lasts for 0 8 s 32 readouts on board producing 8 frames in the down link There are always 5 secons idle time between the calibration block and the on sky part for stabilisation reasons 5 1 Scan mapping mode The scan technique is the most frequently used Herschel observing mode Scan maps are the default to map large areas of the sky for galactic as well as extragalactic surveys but meanwhile they are also recommended for small fields and even for point sources Scan maps are performed by slewing the spacecraft at a constant speed along parallel lines as illustrated in Figure 5 1 The lines are actu ally great circles which approximates parallel lines over short distances The number of satellite scans the scan leg length the scan leg separation and the orientation angles 53 Observing with the PACS photometer in array and sky reference frames are freely selectable by the observer Via a repetition parameter the specified map can be repeated n times The performance for a given map configuration and repe tition factor can be evaluated beforehand via sensitivity estimates and coverage maps in HSpot The PACS SPIRE parallel mode sky coverage maps are driven by the fixed 21 arcmin separation between the PACS and SPIRE footprints This mode is very ineffcient for small fields the shortest possible observati
97. g chopped spectroscopy modes There are hints of systematic differences in the re sponse scaling between chopped and unchopped mode due to response transients within the chop ping pattern These are well within the flux calibration uncertainties listed in Section 4 10 2 1 Back ground subtraction for unchopped spectroscopy measurements is done by subtracting the telescope background spectrum measured at an off position Repeated off position background spectrum measurements show a 4 peak to peak reproducibility in total absolute flux telescope source and a 1 in band shape error for the longest scans we have observed so far In observations of low continuum sources this will dominate the continuum flux accuracy For a 20 Jy source having a 200 Jy telescope background these uncertainties translate to 40 continuum uncertainty and 10 in band continuum shape error Spectrometer sensitivity Photoconductors of the type used in PACS have been demonstrated to have dark noise equivalent powers NEP of less than 9 x 10 wHz for the high stressed array Such a noise level would ensure background noise limited performance of the spectrometer Tests of the high stress detectors done at module level in a test cryostat and with laboratory electronics indicate a significant noise contribution from the readout electronics These measurements can be consistently described by a constant contribution in current noise dens ity from the CREs and a no
98. g with full beam sampling one for compact sources Section 6 1 6 6 which fit within the instantaneous FOV of the spectrometer and one for more extended sources Section 6 1 6 5 All spectrometer observing requests include a calibration block a modulated chopped measurement on the two internal calibration sources with the grating in a fixed position The two sources are heated to different temperatures hence provide different signal levels The grating position is chosen to measure a reference wavelength in the bands that are measured in the sky observation Ta ble 6 1 lists these calibration block wavelengths The calibration block measurement starts during the slew of the spacecraft to the target in order to optimise the use of observing time Data obtained in this block can be used for dark subtraction and for correction of system response changes repres entative at the start of the observation 66 Observing with PACS spectrometer Table 6 1 Key wavelengths The wavelengths observed in the spectrometer calibration block depend on the spectral bands visited in the rest of the observation bands wavelength range um blue key wavelength red key wavelength B2A RI 103 220 60 120 B3A RI 51 73 60 180 B2B R1 71 105 75 150 Based on the continuum and line flux estimates entered by the observer in the HSpot PACS Line Range Editor tables the expected maximum photoconductor signal level is estimated by the
99. hange in scale and a slight rotation to fit to the results of a 32x32 raster on the star o Her In particular these measurements also show that the pos sibilities to further improve the calibration of detailed distortions are limited by short term pointing drifts of the satellite Residual measured inter band offsets between blue green 0 3 arcsec and green red 1 2 arcsec have been characterised and are implemented in the astrometric processing chain Current best point spread functions PSF have been determined on asteroid Vesta The ob served 3 lobe structure of the PSF Figure 3 1 can be explained qualitatively by the secondary mir ror support structure and has been verified in detail by ray tracing calculations taking the telescope design and known wave front errors into account The spatial resolution expressed as encircled en ergy as a function of angular separation from PSF centre is in reasonable agreement with expecta tions from telescope and instrument design Table 3 1 Results of fitting 2 dimensional gaussians to the PSF Note these are fits to the full PSF includ ing the lobes wings Position angles east of North are listed only for beams with clearly elongated core The scan angle was 63 degrees for these observations Band Speed arcsec s FWHM arcsec PA deg 10 5 26 x 5 61 20 5 46 x 5 76 Blue 60 5 75 x 9 0 62 0 60 parallel mode 5 86 x 12 16 63 0 10 6 57 x 6 81 20 6 69 x 6 89
100. he high pass filtering and masking bright source when necessary or an alternative map making al gorithm tool This processing is mostly targeted to detect point sources with good sensitivity Scan maps are in Jy pixel HPPMMAPB amp HPPMMAPR stands for Herschel Pacs Photometer MADMap Blue Red MADmap maps are currently disabled from the HSC piepline until scan and crossed scan maps can be processed together HPPNMAPB amp HPPNMABR stands for for Herschel Pacs Photometer Naive Map Blue Red Averaged signal map after pixel to pixel offset correction This image is used by MADmap as its first value for the sky map and is subsequently improved and optimized iteratively as de scribed above hence the full optimized matrix inversion has not been performed on the data Again these products are not avaialble in recent HSC pipeline maps 7 2 Spectroscopy processing levels and data products There is a Herschel wide convention on processing levels of the different instruments There are dif ferent classes of products created at the Level 0 0 5 1 and 2 of the pipeline A more detailed product description is given in the PACS Data Reduction Guide PDRG and also the Herschel Products Definitions Document HPDD both of which are available via the HIPE help Here we give you a summary of these information For all the products described here the red and blue ranges are separate products They are identified by an R or a B at the end of the product
101. he num ber of observing cycles is used to repeat the entire on off cycle In this example the off position is only 6 away from the target what results only short on off slewing overhead In raster mapping mode the observing block sequence is similar to the chopped mode but here the observer could define the number of raster positions to visit before moving to the off position Note Cee In mapping mode the observing cycle is repeating the entire raster At least one OFF position has ndi to be specified for a single cycle i e the number of raster positions to visit before moving to the OFF position has to be equal or lower than the number of raster positions num raster lines x num of raster points In raster mode PACS executes an obligatory OFF position before the first science block starts at the first raster position For instance in case of a 2x2 raster with a single cycle and with an off position request after 2 raster positions the following spatial pattern will be executed OFF ON ON OFF ON ON OFF The same raster with two cycles OFF ON ON OFF ON ON OFF ON ON OFF ON ON OFF 6 1 9 2 Unchopped bright line mode This bright line option has been made available in HSpot version 5 3 1 and later for science cases where very high signal to noise S N gt 100 line detections are expected applying a single line repe tition factor For such bright targets the integration time can be kept shorter than as is in the default fai
102. he standard way of observing with the PACS spectrometer this mode is re commended especially for faint lines below 4 5 Jy peak to continuum combined with faint or bright continuum Both line flux and continuum level can be recovered from the data The mode could be used in a reliable way only if clean off fields are available within a 6 radius with respect the target coordinates 72 Observing with PACS spectrometer An observing request can contain up to 10 spectral line scans across different bands with the same fixed order selection filter wheel position For each observer defined wavelength in the PACS Line Editor the spectrometer performs grating scans with an amplitude such that a given wavelength is seen successively by all 16 spectral pixels of the detector The sampling density per resolution ele ment is higher than 3 samples per FWHM of an unresolved line at all wavelengths with 43 46 and 48 grating steps in the first second and third order respectively see scan parameters in Table 6 3 A single grating scan is repeated in two directions This up down scan can be repeated up to 10 times for a single line Table 6 4 shows the wavelength range covered in the different bands Table 6 3 Scan parameters in line scan modes Grating settings are shown for three bands and for the faint and bright line options separately the duration of atomic observing blocks are for a single grating up and down scan without overheads the oversa
103. he timescale of the drifts in detector sensitivities should be considered shorter than an hour they will be better corrected with shorter nodding cycle durations The sensitivities for the SED mode and high sampling density mode mode are displayed in Sec tion 4 11 for a single up and down scan and one nodding cycle um Note LEF Sensitivity plots can be easily produced in HSpot go to the Range Spectroscopy AOT define a full n wavelength range in Nyquist or high sampling density and adjust the repetition factors By click ing on Observation estimation and then Range sensitivity plots a pop up window will show both line and continuum sensitivities as a function of wavelength for the integration time defined in the AOR 92 Observing with PACS spectrometer 6 2 6 2 Unchopped grating scan mode The principles of unchopped mode in range spectroscopy is very similar to its implementation in line spectroscopy as described in Section 6 1 9 In range spectroscopy the unchopped grating scan mode can be combined either with high sampling density scan or with the coarser and shallower Nyquist sampling option The unchopped long range grating scan mode does not provide a built in option to visit an off position This may look an additional complexity the observer should deal with when designing the optimal observing strategy but actually flexibility makes the mode more efficient and adaptable to various science cases For instance a grou
104. heat switches which are coupled to the Herschel 3K system with thermal straps control the mode of operations The evaporation of He provides a very stable thermal environment under constant heat load The design of the cooler is well suited for work in space as there are no moving parts and the heat load is small This sorption cooler is nearly identical to the unit developed for SPIRE It provides a stable temper ature environment at 300 mK for more than 48 hours under normal observing and operational cir cumstances The recycling is performed during DTCP periods when the PACS photometer is selec ted for the following observing day and has a hold time of 60h allowing of up to 2 5 consecutive ODs of operations The PACS instrument 2 4 Spectrometer 2 4 1 2 4 2 The power emitted or absorbed by a single spectral line in the far infrared is normally several orders of magnitudes lower than the power in the dust continuum over a typical photometric band Sensit ivity is thus the most important parameter for optimisation with background limited detector per formance the best sensitivity is obtained if the spectrometer satisfies the following conditions the detection bandwidth should not be greater than the resolution bandwidth which in turn should be matched to the line width of the source and the line flux from the source must be detected with the highest possible efficiency in terms of system transmission spatial and spectral
105. here the Lyot stop and the first blocking filter common to all instrument channels can be positioned It allows the chopper through two field mir rors adjacent to the used field of view in the telescope focal surface to switch between a chopped field of view on the sky and two calibration sources see also Figure 2 3 The chopped image is then re imaged onto an intermediate focus where a fixed field mirror splits off the light into the spectroscopy channel The remaining part of the field of view passes into the pho tometry channels A footprint of the focal plane splitter is shown in Figure 2 3 Calibration sources The calibration sources are placed at the entrance of the instrument to have the same light path for the sky observation and internal calibration This is essential for removing detector baseline drifts as well as possible a serious task with a warm telescope and the associated high thermal background To eliminate non linearity or memory problems with the detector readout system the calibration sources are low emissivity gray body sources providing FIR radiation loads at two slightly different signals around the level of the telescope background This is achieved by diluting the radiation from a small black source with a temperature near the telescope temperature inside a cold diffusor sphere with a larger exit aperture The temperature of the radiator 80K is stable to within a few mK The PACS instrument 2 2
106. ise component proportional to the photon background noise where this proportionality can be expressed in terms of an apparent quantum efficiency with a peak value of 26 The NEP of the Ge Ga photoconductor system is then calculated over the full wavelength range of PACS based on the CRE noise and peak quantum efficiency determination at detector mod ule level for the high stress detectors The quantum efficiency as a function of wavelength for each detector can be derived from the measured relative spectral response function Similarly the abso lute responsivity as a function of wavelength is derived from the relative spectral response function and an absolute reference point measured in the laboratory The in orbit performance depends critically on the effects of cosmic rays in particular high energy protons Analysis of in orbit data as well as proton irradiation tests on ground indicate a perman ently changing detector responsivity cosmic ray hits lead to instantaneous increase in responsivity followed by a curing process due to the thermal IR background radiation The HSpot prediction of spectrometer sensitivity in the high sampling mode used in the AOTs line spectroscopy mode and range spectroscopy with the option high sampling are shown in figures Figure 4 29 and Figure 4 30 for the continuum and line detection respectively The HSpot prediction of spectrometer sensitivity in the SED mode used in AOT range spectro scopy with th
107. knowledge contained in numerous PACS technical documents and various discussions 1 4 Acronyms AOR Astronomical Observation Request AOT Astronomical Observation Template CRE Cryogenic Readout Electronics DDCS Double Differential Correlated Sampling mode DMC Detector and Mechanics Controller DICP Daily TeleCommunications Period ESA European Space Agency FM Flight Model FOV Field Of View FPU Focal Plane Unit FWHM Full Width Half Maximum HSpot Herschel planning observations tool ICC Instrument Control Centre ICS Internal Calibration Source ILT Integrated Instrument Level Tests NEP Noise Equivalent Power OD Observation Day PACS Photodetector Array Camera amp Spectrometer QLA Quick Look Analysis QM Qualification Model RSRF Relative Spectral Response Function SED Spectral Energy Distribution SPU Signal Processing Unit Chapter 2 The PACS instrument 2 1 Overview instrument concept The PACS instrument comprises two sub instruments which offer two basic and and mutually ex clusive modes in the wavelength band 55 210 um Imaging dual band photometry 60 85 um or 85 125 um and 125 210 um over a field of view of 1 75 x3 5 with full sampling of the telescope point spread function diffraction wavefront er ror limited Integral field spectroscopy between 51 and 220 um with a resolution of 75 300km s and in stantaneous coverage of 1500 km s over a field of
108. l as for the optimization of the dynamic range Leaving the parameter as the default 0 0 value means the PACS Time Estimator will not perform signal to noise estimation sensitivity estimates are still provided and default integration capacitor may be used providing the smallest dy namic range Continuum flux density mJy Optional parameter Continuum flux density estimate at the line redshifted wavelength The value of this parameter is interpreted by the PACS Time Estimator as flux density for a spectrometer resolution element Leaving the parameter as the default 0 0 value means the PACS Time Estimator will not perform signal to noise estimation sensitivity estimates are still provided and default integration capacitor may be used providing the smallest dy namic range Line width unit This menu gives the flexibility to switch between physical input units sup ported by the PACS AOT logic Line width FWHM Optional parameter The spectral line full width at half maximum value in units specified by the Line Width Units pull down menu Line width in put is used only for checking purposes It helps the observer to ensure the specified line width fits within the requested wavelength range Range repetition Mandatory parameter The relative range strength fraction of on source time per line is taken into account by specifying the grating scan repetition factor for each range A maximum of 10 repetitions in total c
109. l resolution resolving power 60 80 100 120 140 160 180 200 wavelength Figure 4 13 Spectrometer resolving power design values Spectrometer effective spectral resolution velocity 330 300 km s 60 80 100 120 140 160 180 200 wavelength Figure 4 14 Spectrometer effective spectral resolution velocity design values Table 4 1 summarizes the grating characterisation in terms of velocity resolution spectral coverage and typical grating step sizes for a given order wavelength 30 PACS spectrometer scientific capabilities Table 4 1 PACS grating pixel spectral characterisation grating order wavelength FWHM of an unresolved line instantaneous spectral cover pixel per age 16 pixels FWHM um km s um km s um 3 55 114 0 021 1420 0 26 1 20 3 60 98 0 020 1400 0 28 1 06 3 72 55 0 013 580 0 14 1 38 2 75 156 0 039 1720 0 43 1 37 2 90 121 0 036 1220 0 236 1 53 1 105 318 0 111 3030 1 06 1 56 1 158 239 0 126 1650 0 87 2 16 1 175 212 0 124 1340 0 78 2 37 1 210 140 0 098 715 0 50 3 58 The spectral resolution of the instrument measured in the laboratory with a methanol far infrared laser setup follows closely the predicted values of the figures above Spectral lines in celestial standards planetary nebulae HII regions planets etc are typically doppler broadened to 10 40 km s which has to be taken into account for any inflight analysis of
110. l un chopped spectra Flux offsets have been applied for better visualization Some examples of spectral arti facts RSRF wriggles are highlighted on the ON spectra blue ovals These features are present also in the off spectra but disappear when the two are subtracted bottom plots The significantly improves the spectra both in terms of shape and formal rms noise fits me Note EEF In HSpot version 5 3 1 and later a new input field has been introduced Unchopped grating scan ad purpose ON OFF Selector You can indicate with this parameter whether the AOR is targeting on source or alternatively the purpose of the observation is taking a reference off field spectrum This selector has no impact on commanding neither on the way the AOR is executed However the data processing system will be able to recognize the corresponding OBSIDs Observation IDs as on or off scans and the pipeline can arrange automatic off subtraction based on this informa tion The off subtracted spectrum will appear as Level 2 5 product in the Science Archive for the on source OBSID 94 Observing with PACS spectrometer 6 2 7 SED mode This observing mode is intended to cover the full PACS wavelength range in Nyquist sampling to get the far infrared SED Spectral Energy Distribution of a target In HSpot SED options are pre defined templates with fixed wavelength range and with fixed Nyquist sampling density Note Cer A full PACS SED is
111. ll all spectrometer AOTs The scientific signal corresponds to the slope of these in tegrating ramps and for bandwith reasons these are fitted on board Hence a spectrometer obser vation of n seconds always results in 18 x 25 x 8n samples for each camera i e red and blue 18 spectral pixels for each of the 25 spaxels each providing 8 measures per second 11 The PACS instrument spectrograph slit Pdl Pig Lad 3231 UM 01 E E spatial dimension spectral dimension 25 x 16 pixel photoconductor array Figure 2 10 The optical image slicer re arranges the 2 dimensional field along the entrance slit of the grating spectrograph such that for each spatial element in the field of view a spectrum can be simultan eously observed with a 2D detector array On the right part of the red photoconductor array with its area filling light cones and CREs are shown scaled to the schematic picture of the 25 by 16 array 12 Chapter 3 PACS photometer scientific capabilities 3 1 Point spread function The photometer optics delivers diffraction limited image quality Strehl ratio 29596 Therefore PACS preserves the image quality provided by the Herschel telescope and is diffraction limited on it whole energy range The photometer PSF see Figure 3 1 is dominated by the telescope and char acterised by A narrow core which is round in the blue bands but slightly elongated in spacecraft Z direction in red e A
112. ller amp al 2011 for more information on the PACS photometer flux calibration accuracy Absolute flux calibration uncertainties should improve over the mission with better statistics of available celestial calibration observations The calibration itself comprises i flat fielding ii responsivity correction conversion of engin eering units volts to Jy pixel and iii gain drift correction to account to small drifts in gain with time The flux calibration of the PACS photometer assumes that the detectors are linear in the full dynamic flux range observable by PACS e a spectral convention of v F v constant amp reference filter wavelengths at 70 100 and 160um aset of celestial flux calibrators that are mostly stars and asteroids for which model spectra are available that allow either color correction of the measured spectral densities in order to com pare them with the predicted monochromatic model fluxes or computation of the expected meas ured spectral densities 3 4 Photometer bad pixels The flight model bolometer blue array displays about 296 of dead pixels or very low responsivity pixels including one row of 16 pixels as can be seen on Figure 3 6 and Figure 3 7 in the upper right matrix Figure 3 6 FM blue array with low illumination PACS photometer scientific capabilities Figure 3 7 FM blue array with high illumination 3 5 Photometer sensitivity The photometer sensitivit
113. ls array Both channels cover a field of view of 1 75 x3 5 with full beam sampling in each band The two short wavelength bands are selected by two filters via a filter wheel The field of view is nearly filled by the square pixels however the arrays are made of sub arrays which have a gap of 1 pixel in between The incident infrared radiation is registered by each bolometer pixel by causing a tiny temperature difference 2 3 1 Filters The PACS filters in combination with the detectors define the photometric bandpass of the instru ment There are in total 3 bands in the PACS photometer 60 85 um 85 125 um and 125 210 um The PACS filter scheme is shown in Figure 2 4 and the filter transmission of the photometer filters in Figure 3 5 FLRS2 Red BBS Deer FBBS1 2 m FBBS2 1 M FEBS 1 FBBP2 1 FLBS Spectrometer Figure 2 4 Overview of the filter arrangements in PACS The selection of the blue photometer filter is done via commanding of the filter wheel 2 The PACS instrument 2 3 2 Bolometer arrays Figure 2 5 shows a cut out of the 64x32 pixel bolometer array assembly 4x2 monolithic matrices of 16x16 pixels are tiled together to form the short wave focal plane array Figure 2 5 Bolometer matrices assembly 4x2 matrices from the focal plane of the short wave bolometer assembly The 0 3 K multiplexers are bonded to the back of the sub arrays Ribbon cables lead to the 3K buffer electronics In a
114. mini scan map mode intended to replace the chopped noded mode as well as the orginal chopped noded point source mode All photometer configurations perform dual band photometry with the possibility to select either the blue 60 85um or the green 85 125um filter for the short wavelength band the red band 125 210um is always included The two bolometer arrays provide full spatial sampling in each band During an observation the bolometers are read out with 40 Hz but due to satellite data rate limita tions there are onboard reduction and compression steps needed before the data is down linked In PACS prime modes the SPU averages 4 subsequent frames in case of chopping the averaging pro cess is synchronised with the chopper movements to avoid averaging over chopper transitions In PACS SPIRE parallel mode 8 consecutive frames are averaged in the blue green bands and 4 in the red band In addition to the averaging process there is a supplementary compression stage bit rounding for high gain observations required where the last n bits of the signal values are rounded off The default value for n is 2 quantisation step of 2 10 V or 4 ADUs for all high gain PACS SPIRE parallel mode observations 1 for all high gain PACS prime mode observations and 0 for all low gain observations Each PACS photometer observation is preceded by a 30 seconds chopped calibration measurement executed during the target acquisition phase The chopper moves with a
115. mp A submitted Fletcher et al 2010 Fletcher L N Drossart P Burgdorf M et al 2010 A amp A 514 A17 Mueller amp Lagerros 2008 Mueller T G Lagerros J S V 2002 A amp A 381 324 339 M ller amp al 2011 M ller amp al 2011 PACS Photometer Point Source Flux Calibration available on the PACS page of the HSC website Poglitsch et al 2010 Poglitsch et al 2010 A amp A Herschel special issue The Photodetecetor Array Camera and Spectrometer PACS on the Herschel Space Observatory Schulz et al 2002 Schulz B Huth S Laureijs R J et al 2002 A amp A 381 1110 Shirahata et al 2009 Shirahata M Matsuura S Hasegawa S et al 2009 PASJ 61 737 104
116. mpling factor gives the number of times a given wavelength is seen by multiple pixels in the homogeneously sampled part of the observed spectrum band waveleng grating grating over 1 scan grating over 1 scan 1 scan th range step size steps sampling duration steps sampling duration duration um faint factor sec bright factor sec sec un lines faint faint lines bright bright chopped lines lines lines lines grating chop nod scan B3A 51 73 168 48 41 1 384 10 10 0 80 384 B2B 71 105 188 46 36 2 368 10 10 0 80 368 RI 103 220 240 43 27 9 344 10 10 0 80 344 Table 6 4 Spectral coverage in line scan The wavelength range seen in a nominal line scan varies over the spectral bands The column highest sensitivity range refers to the range that is seen by every spec tral pixel band wavelength full range full range highest sens FWHM um FWHM um km s um itivity range km s um B3A 55 1880 0 345 0 095 0 021 115 B3A 72 799 0 192 0 053 0 013 55 B2B 72 2658 0 638 0 221 0 039 165 B2B 105 1039 0 364 0 126 0 028 80 RI 105 5214 1 825 0 875 0 111 315 RI 158 2869 1 511 0 724 0 126 240 RI 175 2337 1 363 0 654 0 124 210 RI 210 1314 0 92 0 441 0 098 140 At every grating position the detector signal is modulated between on and off source via an on off off on on off off on chopping pattern In order to take advan
117. multiplexing Sub traction of the high telescope background has to be achieved by appropriate spatial and or spectral modulation techniques Instrument design The integral field spectrometer covers the wavelength range from 51um to 220um in two channels that operate simultaneously in the blue 51 105um and red 102 220um band It provides a resolv ing power between 1000 and 4000 i e a spectral resolution of 75 300km s depending on wavelength for a fixed grating position the instantaneous coverage is 1500km s It allows simul taneous imaging of a 47 x47 field of view resolved into 5x5 pixels An image slicer employing re flective optics is used to re arrange the 2 dimensional field of view along a 1x25 pixels entrance slit for the grating as schematically shown in Figure 2 6 This integral field concept with spectral and spatial multiplexing allows for the most efficient detec tion of weak individual spectral lines with sufficient baseline coverage and high tolerance to point ing errors without compromising spatial resolution as well as for spectral mapping of extended sources regardless of their intrinsic velocity structure The grating is Littrow mounted i e the entrance and exit optical paths coincide It is operated in first second or third order respectively to cover the full wavelength range The first order covers the range 102 210um the second order 71 105um and the third order 51 73um Anamorphic col limating optics
118. n Figure 5 7 Nod 1 chop A Nod 1 chop B Nod2 chop A Nod2 chop B nod1 chop A nod1 chop B nod2 chop A nod2 chop B Point source AOT footprint on the sky Figure 5 7 Source positions in point source photometry AOT Sketch showing the source positions as a function of the nod and chopper positions The Y axis is to the left the Z axis to the top Chop positions are defined by the internal chopper while nod positions are defined by the satellite pointing Dithering at each chopper position performed with the internal chopper is not represented On each nod position the chopper executes 3x25 chopper cycles The 3 sets of chopper patterns are either on the same array positions no dithering or on 3 different array positions dither option In the dither option the chopper pattern is displaced along Y direction along the chopper direction by about 8 5 arcsec 2 66 blue pixels or 1 33 red pixels Each chopper plateau lasts for 0 4 second 16 readouts on board producing 4 frames per plateau in the down link The full 3x25 chopper cycles per nodposition are completed in less than 1 minute The pattern is repeated on the second nod position In case of repetition factors larger than 1 the nod cycles are repeated in the following way example for 4 repetitions nodA nodB nodB nodA nodA nodB nodB nodA to minimise satellite slew times The minimal duration of this observing mode with calibration and slew overheads is 5 5 min inclu
119. n different flux regimes it is recommended to split the observation into sep arate observations per flux regime Note D Please note in HSpot v5 1 and higher the SED mode templates in the Range Editor Table do ac cept line flux estimates in previous versions you were able to specify only a continuum flux dens ity estimate at a given reference wavelength Line flux estimates should be provided for accepted programs please update AORs via Helpdesk 88 Observing with PACS spectrometer 6 2 2 6 2 3 6 2 4 6 2 5 6 2 6 Spectral leakage regions Spectral regions affected by leakage are discussed in Section 4 8 The measured spectrum in these regions may contain superimposed flux originated in the parallel channel the interpretation of spec tral features unresolved or continuum fluxes should be avoided without consulting a PACS expert i e contact Helpdesk Pointed mode See Section 6 1 4 Pointed with dither mode Warning Ed HSpot v5 0 and later versions allow the reading of AORs in Pointed with dither mode but time es timation has been disabled and submission to HSC is not possible This mode has been decomm sissionned See Section 6 1 5 Mapping mode See Section 6 1 6 Range scan modes Range scan options can be selected in HSpot Wavelength settings Range scan or SED modes pull down menu of the Range Spectroscopy AOT The following diffraction order combinations are available e Range scan
120. n is met throughout all bands While at band borders due to leakage effects and lower S N the RMS calibra tion accuracy is closer to 20 values even better than 10 are obtained in band centres However for point sources the wavelength calibration may be dominated by pointing accuracy Three 4x4 ras ter observations 3 step size in instrument coordinates of the point like planetary nebula IC2501 on the atomic fine structure lines N III 57 um O IIIJ 88 um and C II 158 um have been compared with predictions from instrument design Figure 4 16 shows good agreement between measured spectral line centre positions and the predicted offsets from the relative source position in the slit Observed wavelength offsets for individual point source observations are therefore expected to fall within the dashed colour lines given the nominal pointing uncertainty of the spacecraft consult the Observatory Manual Offset um 50 100 150 200 Wavelength um Figure 4 16 Calculated wavelength offsets for point source positions at the slit border solid colour lines for typical pointing errors up to 2 dashed colour lines and measured line centre offsets for 1 5 dashed black line and crosses and slit border black crosses for three spectral lines on the point like planetary nebula IC2501 Instrumental profile When observing point sources for a single spectral line we expect to see a Gaussian instrumental profile IP in all spaxels
121. nded objects For extended objects mapping with oversampling i e with step size smaller than one spaxel may 71 Observing with PACS spectrometer be very time consuming Therefore this mapping strategy is suggested with step sizes larger than one spaxel but such that the beam is Nyquist sampled Taking into account that what is defined as raster point step in HSpot corresponds to the spacecraft Z axis and the line step to the Y axis the re commended raster step sizes see Figure 6 2 for Nyquist sampled maps of larger areas translate to the following HSpot settings e Blue point step 16 0 line step 214 5 Red point step 24 0 line step 222 0 e o o o 9 o o e o gt LOI gt ee E ets aT aus Te o g woe gle Leg s s e o C is 4 6 et 6 4 9 ar 40 4 eee ate e 9 99 9 d e V u rg ele o e E e e e e e e 9 e o e 4 a T Sere ka 9 o e E B e ef o v y o C e sot eo F o o a s T ef 16 byt elh tgi eee e 9 a OR ee eet oe oot o ee B e we fe t 06 e o a 60 B Ge eve ogc L e en v eee p oF eote ee amp o o e eo ef o a 2 Ser eee E e oo oto 6 o ee e o OF e e oe 9 ec eet pe 909 6 E e oo otoo ote e ote otoo ot
122. ng to this level contains actual spectra and is highly AOT dependent For op timal results many of the processing steps involved to generate level 2 data may require human interaction based both on instrument understanding as well as understanding of the scientific aims of the observation These data products are at browse quality level and should be suitable for publishing following an interactive enhancement The level 2 data product contains noise filtered regularly sampled data cubes HPS3DRB R of dimensions lambda x 5 x 5 and a combined cube projected on the WCS HPS3DPB R of dimen sions lambda x N x M where N and M are the spatial dimensions of the projected map This cube is constructed by rebinning the integral field cube oversampled in wavelength different projection per wavelength layer due to distortions onto the same sky wavelength grid of the instrumental resolution The cube flux values are Jy pixel the wavelength is in microns In case of mapping observations rasters the pipeline loops over all raster positions and com bines the rasters into a single grid by adding up for each new spaxel the fluxes of the contribut ing old spaxels multiplied by their overlap weights The HPS3DP product is worth using even for pointed observations i e no rastering because it does not just add together or mosaic mul tiple pointings but also sets the correct spatial grid for each wavelength of your cube for the PACS spectrometer ea
123. nt line mode even on the expense of compromised observing performance In this mode grating scan parameters and observing logic remain unchanged except the number of grating steps visited by a single scan Instead of applying the default 75 steps a bright line scan is done through 50 steps resulting a factor 1 3 reduction in integration time The shorter duration scan is therefore achieved via a shorter wavelength coverage the scan speed number of integration ramps per grating position has not been changed Due to instrument and spacecraft overheads the 33 gain in integration time does not apply to the total duration of a bright line AOR Typically a bright line AOR is shorter by 25 30 than its faint line homologue We show in Figure 6 10 the spectral coverage of the bright line and standard unchopped mode 81 Observing with PACS spectrometer Bright Unchopped Faint Unchopped 5000 Erp PTT 4500 mr 4500 E 4000 F 4000 F T ITTTTR TTE ETT 3500 T 3500 F E F 3000 E n 3000 E qj 2500 E E Es me j gt 2000 umo 4 mere j 1500 E M MEORE j 1000 A 4 1000 E a 500 Nm j 500 SAO NYO WWE POOP Loa naa Lia Lans FETE OID FOYE ober Lisa Lisa asa Lira Las 87 8 87 9 88 0 88 1 88 2 88 3 88 4 88 5 88 6 88 7 88 8 88 9 87 6 87 8 880 882 884 886 888 890 Wavelength u m Wavelength u m Figure 6 10 The unchopped bright line mode left 50 grating steps covers
124. o T e e re ERG tree oe 4 v w9 CME 97 9498 99 4 0 4 9 e ee oto of oe te oF ete otoo of o ga Ret ga tat s e ee oto ef o fo ototo e oe of e E o Moe eee o e oo oo ef o te oF eo Pe oF ee oF o 60 Figure 6 2 Spatial sampling by all PACS spaxels when using a 5x5 raster with step size 14 5 16 for the blue left and a 3x3 raster with step size 22 24 for the red right 6 1 6 6 Full PACS spatial resolution of compact objects 6 1 7 In order to map the sky at full PACS spatial resolution step sizes smaller than a spaxel have to be used Since this increases the observing time this mode is strongly suggested to be used only for mapping point like or almost point like objects In order to recover the best PACS resolution we re commend the following minimum number of raster positions and maximum step sizes e Blue 3x3 raster with step size equal to 3 0 in both directions e Red 2x2 raster with step size equal to 4 5 in both directions Note ao For oversampled maps Section 6 1 6 5 and Section 6 1 6 6 the optimum separation between raster ndi points has been optimised to a reference beam size in the blue and red channels In case an AOR contains a mix of blue and red lines ranges then it is recommended to adopt the blue settings Standard chopping nodding mode This mode is called Chopping nodding in the observing mode settings panel of HSpot The chopping technique is t
125. ob serving logic For range spectroscopy the expected flux at the maximum response is extrapolated via a Rayleigh Jeans law from the reference wavelength and corresponding flux estimate The ap propriate integrating capacitance of the CRE is then chosen for the entire observation to avoid satur ation This way observer s flux estimates can adjust the dynamic range of the observation Warning e If continuum and expected line fluxes are higher than the saturation limits for the default capacit ance it is mandatory to enter the expected continuum and line flux for every line range in HSpot Observations that are saturated because no HSpot flux estimates were entered by the observer will not be considered as failed for technical reasons Saturation limits are presented in Section 4 12 In case the target flux estimates exceed PACS capabilities i e you get an HSpot message stating the observation will be saturated then please contact Helpdesk for further instructions 6 1 Line Spectroscopy AOT This AOT is intended to observe one or several unresolved or narrow spectral line features on fixed wavelength range of about 1 micron but varying from 0 35 to 1 8 um depending on the wavelength and the grating order Only lines in the first 103 220 um and second order 71 105 um combination or first and third order 51 73 um can be observed within a single AOR to avoid filter wheels movements If lines of second and third grating order are to be
126. observations Roughly two thirds of the observing time are open time and will be offered through a standard competitive pro posal procedure The Photodetector Array Camera amp Spectrometer PACS is one of the three science instruments of the Herschel observatory PACS provides the Herschel Space Telescope with the capabilities for spectroscopy and imaging photometry in the 55 210 um range PACS has been designed and built by a consortium of institutes and university departments from across Europe under the leadership of the Principal Investigator Albrecht Poglitsch at Max Planck Institute for Extraterrestrial Physics Garching Germany Consortium members are from Introduction Austria UVIE from Belgium IMEC KUL CSL from France CEA OAMP from Germany MPE MPIA from Italy IFSI OAP OAT OAA CAISMI LENS SISSA from Spain IAC For a detailed description of the PACS instrument and its in flight performances we refer to Poglitch et al 2010 1 3 Acknowledgements The PACS instrument is the result of many years of work by a large group of dedicated people in several institues and companies across Europe It is their efforts that have made it possible to create such a powerful instrument for use in the Herschel Space Observatory We would first like to ac knowledge their work This manual is edited by Bruno Altieri and Roland Vavrek ESAC on behalf of the PACS ICC con sortium This Observer s Manual also uses the
127. observed on the same target at the same time two AORs shall be concatenated Depending on the requested wavelength grating order only the data of one of the two detector arrays is normally of interest to the observer The fixed wavelength and its immediate neighborhood is observed for each chopper and grating po sition For improved flat fielding especially for long integrations the grating is scanned by a num ber of discrete steps around a specified centre position such that drifts in the detector responsivity between individual pixels are eliminated The centre position of a grating scan is the corresponding line peak wavelength These grating scans provide for each line and for each of the 5 by 5 spatial pixels a short spectrum with a resolving power of 1700 in its highest resolution covering 1500 km s but dependent on the wavelength and order Up to 10 lines can be studied within one observation The relative sensitivity between the lines is controlled by using the line repetition factor in the PACS Line Editor of the Wavelength Settings in HSpot that allows to repeat a line scan several times While the absolute sensitivity is controlled by the number of observing cycles in the Observing Mode Settings by dedicating a larger amount of time to this observation integer multiples Note LEFT A maximum of 10 repetitions in total can be specified in the table For instance in the case that 10 lines are selected the Line repetition
128. obtained within 1 hour in order 1 red detector and order 2 blue detector d with two PACS range spectroscopy AORs of a single repetition In these scans both nominal and parallel data have to be taken into account 6 2 7 1 Predefined set of spectral band combinations Three SED options are offered SED B2B long R1 70 105 um 140 220 um data obtained in the first and second diffrac tion orders total duration 1s 2438 seconds SED B2A short R1 51 73 um 102 146 um in the range 55 and 73 um data obtained in the first and second diffraction orders but covering the short part total duration is 1310 seconds This scan offers a better continuum sensitivity in the blue range than the SED B3A option but a worse line sensitivity because of the much worse spectral resolution of the 2nd order compared to the 3rd order SED B3A long R1 47 73 um 140 219 um For sources where the order 3 spectral resol ution is required e g because you look at a source with a rich line spectrum where lines can be blended this additional AOR of 3110 seconds can be added Note To cover the full PACS spectrometer wavelength range 51 220 um two AORs in SED B2B and a SED B2A and or SED B3A have to be concatenated A single AOR can have only a single SED option defined For deeper exposure increase the range repetition factor A spectral dithering scheme has been im plemented for SED scans similar to the Nyquist sampled
129. ographs a different position of the source photocenter in the disper sion direction of the slit induces wavelength shifts and line profile skews Section 4 7 3 Co adding line profiles in different spaxels will therefore result in broadened and skewed line shapes Work on proper optimal extraction and fitting of spectral lines is under way in the PACS ICC 4 10 2 Flux calibration accuracies The PACS spectrometer flux calibration accuracy is limited by detector response drifts and slight pointing offsets These limit both the absolute flux accuracy and relative accuracy within a band Corrections for both effects are under study by the PACS ICC and will be provided to the user in forthcoming HIPE software versions Awaiting these corrections the following accuracies shall be assumed when interpreting PACS spectroscopy data 4 10 2 1 Absolute flux calibration accuracy This accuracy applies for single line fluxes or continuum flux densities at a given wavelength in any spaxel Beware of the correction needed for flux falling out of the pixel for point sources The abso lute flux calibration accuracy was determined from observations of 30 absolute flux sky calibration sources fiducial stars asteroids planets of which some are shown on Figure 4 25 Figure 4 26 Fig 38 PACS spectrometer scientific capabilities ure 4 27 and Figure 4 28 These figures show the observed flux of calibrators at key wavelength in the four spectral bands
130. on 57 33 microns line Fitted profile on 63 1852 microns line Fitted profile on 88 3564 microns line 57 3 microns E set 63 18 microns 1400 E 88 36 microns 4 N Ill 2P3 2 2P1 2 of O13P1 3P2 1200 O III 3P1 3PO j Wavelength wm Wavelength wm Wavelength wm Fitted profile on 121 8 microns line Fitted profile on 145 535 microns line Fitted profile on 157 740905045 16303 microns line 157 74 microns CII C 145 53 microns 4 O I 3P0 3P1 121 8 microns N II 3P2 3P1 Flux Jy Lusbuslulbuslusl Wav elength lu ml W avelength iem Figure 4 19 Frequently observed lines in the PACS bands in this example from observations on planet ary nebula NGC6543 The rebinned spectra taken from the central spaxel are produced by the standard pipeline the fitted model is a Gaussian plus zero order polinomial representing the continuum Data were taken in faint line mode high grating sampling density applying single line repetition and single nodding cycle therefore S N strongly varies with lines in this example 4 8 Spectral leakage regions The precision of the relative spectral response is affected by spectral leakage order overlap due to 34 PACS spectrometer scientific capabilities finite steepness of order sorting filter cut off edges from grating order n 1 into grating order n At wavelengths of 70 73um 98 105um and 190 220um the next higher grating order wave lengths of 52 5 54 5 um 65 7
131. on requires about 45 min observing time Available satellite speeds are 20 or 60 arcsec s The highest 60 s speed default value is dedic ated for large galactic surveys with a degradation of the PSF in the blue channel due to the on board averaging of 4 frames final 10 Hz sampling Cross scan distance Scan leg length Figure 5 1 Example of PACS photometer scan map Schematic of a scan map with 6 scan line legs After the first line the satellite turns left and continue with the next scan line in the opposite direction just like in the raster map case The reference scan direction is the direction of the first leg Note that the turn around between line does take place as simplistically drawn in the figure During the full scan map duration the bolometers are constantly read out with 40 Hz allowing for a complete time line analysis for each pixel in the data reduction on ground Important o A combination of two different scan directions preferably orthogonal is recommended for a better field and PSF reconstruction and to remove the stripping effects of the 1 f noise For this purpose two AORs can concatenated in HSpot In the second AOR the map orientation angle is then in creased by 90 degrees to get an orthogonal coverage for instance 45 and 135 degrees orientation angle when in instrument reference frame Most of the PACS prime observations are performed with the 20 arcsec s scan speed where the bo lometer perf
132. onse Proton irradiation tests performed at the synchrotron source of the Universite Catholique de Louvain Louvain la Neuve Belgium complemented by a y ray radiation test programme at MPIA indicated that NEPs close to those measured without irradiation should actually be achievable in flight In flight meas urements performed during the commissioning phase of Hershel confirmed these findings Figure 2 9 Array close ups The 25 stressed and low stress modules corresponding to 25 spatial pixels in the red and blue arrays are integrated into their housing Stress is applied to the whole stack of 16 Ge crystals providing the instantaneous spectral coverage for each of the 25 spatial fields on the sky Light cones provide for area filling collection onto the individual detectors Each module is attached to an 18 channel cold readout electronics CRE amplifier multiplexer cir cuit in CMOS technology The photocurrent from the detector crystals is integrated on a capacitor The capacitance is switchable between 4 values from 0 14 to 1 15pF to provide sufficient dynamic range for the expected flux range The integration process is reset after preset interval During the integration the voltage signal is regularly read in a non destructive way with a frequency of 1 256s leading to an integration ramp with 256 reset interval samples The extensive PV program aimed at optimising the AOT parameters led to ramps of 32 samples i e 1 8 second for a
133. orders As a consequence the B3A RSRF product is also affected resulting in an inaccurate flux calibration Despite the lower spectral resolution in band B2A the detectability of unresolved lines is not compromised Please note that the PACS Line Spectroscopy AOT does not provide access to band B2A so this up date of AORs require the use of PACS Range Spectroscopy AOT instead You can do this so e If an unresolved line in the range of 51 55 micron in band 51 73 and 103 220 microns 3rd 1st orders was requested with the Line Spectroscopy AOT then you need to re define this as a short range in high sampling density mode in the band Range scan in 51 73 and 102 146 microns 2nd Ist orders The width of the range for an unresolved line should be at a minimum 48 grating steps to allow full coverage of the entire line profile with all the 16 spectral pixels For instance observing the ONI line at its rest wavelength 51 8 mu the Blue edge should be at 51 44 mu and the Red edge at 52 16 mu Such a 0 72 mu range provides sufficient coverage for unresolved lines in the 51 55 mu range you can verify the number of grating steps in the PACS time estimator mes sages under Info for range xx yy mu Please note that the Range Spectroscopy AOT does not recalculate wavelengths for redshifted lines the red blue edges need to be set up appropriately 69 Observing with PACS spectrometer 6 1 4 6 1 5 6 1
134. ormance is best and the pre flight sensitivity estimates are nearly met in the blue chan nel For larger fields observed in instrument reference frame there is an option to use homogeneous coverage which computes the cross scan distance in order to distribute homogeneously the time spent on each sky pixel in the map For short scan legs below about 10 arcmin the efficiency of this mode drops below 50 due to the relatively long time required for the satellite turn around deceleration idle time acceleration between individual scan legs which takes about 20 s for small leg separations of a few arcseconds Nevertheless this mode has an excellent performance for very small fields and even for point sources The advantages of the scan mode for small fields are the better characterisation of the source vicin ity and larger scale structures in the background the more homogeneous coverage inside the final map the higher redundancy with respect to the impact of noisy and dead pixels and the better point source sensitivity as compared to a chop nod observation of similar length PACS scan maps can be performed either in the instrument reference frame or in sky coordinates 54 Observing with the PACS photometer 5 1 1 Scan maps in instrument reference frame When scan maps are performed in instrument reference frame an array to map angle is chosen ob server The array to map angle is the angle from the spacecraft Z axis to the line
135. osition duration could be kept identical to the on source duration by specifying single line repetitions and higher number of observing cycles However this parameterization is inefficient in terms of slew ing overheads os Note LET While the scheme in the block diagram Figure 6 9 can be executed it may turn more advisable to rather do this line by line particularly for rasters with not too frequent off positions 80 Observing with PACS spectrometer am Note LET A shorter integration time in the OFF position results in a RMS noise of the reference continuum d higher than that of the source continuum To avoid to increase the RMS noise when subtracting the OFF position the OFF position data can be binned using larger wavelength intervals In case of flat continuum a single value can be computed for the reference continuum START observation On source position On target slew Off source position A grating up down scan Repeat N times number of cycles 809 sec END If mapping move to next raster observation position and repeat off position after nth raster point here n 1 Figure 6 9 Instrument observing blocks are shown for a typical unchopped grating scan measurement combining two lines with single repetition in B3A and a third line with two repetitions in band R1 3rd 1st orders setting Line repetitions adjust the relative depth required for spectral lines while t
136. osition 3 Position 7 Position 11 Position 15 Position 19 Position 23 Position 27 Position 31 Position 35 Position 39 eevee o E 9 f Figure 6 6 Visualization of the line scan AOT on an unresolved PACS line here given by a Gaussian 77 Observing with PACS spectrometer 6 1 8 The grating step size and number of grating positions used is the nominal one currently coded in AOT design for standard faint lines summarized in Table 6 3 Top row is for a blue line at 60um bottom row is for a red line at 205um In bright line mode grating step sizes are identical but the instrument scans over only the 10 central positions These scan patterns are identical for chopped and unchopped modes PACS footprint and S C boresight positions T T T T T T T T T T T T T T 66 43 00 66 42 00 66 41 00 ess 66 40 00 66 39 00 66 38 00 NT Lu 4 pe 66 37 00 m uet aie 66 36 00 6673500 ij 66 34 00 Dec 66 33 00 EN L 1 1 1 1 1 1 1 l L 1 1 18 00 00 17 59 30 17 59 00 17 58 30 17 58 00 17 57 30 A off Bon 4 B off Herschel boresi ght E Source Figure 6 7 The PACS spectrometer footprint of a chop nod pointed observation is shown for the blue band The four fields of nod A and B positions combined with chop on and off fields are plotted a
137. p is per formed in instrument reference frame array Cross scan distance Distance between two scan legs maximum 210 arcsec i e the long side of the bolometer array square map If selected Yes HSpot computes the number of scan legs in order to complete a square map in the sky which is recommended for scan maps performed in instrument reference frame where the orientation of the map in the sky is not known in advance However do not use in the mini scan map case Number of scan legs Repetition factor Number of parallel line legs in the scan map the maximum 1500 but there is additional limit of 4 degrees for the with of the scan map i e the total cross scan distance In case of repetition factor 1 it is recommended to use an even number of scan legs to minimize the satellite slew overheads number of times to repeat the scan map to adjust the absolute sensitivity maximum 100 Source flux estimates Optional point source flux density in mJy or surface brightness in MJy sr for each band It is used for signal to noise calculations and to adjust the ADC to low gain if the flux in one of the two channel is above the ADC saturation threshold See Section 5 4 for more details 5 1 3 Scan maps sensitivity HSpot returns the both the averaged point source predicted sensitivity across the map in the column Averaged point source sensitivity in the instrument performance summary win
138. p of targets may share the same off position scan or scans and the optimal sequence of ON OFF ON can be optimized depending on the duration of indi vidual AORs in such a chain Note DEF It is mandatory to use identical wavelength settings for on and off source AORs An off observation should be taken either before or after a long range scan observation In order to prevent data from the impact of long term drifts of system response it is recommended to have one off scan in every 60 minutes at least Total integration times over the reference field needs to be equal to the on source integration however the off integration can be split up to various individual scans AORs if the scan is defined in high sampling density mode For instance an on source integration with 4 scans could have two off scans of each 2 repetitions one before and the other after the on source AOR In such a case the three AORs need to be concatenated OFF1 ON OFF2 The equal on and off integration time in total is a mandatory requirement for range scans and SED mode observations too it has been found that the subtraction of the off taken with sufficient S N will significantly improve continuum S N by correcting for 2nd order modulations i e wriggles on the RSRF Warning pg Contrary to high density settings in case the AOR is defined in Nyquist sampling or SED mode then the off postion AOR s should not be split up i e the duration of an off scan shoul
139. r of grating steps is fixed to 75 irrespective the spectral band in which the line is observed Other grating scan parameters and block duration are shown in Table 6 3 A key element in the unchopped mode is to make this technique robust against instantaneous re sponsivity changes caused by cosmic ray hits in the Ge Ga detector pixels For this purpose the up down grating scan have been made much faster than in the chopped mode and every line repetition requested by the observer is doubled internally by PACS On every grating position four integration ramps are taken Each integration lasts 1 8 s resulting in a grating scan which is four times faster than the one in chop mode The sensitivity of an observation performed in the unchopped line scan mode is quite similar to that obtained in chop nod when the time spent on source is taken into account This conclusion is based on actual observations of several galaxies in which bright lines were observed both in chop nod and in unchopped mode Both the line shape and the strength of the line when differences in the calibra tion procedures are taken into account are very similar Figure 6 8 shows the observation of a line in the red spectrometer observed in both chop nod and unchopped mode Except for differences in the continuum level the line is well reproduced in both cases Note that in this example both un chopped observations over estimate the continuum by 3096 compared with the chopped case an
140. red on the central spaxel are not affected The central spaxel of the IFU is not con taminated by 2nd pass ghosts 36 PACS spectrometer scientific capabilities T aee 3 BIENES P 1 2 zz 3 2 4 2 a 0 0 1 0 20 TN 4 0 Eo Figure 4 23 Location of the spaxels where a second pass ghost might appear In black are the module numbers this is the numbering in the PACS frames product in white the row and column numbers in the PACS cube products The arrows indicate the spaxel where the originating real emission is located Module 15 zt E dd E OF 3 EB E 3 n E 3 Ui QE i 7 j 145 159 1 55 160 Wavelength um Module 10 Q 0 20E Zap OT OE j Oo E j amp EOS p YM Q 00E j 110 115 120 1 Wavelength um N GH Figure 4 24 Example of the second pass spectral leak the strong real line emission at 145 5 OI and 157 7 CII in module 13 leak into module 10 where they are seen as broadened spectral lines around 108 and 122 micron 37 PACS spectrometer scientific capabilities Table 4 2 Example 2nd pass ghost wavelengths corresponding to prominent fine structure lines in the PACS wavelength range Spectral band Parent line Wavelength Ghost wavelength micrometer micrometer B3A 50 70 micrometer OI 63 2 54 B2B 70 100 micrometer OIII 88 4 72 R1 100 220 micrometer OI 145 5 108 R1 100 220 micrometer C
141. relation matrix Investigations on the noise filters are underway from the in flight data Current version of the data processing environment relies of pre launch estim ates from ground based tests Users wishing to use MADmap for their data processing needs should contact the helpdesks at ESA or NHSC for further information 7 1 2 Level 2 pipeline products for scan maps gener ated with HCSS in the HSA archive There are 3 types of products in the level 2 produced by the pipeline for the PACS photometer in scan map mode MADmap as been disabled from the HSC pipeline with HCSS 2 0 hence the second product HPPMMAP is not available in recently processed scan maps observations These maps are produced by automatic pipeline scripts and shall only be considered as a preview and not for science directly HPPPMAPB amp HPPPMAPR stands for Herschel Pacs Photometer PhotProject MAP Blue Red This refers to maps produced by the photProject task i e a simple projection of each frame 10Hz after running a temporal high pass filter with a width of n 20 i e subtracting a median with a width of 2 n 1 frames This allows to filter a signicant part of the 1 f noise at the ex pense of removing completely ALL spatial scales larger than this width i e typically larger than arcmin and creating negative undershooting around bright sources along the scan direction To preserve extended emission the pipeline script shall be re run with higher width in t
142. rphic re imaging op tics and a dichroic beam splitter for separation of diffraction orders The blue channel contains an additional filter wheel for selecting its short or long wavelength part two photoconductor arrays with attached cryogenic readout electronics CRE The PACS instrument Spectrometer Fields 0 8 x 0 8 Photometer Fields 3 5 x 1 75 Calibrator Fields 3 5 x 3 0 PACS focal plane usage Long wavelength and short wavelength photometry bands cover practic ally identical fields of view The spectrometer FOV is offset in the Z direction closer to the optical axis of the telescope Chopping is done along the Y axis left right in this view and also allows ob servation of the internal calibrators on both sides of the used area in the telescope focal plane The maximum chopper throw for sky observations is 3 5 arcmin for photometry and 6 arcmin for spec troscopy In photometry object and reference fields are almost touching at 3 5 arcmin throw Figure 2 3 PACS field of view footprint in the telescope focal plane 2 2 Common Optics 2 2 1 Entrance Optics 2 2 2 The entrance optics fulfill the following tasks they create an image of the telescope secondary mir ror the entrance pupil of the telescope on the focal plane chopper this allows spatial chopping with as little as possible modulation in the background received by the instrument It also provides for an intermediate pupil position w
143. s well as the instrument boresight grey and target position red In this example observation large chopper throw 6 was used which is a result of a symmetrical chopping pattern around the instrument boresight Bright lines chopping nodding mode This mode is devoted to bright lines where it is not needed to spend as much time per line as in the standard chopping nodding mode with a single repetition Up and down grating scans are performed but only with 10 grating steps i e the on sky time about 3 4 times shorter than in the standard chop ping nodding mode depending on the band A comparison of bright line mode scan parameters can be found in Table 6 3 The observing efficiency of this mode is rather poor because of fixed incompressible overheads nod slew time and instrument overheads The minimum total observing time one line one cycle is 78 Observing with PACS spectrometer 6 1 9 6 1 9 1 303s versus 583s in the standard chopping nodding while it is about twice less sensitive the sensit ivity ratio per resolution element in the highest sensitivity part of the coverage is about 0 6 Sensitivity ratio is driven by the respective oversampling factors sqrt 10 27 9 If several lines are observed the modes becomes more attractive but line repetition factor as well as nodding cycles should be equal or less than 3 In bright line mode even for unresolved lines the central 3 pixels in a 16 pixel module do not see
144. similar way 2 matrices of 16x16 pixels are tiled together for the long wavelength focal plane array The matrices are mounted on a 0 3K carrier which is thermally isolated from the surrounding 2K structure The buffer multiplexer electronics are split in two levels a first stage is part of the indi um bump bonded back plane of the focal plane arrays operating at 0 3K Ribbon cables connect the output of the 0 3K readout to a buffer stage running at 2K For science observations the multiplexing readout samples each pixel at a rate of 40 Hz Because of the large number of pixels data compression by the SPU is required The raw data are therefore binned to an effective 10 Hz sampling rate After that the same lossless compression algorithm is applied as with the spectrometer data 2 3 3 Cooler The photometer operates at sub Kelvin temperatures which are achieved using a He cooler This type of refrigerator uses porous material which absorbs or releases gas depending on the mode cooling or heating The use of the He isotope instead of the common He is dictated by two reasons it is not super fluid at cryogenic temperatures below 2 2 K and it is a superior cryogen This sorption cooler is run from a cold stage provided by the Herschel cryostat The refrigerator contains 6 litres of He and can in principle be recycled infinitely with an efficiency of more than 95 with a life time limited only by the cold stage from which it is run Gas gap
145. so some deviation of the parallelism of scan legs for some observations blurring the PSF Since star tracker CCD tem perature was lowered no such anomaly has been observed since 20 Chapter 4 PACS spectrometer scientific capabilities 4 1 Diffraction Losses The image slicer is the most critical element of the PACS optics in the figures below the effect of diffraction vignetting by the entrance field stop and Lyot stop have been included For the Lyot stop a worst case loss of 1046 is used For the losses in the spectrometer the fraction of power arriving at the detector is shown in Figure 4 1 0 8 0 4 Diffraction throughput c 60 80 100 3120 140 160 180 200 Wavelength um Figure 4 1 Diffraction throughput of the spectrometer optics the diffraction losses mainly occur in the image slicer 4 2 Grating efficiency The calculated grating efficiency i e the fraction of the incident power that is diffracted in the used grating order as a function of wavelength is shown in Figure 4 2 21 PACS spectrometer scientific capabilities 0 8 E c Grating efficiency c p 02 60 80 100 3120 140 160 180 200 Wavelength Lum Figure 4 2 Calculated PACS grating efficiency 4 3 Spectrometer filters The transmission of the filter chain in each of the instrument channels has been calculated from measurements of the individual filters see Photometer filters section The filter transmission curves for
146. spectra e cont EJ TT Tapa poppa R1 low flux source B2B low flux source 1 Normalized flux density Normalized flux density 171 172 n 7 74 75 76 Wavelength py m Wavelength py m Normalized continuum spectra Normalized continuum spectra pape 1 10 r7 T TU B2A low flux source 15 Jy cont p ME B2B high flux source 260 Jy cont 1 06 f r z x Normalized flux density Normalized flux density t d Eos 1 1 i rislissilis 62 6 62 7 62 8 62 9 63 0 63 1 63 2 63 3 63 4 63 5 63 6 63 7 87 0 87 5 88 0 88 5 89 0 89 5 90 0 Wavelength y m Wavelength u m Unchopped ONI normalized _ Unchopped ON normalized Unchopped ONT normalized _Unchopped ON normalized e Unchopped ON1 OFFI normalized Unchopped ON2 OfF2 normalized Unchopped ON1 OFFI normalized Unchopped ON2 OFF2 normalized Chopped normalized Onpped mrmakzed Figure 6 15 Examples of spectra which show two main features of unchopped scans a the improve ment of continuum RMS after applying OFF subtraction and b the excellent reproducibility of two ON blocks on the timescale of 1hr respectively The red spectra are chopped SED observations light and deep blue are spectra obtained on the ON1 and ON2 blocks each has the same integration time as the chop ON frames and in the bottom line the two green curves represent the OFF subtracted fina
147. stal The light cones also act as a very efficient means of straylight suppression because their solid angle acceptance is matched to the re imaging optics such that out of beam light 10 The PACS instrument is rejected Responsivity measurements of both stressed and unstressed modules show sufficiently homogen eous spectral and photometric response within each module and between modules Absolute re sponsivity calibration for optimum bias under in orbit conditions is under way and will most likely give numbers of 10 A W for the unstressed detectors and 40 A W for the stressed detectors The detectors are operated at stressed or slightly above unstressed the Level 0 cryostat temperature 71 65 K Each linear module of 16 detectors is read out by a cryogenic amplifier multiplexer cir cuit CRE in CMOS technology The readout electronics is integrated into the detector modules but operates at Level 1 temperature 3 5 K Measurements of the NEP of both arrays after integ ration into the instrument flight model at characteristic wavelengths and with representative flux levels have confirmed the performance measured at module level Only a small fraction of pixels suffers from excess noise Median NEP values are 8 9 x 10 WHz for the stressed and 2 1 x 10 WHZz for the unstressed detectors respectively The achievable in orbit performance was ex pected to depend critically on the effects of cosmic rays on the detector resp
148. step follows an AABBBBAA pattern where A is a detector integration at the initial wavelength and B is a de tector integration at the wavelength switching wavelength This cycle is repeated 20 times in one direction and repeated in the reverse wavelength direction The switching amplitude is fixed for every spectral band In order to reconstruct the full power spectrum a clean off position is visited at the beginning and the end of the observation On this position the same scan is performed In between the scan is per formed at two or more raster positions 0 10 0 05 0 00 FEED Signal i 0 05 V 0 10 157 0 157 5 158 0 158 5 Wavelength um Figure 6 12 Reconstruction of emission line flux in the differential wavelength switching mode by fit ting a differential Gaussian Wavelength switching can be used for large extended sources since no clean reference is needed But it shall be used with caution by definition this technique eliminates the continuum information Besides the baseline estimates line profiles are not be reliable if 84 Observing with PACS spectrometer anoticeable gradient is present in the continuum flux over the performed wavelength throw e blends of line forests disturb the wavelength switch interval Table 6 5 User input parameters for Line Spectroscopy AOT Parameter name Wavelength ranges Signification and comments Which of the two order combinations to use
149. t the HSpot Users Manual for a detailed description how to set time constraints and what con sequences apply If the map is defined in sky coordinates the map orientation angle in the Observing Mode Set tings HSpot panel can be used to rotate the raster map this angle is counted from the celestial north to the raster line direction counterclockwise If the map is defined in sky coordinates and the observer wants to cover a contiguous area in the sky under any position angle he or she shall not define a step size larger than 34 arcsec i e the size of the array 47 divided by V2 6 1 6 3 Map orientation in chopped modes Chopped rasters cannot be rotated with a specific orientation angle the chop direction is hard coded in instrument reference frame with zero angle orientation i e the chop direction is perpendicular to a raster line If the target is at higher ecliptic latitude then put a time constraint on the AOR This way the array and the whole raster can be rotated to the desired angle by a time dependent array pos ition angle 6 1 6 4 Sky mosaics For raster maps with stepsize greater than 30 i e tiling the sky rather than oversampled rasters there are no particular recommendations for step sizes Typical step sizes are 47 no overlap between the different raster positions and 38 approximately one row or column of spatial pixels overlap between the different raster positions 6 1 6 5 Nyquist sampling map of exte
150. t affected by that constraint The user is thus invited to assess himself the impact on the visibility of that constraint Table 5 2 lists the user inputs required in HSpot Table 5 2 User input parameters for the point source AOT mode Parameter name Meaning and comments Filter which of the two filters from the blue channel to use In case observations in the two blue filter bands are required to be performed consecutively two AORS shall be concatenated Dithering On dithering enabled or Off dithering disabled A fixed dithering pattern is applied with an amplitude of 2 33 blue channel pixels with the chopper This is intended to improve the flat field accuracy and to produce better photometric results for faint targets Chopper avoidance angle Interval of position angles for the chopper avoidance zone modulo 180 de grees The position angle is counted positive east of north i e counterclock wise in the sky from the north to the direction of the object to avoid Repetition factor Number of AB nod cycles to adjust the absolute sensitivity maximum 120 Source flux estimates Optional point source flux density in mJy or surface brightness in MJy sr for each band It is used for signal to noise calculations and to change the ADC to low gain if the flux in one of the two channel is above the ADC saturation threshold See Section 5 4 for more details 5 4 Gain setting for bright sources
151. tage of the best spectrometer 73 Observing with PACS spectrometer sensitivity at every chopper plateau two 1 8 second integrations of the photoconductor signals are recorded These integration ramps of 1 8 second length are fitted on board PACS therefore the sig nal transmitted to the ground is represented by the slope of the ramp in units of ADU reset inter val The observer can choose a chopper throw of Small Medium and Large what refering to 6 3 or 1 5 respectively The apparent chopper throw is curved on the sky This rotates the on source spectral footprint in the two nod positions and this rotation becomes larger with increasing chopper throw see Section 4 5 m Note LEFT It is recommended to select the Small 1 5 chopper throw if applicable for the target i e well ndi isolated point sources in order to reduce the effect of field rotation between the two chop positions see Figure 6 3 ECT 73 50 082 oo oo 50 14 E 5 Oe 50 12 M 50 050 J 50 10 E E j 50048 F d 4 50 08 4 4 50 06 E 50 046 E ne 4 o E j o E 4 Bods E 3 a 50 044 E i j 50 02 i snas i 50 00 4j 90 082 p amp 4r 1 49 98 E n E 80 040 j 49 96 E d E 4 J E j 50 038 49 94 E E RI E i El efe eraa MA 50 056 Cii 1 1 a a au 1 4 l J 266 10 266 30 266 35 266 22 266 23 266 24 Aon Bon Aoff Boff B Source Bon Aon
152. tance the 80 90 microns range is observed with step size 188 in the 2nd order but could be covered as well in parallel to the 160 180 microns 1st order range with 240 grating step size The latter will result sub optimal wavelength sampling in the 2nd order therefore observers should always define the primary range for the wavelength of the main scientif ic interest PacsRangeSpectroscopy AOT is suitable for broad lines or for long range coverage and conceptu ally this template is not meant for observing narrower ranges than what the default wavelength cov erage of Line Spectroscopy AOT provides designed for unresolved lines This would mean an in adequate use of the system however HSpot has no hard protection to prevent observers doing this In practice Range Spectroscopy AORs should never be created with fewer grating steps than what Line Spectroscopy would provide at the same central wavelength in the same diffraction order The actual coverage in grating positions can be found in the Time Estimator Message under Info for range XX um For instance an info print line such as Grat Order StepSize NbSteps 2 2400 262 the last number is in your interest From this line the value of NbSteps can be compared with the Line Spectroscopy design values provided in the 3rd column of Table 6 3 In essence the number of grating steps in Range Spectroscopy has to be always larger than 168 188 or 240 in high sampling density in bands B3A B2B or R
153. ted area e g a nearby 86 Observing with PACS spectrometer Parameter name Signification and comments edge on galaxy then the observer might have no other option then using sky reference frame and turn the raster to the right direction If the target is at higher ecliptic latitudes then you may select instrument reference frame and put a time constraint on the AOR The appropriate time window can be identified in HSpot Overlays AORs on images option by changing the tentative epoch of observation This way the array can be rotated to the de sired angle by the time dependent array position angle Unchopped grating scan off The off position can be specified by relative offset in arcminutes with re spect the target coordinate or alternatively you can define an absolute posi tion within a two degrees radius In case you prefer to use the absolute posi tion option then it is highly recommended to specify RA Dec via the stand ard HSpot target definition window You can have access to this window by clicking on the Choose Position button This panel is enabled only for the unchopped grating scan mode 6 2 Range Spectroscopy AOT Similarly to the Line Spectroscopy AOT PACS in this mode allows to observe one or several spec tral line features or broad ranges up to ten but the observer can freely specify the explored wavelength range or use the predefined full range templates SED mode Only lines
154. the fraction of the PSF falling onto the central spaxel can vary substantially This is presently the main limitation of the flux calibration The full wavelength dependent characterisation of the PSF is ongoing at the PACS Instrument Contol Centre P ICC 27 PACS spectrometer scientific capabilities Figure 4 10 PACS spectrometer detector sampling of the telescope PSF at 75 um left and 150 um right Color scaling of the PSF is chosen toenhance the lobes and wings of the psf 4 6 3 Measured beam efficiencies A full characterisation of the spectrometer beam efficiencies is ongoing The beam efficiencies have been measured via raster maps on Neptune at a few selected wavelengths The Neptune visibility window in spring 2011 is being used to complete these measurements at different wavelengths in every spectral band Together with the detailed telescope PSF models this will allow us to provide reliable model beam efficiencies well sampled across the PACS spectral coverage In Figure 4 11 the measured beam efficiencies are shown at 62 75 125 and 150 um The pixel size dominates the width of the beam efficiency up to 150 um Figure 4 12 shows the Gaussian width of the measured beams as a function of wavelength The pixel size dominates the width of the beam efficiency up to 150um Figure 4 11 PACS spectrometer beam efficiency as measured from raster maps on Neptune From top left to bottom right 62 um 75 um 125 um 150 um The
155. the spectral resolution The double differential chop nod observing strategy leads to slight wavelength shifts of spectral profile centres due to the finite pointing performances Averaging nod A and nod B data can therefore lead to additional profile broadening causing typical observed FWHM values that are up to 10 larger ILT with gas laser Effective resolution km s Spectral Resolution km s H 2 2 m 100 no Mo 160 180 wo i xx a 15 e Wavelength 7 m Wavelength um Figure 4 15 Comparison of pre flight spectral resolution left with in orbit performance right Con tinuous curves represent the design values respectivel signs overplotted shows the measured laser line widths in the laboratory during the PACS flight module tests and on the right data are derived from planetary nebulae measurements and corrected for internal velocity broadening 31 PACS spectrometer scientific capabilities 4 7 2 Wavelength calibration 4 7 3 The wavelength calibration of the PACS spectrometer relates the grating angle to the central wavelength seen by each pixel Due to the finite width of the spectrometer slit a characterisation of the wavelength scale as a function of point source position within the slit is required as well The calibration derived from the laboratory water vapour absorption cell is still valid in flight For ideal extended sources the required accuracy of better than 20 of a spectral resolutio
156. the three grating orders are plotted in Figure 4 3 22 PACS spectrometer scientific capabilities PACS spectrometer system transmission transmission 60 80 100 120 140 160 180 200 220 Wavelength um Figure 4 3 Transmissions of the spectrometer filter chains The graph represents the overall transmis sion of the combined filters in each of the three grating orders of the spectrometer The vertical lines mark the edges between spectral bands 4 4 Spectrometer relative spectral response function The relative spectral response function RSRF gives the combined system efficiency including fil ter transmission and detector sensitivity The on ground measured RSRF during PACS flight module black body measurements is displayed in Figure 4 4 Besides the overall trend of the RSRF one of the most important issue is to calibrate with a high accuracy the ripples on short wavelength scales It is particularly important for faint line detection and identification The RSRF is very stable over the mission lifetime however an updated version will be made available to PACS observers if it is appropriate 23 PACS spectrometer scientific capabilities Response ADU 25sec Jy gt 10 y Hs bain ps i a 60 Ga 100 120 4 160 18C 209 220 Wavelength pam 1 Figure 4 4 PACS spectrometer relative spectral response function measured on ground colored signs show the key wavelengths measured in the calibra
157. tion block of every single observation The wavelength dependence of the absolute response of each spectrometer pixel is characterised by an individual relative spectral response function Since this calibration file has been derived from the extended laboratory black body measurements its application to point sources requires addition al diffraction corrections The required correction curve is provided in Figure 4 5 however partly extended sources may consequently show deviating spectral shapes according to their size and mor phology see more in Section 4 10 In the data reduction environment HIPE this curve is available as a calibration product For a point source the flux derived from the central spaxel of the 5x5 intergral field array has to be multi plied by the correction factor at the corresponding wavelength This flux represents the total line flux of the source however for slightly extended sources the total flux must be extracted from the 25 spaxels even if S N is lower for the neighbouring spaxels comparing to the central one 24 PACS spectrometer scientific capabilities e Co EE p e WC e A E a ae u e OS e e ET o 9 y yN o c gt e a Flux fraction seen in 1 spaxel 40 60 80 100 120 140 160 180 200 220 240 Wavelength u m Figure 4 5 Fraction of the total beam flux seen in one PACS spaxel as a function of wavelength This as sumes perfect centering of the point source on the spaxel
158. tral pixel 8 i e the predefined range is not covered by all spectral pixels and the ac tual range in the data has S N going up at the edges The spatial layout of a pointed measurement is shown on Figure 6 7 where the 25 spaxels for the foure chop nod position are projected onto the sky Chopping and nodding is imposed by the design of the AOT in other words if chopping nodding is deselected the unchopped grating scan is selected instead as both observing techniques are mutu ally exclusive 76 Observing with PACS spectrometer order 3 fwhm 0 02 Flux Arbitrary units 60 77 8 amp 0 BF rao 10 20 30 40 SO 0 59 76 59 82 59 88 59 94 60 00 60 06 60 12 60 18 60 24 Wavelength microns Line Position 2 Position 1 Position 6 Position 7 M Position 10 Position 11 Positisn 14 Position 15 Position 18 Position 19 Position 22 Pasition 23 Position 26 Pasition 27 Pasition 30 Position 31 Pusition 34 Position 35 Position 38 Position 39 Pasition 42 Position 43 Position 46 Position 47 order 1 fwhm 0 10 Flux Arbitrary units 10 20 30 40 50 60 70 80 90 100 0 204 0 204 3 204 6 204 9 205 2 2055 2058 206 Wavelength microns Position O Position 1 Position 4 Position Position amp Postion 9 Position 12 Postion 13 Position 16 Position 17 Position 20 Poskion 21 Position 24 Position 25 Position 28 Postion 23 Position 32 Postion 33 Position 36 Postion 37 Pasition 40 Pasirian 41 Line P
159. tri lobe pattern seen at the several level in all bands most clearly in the blue with its strongest signal and ascribed to imperfect mirror shape due to the secondary mirror tripod e Knotty structure at sub percent level clearly seen in blue and indicated in green Figure 3 1 The photometer PSF in blue green and red top to bottom derived from scans performed at 10 arcsec s Left hand panels display the image with a linear scale up to the peak while right hand pan els show up to 10 of the peak For fast scans in normal and parallel mode this PSF structure is smeared by detector time constants 13 PACS photometer scientific capabilities and data averaging Quantitative information on the PSF is given in Table 3 1 These PSFs and derived quantities reflect the intrinsic optical quality of Herschel PACS In a scan map reduction they will be very slightly smeared in particular at short wavelengths because of the data averaging on board from 40Hz to 10Hz sampling but also due to detector time constants tele scope pointing jitter and drifts For more detailed information on the PACS photometer PSF we refer to the PACS technical note PICC ME TN 033 version 2 0 Apr 04 2012 The photometer focal plane geometry initially established on ground by scanning a back illu minated hole mask across the bolometer arrays has been adapted to the actual telescope by optical modeling The in flight verification required a small c
160. trom etric ACCULACY i iris e preter Poe Hd pee oases Prep soe Peas PTRS ERE EE RI e er mes 20 4 PACS spectrometer scientific capabilities 2 00 0 cee cece eee ce cece ce cene cen mH Herr 21 4 1 Diffraction Losses 5 sien de sont ERI NEN HAE 21 A D Grating ELHICIENCY EIS 21 4 3 Spectrometer Uteris 4 tuer Eo ettet te pi opes tede nee tee Lov etie norte 22 4 4 Spectrometer relative spectral response function sssses eeeeeeeeeeeaeeeaes 23 4 5 Spectrometer field of view and spatial resolution e 25 4 6 Spectrometer Point Spread Function PSF esses 26 4 6 1 Measured vs model PSF 00 0 0 eee cece nee ceca cece cena e Hem eme emere 26 4 6 2 Detector sampling of the PSE 02 scc scssestssss esses ceseseadessaeessessesvesbecnoagsesdeossdennses 27 4 6 3 Measured beam efficiencies 2 1 0 0 cece cece eeee cece cece cena e em e He ee men eren 28 4 7 Spectrometer spectral resolution and instrumental profile esseeeee 29 4 7 1 Spectrometer spectral resolution sssse HH me 29 4 7 2 Wavelength calibr tion tret ER Et Ree TEE Rena 32 4 7 3 Instrumental profile erse e EE RERE ee eot epe eI E Ee rr Hoe Pee e ghe Pp summed ys 32 4 8 Spectral leakage regions esses e eme emen mee hee nhe nhe nhe nee rennen 34 4 9 Second pass spectral ghost 5 d eie has etd br etit odes meter ier 36 4 10 Spectrometer flux calibration 2 0 e
161. ult of extended rasters on Neptune has been the verification of the point spread function of the spectrometer Remarkable agreement with predictions from telescope and instrument model ing has been found A measurement for a typical spatial pixel of the PACS spectrometer can be compared in Figure 4 8 and Figure 4 9 to a convolution of a calculated PSF from actual telescope model including known wave front errors with a square pixel of 9 4 x9 4 26 PACS spectrometer scientific capabilities 0 0 0 2 0 4 0 6 0 8 1 0 Figure 4 8 Calculated spectrometer PSF at 62 um left and measurement on Neptune right done at the same wavelength Both are normalised to the peak and scaled by square root to enhance the faint wing pattern The calculation includes the predicted telescope wave front error which dominates the overall aberrations 0 0 0 2 0 4 0 6 0 8 1 0 Figure 4 9 Same as Figure 4 8 at 124 um 4 6 2 Detector sampling of the PSF The PACS spectrometer spaxels sample a part of the PSF delivered by the Herschel telescope The telescope PSF becomes larger with wavelength and shows substantial departure from a Gaussian profile due to the telescope wavefront errors mainly caused by the three point mount of the tele scope dish At different wavelengths different fractions of the PSF structure are seen by the differ ent spaxels This is illustrated in Figure 4 10 Given the pointing accuracy of the spacecraft this means that
162. ure 4 37 Saturation limit for unresolved lines on a zero continuum for the default smallest 0 14 pF integration capacitance PACS spectrometer scientific capabilities PACS spectro saturation limit Capacitance 8 telescope background included 10 TO yey 10 Saturation limit Jy 40 60 80 100 120 140 160 180 200 220 240 Wavelength u m RI B2B B3A B2A Figure 4 38 Saturation limit for the second 0 24 pF integration capacitance including 80 safety margin PACS spectro saturation limit Capacitance 8 telescope included 10 Saturation limit W m 1o terete Pt Li Lixcbopbor eb 40 60 80 100 120 140 160 180 200 220 240 Wavelength u m Ri B2B B2A B3A Figure 4 39 Saturation limit for unresolved lines on a zero continuum for the second 0 24 pF integrat ing capacitance inc 80 safety margin 49 PACS spectrometer scientific capabilities PACS spectro saturation limit Capacitance 4 telescope background included Saturation limit Jy 40 60 80 100 120 140 160 180 200 220 240 Wavelength u m RI B2B B3A B2A Figure 4 40 Saturation limit for the third 0 46 pF integrating capacitance inc 80 safety margin PACS spectro saturation limit Capacitance 4 telescope included 10 Saturation limit W m go AA EA A A A E A A Dr 40 60 80 100 120 140 160 180 200
163. verlapping footprints in a small raster produce deeper coverage towards the centre of the map Flux estimates and dynamic range The uplink logic automatically selects the integrating capacitance based on estimated continuum and line fluxes If an observation contains lines that fall in different flux regimes then the largest applic able capacitance will be chosen for the entire observation If lines in the same observation fall in dif ferent flux regimes it is recommended to split the observation into separate observations per flux re gime The HSpot Time Estimator Message click on Observation Estimation PACS Time Estimat or Messages indicates if other than the default capacitance has been selected for a given combina tion of lines Please note a single bright line could trigger capacitance switching in both red and blue channels i e capacitances are always kept in synch between the two channels In case a bright red line is grouped together with faint lines in the blue channel then the observation will be sub optimal in the blue In such a case it is recommended to regroup lines per channel in two separate AORs Spectral leakage regions Spectral regions affected by leakage are discussed in Section 4 8 The measured spectrum in these 68 Observing with PACS spectrometer regions may contain superimposed flux originated in the parallel channel the interpretation of spec tral features unresolved or continuum fluxes shoul
164. view of 47 x47 Photometer Optics Filter Wheel Blue Bolomete Slicer Optics sGe Ga Detector Red Spectromete Chopper sGe Ga Detector Calibrators and II Blue Spectrome Entrance Optics Filter Wheel II Figure 2 1 Optical layout After the common entrance optics with calibration sources and the chopper the field is split into the spectrometer train and the photometer trains In the latter a dichroic beam split ter feeds separate re imaging optics for the two bolometer arrays In the spectrometer train the image slicer converts the square field into an effective long slit for the Littrow mounted grating spectrograph The dispersed light is distributed to the two photoconductor arrays by a dichroic beam splitter which acts as an order sorter for the grating Figure 2 1 shows how the functional groups are distributed in the spatial instrument envelope Figure 2 2 shows an optical circuit block diagram of the major functional parts of PACS At the top The PACS instrument the entrance and calibration optics is common to all optical paths through the instrument On the right the spectrometer serves both the short wavelength blue and long wavelength red pho toconductor arrays A fixed dichroic beam splitter separates blue from red spectrometer light at the very end of the optical path On the left the bolometer fixed dichroic beam splitter comes before the blue and red imaging branches
165. y in unchopped grating scan mode Please note in unchopped grating scan mode the HSpot default option is sky reference but we highly advise to switch to instrument mode except for cases described in Section 6 1 6 2 Note Ce Time constraints as well as the chopper avoidance angle may be used to put restrictions on the map i orientation angle if the map is defined in instrument reference frame The time constraint can in directly limit the array position angle in the optimal range while the chopper avoidance can be used to avoid a certain chopping direction with respect to the target position see details in the HSpot Users Manual Map orientation direction restrictions are possible only for targets at high ecliptic latitude here the spacecraft orbit may result different position angles at different observing epochs 6 1 6 2 Map orientation in unchopped modes In unchopped grating scan mode if an AOR raster covers an elongated area e g a nearby edge on galaxy then the observer might have no other option than using sky reference frame and turn the raster to the right direction If the target is at higher ecliptic latitude then you may select instrument reference frame and put a time constraint on the AOR The time window can be identified in HSpot Overlays AORs on images option by changing the tentative epoch of observation This way the array can be rotated to the desired angle by the time dependent array position angle Please consul
166. y is driven by the foreground thermal noise emission mostly from the tele scope and the electrical noise of the readout electronics The point source sensitivities have been updated in HSpot 5 0 to the in flight measured sensitivities on deep fields maps However the achieved sensitivity is a strong function of data processing and scan strategy The given sensitivies in Table 3 2 refer to scan maps processed with a high pass filtering with a very short width typically a half width of 16 in the blue green bands and 25 in the red band to mit igate the effect of 1 f noise and gain detector responsivity drifts in time at the expense of removing any extended emisison beyond the scale of this filter length Maps processed with MADmap which preserve extended emission at any scale cannot reach this sensitivity yet Based on in flight performance we recommend that observers Use the medium 20 sec scan speed to get optimum point source sensitivity The fastest 60 sec scan speed should be used if a wide area is to be mapped e Use concatenated cross scans for observations that cannot benefit from high pass filter reduc tions e g fields with spatially complex extended and diffuse emission The cross scans are also useful to obtain higher photometric accuracy for faint sources Table 3 2 PACS photometer sensitivity central wavelength 70um 100um 160um scan mapping 16 1second 30 6 36 0 68 5 mJy mini scan mapping 50 4

PACS Observer`s Manual - Herschel

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