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1. 66 MODS Instrument Manual Introduction 1 1 Scope This document describes the properties of the Multi Object Double Spectrograph MODS and how to use it for observing on the Large Binocular Telescope A companion manual gives the details of the MODS Observing Scripts that are the primary way to control MODS at the telescope This early version of the manual specifically describes the MODSI spectrograph that was delivered to the LBT in May 2010 and was commissioned between September 2010 and May 2011 going into early science operations during Semester 201 1B The MODS2 spectrograph is scheduled for delivery to LBT in autumn 2012 for a late 2012 early 2013 installation on the telescope The current manual describes monocular single MODS properties and operations Future versions of this manual will describe binocular MODS operation This document and the companion MODS Observing Scripts document together constitute the user manual for MODS 1 2 Citing and Acknowledging MODS The current literature citation describing the MODS instruments is Pogge et al 2010 SPIE 7335 9 BibCode 2010SPIE 7735E 9P All papers that use MODS data are required to cite the paper above and to include the following acknowledgement of the funding agencies that made these spectrographs possible This paper uses data taken with the MODS spectrographs built with funding from NSF grant AST 9987045 and the NSF Telescope System Instr
2. The main color free component of spectra flat field images is pixel to pixel variation in gain including gain differences between the quadrants of the device and between the even and odd channels within each quadrant the even odd effect noted above The procedure that we have found works best for grating spectral flats is one we have adapted from the practice with the Keck LRIS instrument using slit less spectra of flat field lamps to create color free pixel flats These capture enough light in the UV and far red ends of the spectral range to get us well into the fixed pattern noise limited regime for most of the spectral coverage of the detectors Blue channel pixel flats are created using a combination of slit less flats of the QTH lamps acquired with the clear and UGS filters 5 each the latter to suppress the red end of the spectra in favor of more blue light all flat field lamps are intrinsically very red Dividing by the color terms removes the wiggles from the UGS filter Twilight sky illumination is not recommended as there is too much UV in the twilight sky and we get unacceptable UV fixed pattern artifacts in the resulting pixel flats Red channel pixel flats are taken with the clear filter and the internal VFLAT lamp While we have had success taking slit less twilight flats in the red they are not demonstrably better than internal lamp flats and are not worth the pain of chasing the twilight exposure times better to use t
3. 2 11 2 Shutter Lag There is a measured time lag of 1 62 0 05 seconds between the time recorded in the FITS header DATE OBS datum and when the shutter 1s actually opened by the CCD controller such that t DATE OBS 1 62 sec actual The range of shutter lag is between 1 55 to 1 68 seconds There is no significant difference in the lags measured for the blue and red camera shutters and no correlation with the size of the image being read out The origin of the shutter lag is buried in the depths of the CCD controller code and eliminating this lag time will be a goal of future CCD controller software updates Please keep this in mind if your observations require precision 1 minute timing The rms of the measured shutter lag of 5Omsec is typical of system times derived from synchronizing clocks to a local Stratum 0 GPS time server via NTP typically 10 20msec as is the case with the MODS computers on the LBTO mountain network 2 12 MODS Data MODS data are written onto the archive disk as standard FITS files Images have names like mods1b 20111213 0001 fits The format is modsIb for MODS1 Blue CCD the 8 digit number is the UTC date of the observation in CCY YMMDD format and the 4 digit number is the image sequence number during that UTC day Raw images appear on the mountain 34 MODS Instrument Manual OSU MODS 201 1 003 Version 1 2 archive disk newdata within a few seconds after they are written to the data takin
4. TT TAS Se SG LBT Weather Station Link State LBT Ambient Pressure hPa LBT Ambient Temperature deg C LBT Relative Humidity percent LBT Dew Point deg C 73 MODS Instrument Manual Appendix B Filter Bandpass Parameters The MODS effective filter bandpass parameters tabulated 1n this manual were computed following the prescriptions of Schneider Gunn amp Hoessel 1983 Their original formulation was in frequency units so we adopt the reformulation of these filter parameters in wavelength units following Koornneef et al 1986 These are the same definitions adopted by the Hubble Space Telescope and Sloan Digital Sky Survey so they are now in broad use throughout the astronomical community at least for ground and space based optical and near IR photometry The filter parameters we use are formulated in terms of the average throughput of the filter defined as the product of the measured filter transmission curve T and the instrumental throughput Q T is derived from laboratory measurements of the filter transmissions usually provided by the filter vendors Q is estimated from the measured total transmission including AR coatings of the MODS lenses the measured reflection coatings of the MODS mirrors and the measured quantum efficiencies of the MODS CCD detectors For the effective wavelength of the filter we adopt the pivot wavelength 2p defined as 1 2 fQ 7 4da jor p The pivot wavelength has the
5. carries the standard LBTO off axis guide acquire and slow Shack Hartmann wavefront sensor WFS cameras These are the same guide and WFS cameras as used by the AIP AGw units on other LBT focal stations and uses the same LBTO Guide Collimation System GCS software Because the system resides inside the MODS common focal plane area it has a different and smaller guide patrol field than LUCI The stability of MODS is ensured by an internal closed loop Image Motion Compensation System IMCS that monitors the alignment of the optics in each channel in real time during exposures An IR laser beam A 1 55um is launched from below the focal plane and passes through the same optics as the science light onto a Germanium quad cell mounted off axis just above each channel s science CCD Error signals from the quad cells are used to steer the collimator mirrors in each channel to null image motion due to gravity induced flexure of the structure stochastic ticks and pops in the structure and any other sources of optical misalignment that occur along the beam path from the focal plane to the detectors Basic Parameters Optical Design Seeing limited dichroic split double beam grating spectrometer Wavelength Coverage 3200 10000 Field of View 6x6 arcminutes 2900x2900 pixels Pixel Scales Blue 0 120 arcsec pixel Red 0 123 arcsec pixel Operating Modes Direct Imaging SDSS ugriz filters ug in Blue riz in Red Medium Dispersion Gra
6. mas per yr GUIPMDEC 0 00 Guide Star Dec proper motion mas per yr GUIEPOCH 2000 00 Guide Star Epoch Telescope Pointing and Rotator Information These keywords give the telescope pointing and rotator information read from the Telescope Control System TCS at the start of the exposure TCSLINK Live i TCS Communications Link Status DATHSOBS 2010 Li 19T1021229306 9907 35 UTC Date at start Of obs UTC OBS I0312 3064 980 UTC Time at start of obs TIMESYS UTC i Time System horocoBS UOA paaa 104 Local Sidereal Time at start of obs MJD OBS 55519 425428 Modified JD JD 2400000 5 at start of obs RADECSYS FK5 Coordinate System EQUINOX 200040 Z Equinox of coordinates TELRA 05 54 20 905 Telescope RA DELDEC 5222009594996 Telescope DEC POSANGLE 0 00000 Instrument Celestial Position Angle deg TELALT 70 96853 Telescope Altitude at start of obs deg TELAZ 240 25531 Telescope Azimuth at start of obs deg PARANGLE 52 29008 Parallactic Angle at start of obs deg ROTANGLE 100 34461 Rotator Angle at start of obs deg ROTMODE POSITION Rotator Mode HA 01 12 24 101 Hour Angle at start of obs ZD 19 03 Zenith Distance at start of obs deg 70 MODS Instrument Manual AIRMASS OSU MODS 2011 003 1 06 Airmass Version 1 2 secZD at start of obs These parameters except for the time are read directly from the telescope control system The time 1s the
7. roughly 150x86 in size The guide WFS star is marked in the center of the WFS pickoff field In round numbers if your guide star is located gt 223 south of the science field center at PA 0 the guide probe will not shadow the science field Closer to the center of the science field a conservative zone of avoidance for PA 0 is such that you want no slits within lt 23 North lt 106 East lt 44 West of the location of guide WFS star For observations of single targets roughly centered in one of the facility long slit masks you can utilize a fair amount of the science field to obtain guide WFS stars so long as you keep the long slit outside the conservative boundaries above 2 9 2 Guiding and WFS Star Brightness Limits The MODS AGw system is as sensitive as the LUCI AGw unit so similar recommendations on guide WFS star brightness apply Bright Limit Re12 Faint Limit R 16 5 S The limits apply for good seeing of 0 8 FWHM with the Clear or F525LP filter The bright limit 1s set at the point where Shack Hartmann spots begin to saturate on the WFS camera when seeing is 0 6 The WES limit is independent of the guide camera filter because the WFS pickoff beam is located in front of the guide camera filter wheel For using the B Bessel filter to guide in blue light the bright limit is about 1 magnitude brighter because of the lower QE of the LBTO guide camera CCDs in the blue In practice guiding
8. 5 pixels rms for every 15 of elevation change while tracking instrument elevation is the dominant term in the flexure budget There still remains a small 1 2 pixels of residual image motion across the entire range of telescope elevation 30 90 and instrument rotator angle 0 360 that will show up as a field to field zero point shift Future refinements of the IMCS will be made with the goal of improving the absolute compensation In practice we often see 0 5 pixels of shift between spectra taken over 2 hour integrations of continuous tracking When moving the telescope pointing from one position to another the typical time for the IMCS to correct for post pointing flexure is 60 seconds but it can be as long as 90 seconds if going to very low elevations 30 The IMCS zero points for the collimator tip tilt focus actuators are measured at 60 elevation with the rotator at the nominal 0 position and stored in the instrument configuration files These are the starting points for corrections made after either pointing the telescope or reconfiguring the instrument IMCS operation is automatic when using MODS in the recommended way with the scripting interface If running MODS by hand e g during technical nights you must always follow a PRESET or INSTCONFIG command with IMCSLOCK to re align the optical system 2 11 Camera Shutter Each camera has an integrated shutter consisting of two graphite epoxy blades in a barn door configura
9. Acquisition Guide and Wavefront sensing unit Anti Reflection coating Charge Coupled Device Digitized Palomar Observatory Sky Survey Flexible Image Transport System image data format Field of View Full Width at Half Maximum Guider and Collimation System LBTO telescope subsystem Global Positioning System Graphical User Interface Hour Angle Identification Image Motion Compensation System MODS subsystem Instrument Messaging Protocol version 2 Infrared Instrument Support Structure LBT focal station rotator component Large Binocular Telescope Light Emitting Diode LUCI Mask Simulator slit mask design program Liquid Nitrogen LBT Near IR Imager Spectrometer aka LUCIFER MODS Mask Simulator version of LMS for MODS Multi Object Double Spectrograph Multi Object Spectroscopy Neutral Density Near Infrared here 7000 10500A aka suboptical Network Time Protocol The Ohio State University Position Angle measured North thru East in degrees Pointing Control System LBTO telescope subsystem Quantum Efficiency Quartz Tungsten Halogen lamp Root Mean Square Region of Interest Sloan Digital Sky Survey Telescope Control System University Research Instrument Center University of Arizona Coordinated Universal Time Ultraviolet specifically the terrestrial near UV 3200 4000A Wave Front Sensor MODS Instrument Manual 2 Instrument Characteristics The Multi Object Double Spectrographs MODS
10. Channel Filter s 0 6 Slit Readout 3088x3088 SDSS ug 3300 6000 SDSS riz na 5000 10000 Some O Re moe Meum p py 8288x3088 1850 3200 6000 GG495 2300 5000 1000 Prism Dual B es B 420 Nap 3200 10000 Blue Cae 420 140 3200 6000 GG495 500 200 5000 10000 ee NN 1 Instrument configurations are requested in MODS observing scripts by specifying the channel configuration and mode name For example instconfig dual grating or instconfig red imaging 2 See 2 3 1for detailed properties of the current MODS filter set 3 See 2 5 for the detailed properties of the current complement of gratings and prisms 4 The CCD readout region is the default size for the mode For imaging it includes the full FoV plus imaging stop and for the spectroscopy modes it 1s sized for the blue and red extremes when used with MOS masks with slits extending to the outer edges of the mask See 2 6 for detailed properties of the CCDs 5 The gratings are tilted to give the optimal wavelength coverage for the full spectral range of MODS There are no immediate plans to offer other grating tilts 6 The GG495 filter is a long pass filter that cuts on at 4950A and is used to block second order light from the red grating 2 2 Instrumental Throughput 2 2 1 Imaging Throughput Estimates of the imaging sensitivity of MODS are based on measurements of secondary photometric standard stars in the SDSS AB m
11. Instrument Manual Of particular interest to observers are the dewar temperature and pressure and the CCD temperature If the LN reservoir has run dry DEWTEMP and DEWPRES will rise above the nominal values depicted above CCDTEMP 1s the temperature of the CCD detector mount If the detector is warming up and producing high dark counts or other warm detector artifacts this number will be warmer than the nominal value shown above Instrument Temperature Sensor Data These keywords report the values of various instrument temperature sensors in the instrument at the start of the exposure Sensors are located inside the instrument electronics boxes Mechanism controller and utility boxes on the instrument main structural truss and sampling the air temperature inside the instrument AMBTEMP 0 3 Outside Ambient Air Temperature deg C TAIRTOP 1 6 MODS Inside Air Temp at Top deg C TAIRBOT 0 5 MODS Inside Air Temp at Bottom deg C ICOLLTOP 2 l J Qollimator Truss Tube Top Temp deg TCOLLBOT 0 8 Collimator Truss Tube Bottom Temp deg C IEBTEMPB 4 1 Blue IEB Air Temperature deg C IEBGRT B 0 0 Blue IEB Glycol Return Temperature deg C IEBTEMPR 2 5 Red IEB Air Temperature deg C IEBGRT R 0 2 Red TEB Glycol Return Temperature deg C IUBTAIR T4 7 Utility Box Arr Temperature deg C AGHSTEMP 0 0 AGw Controller Heat Sink Temp deg C Glycol Cooling Sensor Data These keywords report the glycol coolant s
12. MODS Internal Calibration Lamps Krypton Pen Ray Lamp Dual channel wavelength calibration OTHI 10W Quartz Tungsten Halogen Lamp Blue spectral flat field u imaging flats OTH2 10W Quartz Tungsten Halogen Lamp Blue spectral flat field Variable intensity Incandescent Lamp Red spectral and Dual imaging flats Spectral line finder plots and tables of identified spectral comparison lamps for the five wavelength calibration lamps are available on the MODS website Plots of representative spectra are given in Appendix C The QTH lamps are used individually or together for blue UV spectral flats The VFlat variable intensity incandescent lamp is used for imaging flats 1n the g z bands and red spectral flat fields 1n the grating and prism modes Its intensity can be varied over integer levels of 1 to 10 to select a brightness that won t saturate the detectors Typical values are VFlat 2 for the SDSS r band imaging flats and VFlat 4 for the SDSS g filter imaging flats with greater values used for spectral calibration VFlat 9 10 1s typical of red grating flats The brightness of the lamp 1s non linear with intensity value as shown in Figure 21 and the lamp gets slightly bluer with higher intensity VFlat intensity is proportional to the applied voltage and the lamp filament gets hotter with greater voltage 28 MODS Instrument Manual OSU MODS 2011 003 Version 1 2 MODS Variable Intensity Flat Lamp VFlat 0 8 0 6 0 4 i Re
13. and WFS stars fainter than R 16 5mag are challenging to use for guide WES stars even in good seeing with the current GCS algorithms for guide star tracking 3l MODS Instrument Manual and WFS measurement Future work on the GCS may improve the lower limit and expand the number of available guide stars 2 10 Image Motion Compensation System IMCS MODS is a long instrument 4m and as it is pointed around the sky it is subject to gravity induced flexure of the structure stochastic ticks and pops in the structure s welds and bolts print thru from the instrument rotator bearing and other sources of mechanical flexure There are also small thermal components of flexure as the ambient temperature changes All of these factors work together to slightly misalign the optics as a function of instrument elevation angle and rotator angle leading to undesirable motion of images across the science CCDs Computer models and measurements at the telescope show that uncompensated image motion can be as much as 100 pixels when the instrument tracks from horizon to zenith To eliminate most of this image motion MODS uses an internal closed loop Image Motion Compensation System IMCS Marshall et al 2006 The IMCS measures the alignment of the optics in each channel in real time during exposures and then steers the collimator mirrors in tip and tilt to null the image motion An IR laser beam A 1 55um is launched from below the focal pl
14. and observing scripts and flux tables are on the MODS website We are in the process of compiling a library of MODS spectra taken in all modes of the instrument which will be available online with the recommended flux tables as we progress with reducing the data We recommend observing all flux standard stars with the 5 arcsec spectrophotometric slit mask LS60x5 This will provide spectra across the full wavelength coverage and you will have nearly zero losses due to atmospheric dispersion and seeing Note that resolution in the 5 arcsec slit will be seeing dependent but given that the flux calibrations are typically in large bins this will have little impact on the derived response curves which have structures on much large scales than the resolution Standard star spectra observed with one of the narrower slit masks will impose a substantial penalty in losses due to atmospheric dispersion at the far blue end In part this is because a narrow slit will always have some slit losses at the blue end exacerbated by the fact that 66 MODS Instrument Manual OSU MODS 201 1 003 Version 1 2 many of the standard stars are in sparse fields that lack sufficient numbers of guide stars to always allow you to orient the slit along the parallactic angle We do not recommend using anything but the wide slit for spectrophotometric standards We have prepared a set of standard scripts for acquiring wide slit flux star spectra with selections of guide stars
15. are loaded into the mask cassette by observatory instrument support personnel before your observing run The slit mask insert retract time is 10 seconds The maximum time to extract an old mask and select insert a new mask is 35 second for the largest cassette excursion 2 7 3 Multi Slit Mask Field of View MODS is designed to deliver good images inside the central 4x4 field of view and reduced image quality in a 6x6 extended science field In practical terms the image quality decreases rapidly outside a 5 6 arcminute diameter circle primarily due to a combination of astigmatism from the off axis paraboloid collimator mirrors and off axis aberrations in the LBT f 15 direct Gregorian focal plane For purposes of locating MOS mask slits and alignment stars we recommend that you keep primary science target slits within a 5 6 circle and alignment stars within a 5 0 circle as shown in Figure 20 Note that the LUCI 4x4 is inscribed inside the 5 6 circle so the primary science fields of MODS and LUCI are the same 3000 2000 m Y pixels 1000 1000 2000 3000 X pixels Figure 20 MODS multi object slit mask effective field of view The outer circle is 5 6 in diameter the inner 5 The short lines show an exaggerated representation of image distortion due to astigmatisms at those field locations Slits may be located outside the 5 6 circle but with a penalty of greater image aberration re
16. calibration cal and instrument setup procedure pro scripts This division recognizes the different requirements of target and data acquisition for spectroscopy modern active optics telescopes require very close choreography between telescope instrument and guiding active optics systems during target acquisition but once science data acquisition begins most of the activity 1s centered on the instrument and consists primarily of monitoring the exposure progress MODS scripts are highly re entrant an observing script interrupted by errors or other problems can be restarted from any point within the script without editing the source file The MODS script execution engines also feature robust error trapping and a flexible point of fault 54 MODS Instrument Manual OSU MODS 201 1 003 Version 1 2 recovery abort retry 1gnore facility to notify observers of problems and provide pathways to quick resolution To help observers create MODS scripts the modsTools suite of script preparation tools are available for observers to install on their computers These tools create scripts that can be used as is or as templates for crafting more sophisticated observing sequences They ensure that observers start with syntactically correct script files as they prepare for MODS observing For each MODS spectral target you create two scripts an acquisition acq script to point the telescope and align the slit mask with the target and an observing ob
17. dt Focus 3220 um TTF A 17378 B 17804 C 19922 um Focus 1200 um TTF A 15880 B 14860 C 12844 um Litilities Exposure Control Exposure Cantral Name VFlat Lamp Mame VFlat Lamp A Type Object ExpTime 2 B i Images 1 Type Objact ExpTime amp sec it Images i Binning X 1 Y 1 ZCDHReadout zKxzkK Binning X 1 Y 1 COD Readout BKx3K MaxtFil amp modsib 20110810 0085 LastFile modsib 20110810 0034 Nestrile modsir 20110810 0017 Lasit ile mods1r20110810 0016 Pause Shutter Closed Image Pause Shutter Closed Image GO Exposure GO Exposure Stop Figsdaut zin Readout IMCS idle Use IMCS Lock On MGS bile Use IMCS Lock On Command Status Helresh ABORT View Log Figure 47 MODSI dashboard screen The dashboard is laid out from top to bottom in the order that photons make their way into the instrument and through the optical system into the detectors in each channel White boxes are entry boxes where you may type new parameters When you make an entry the box background will turn pale yellow until you hit the Enter key to commit the change Gray boxes with blue text are information displays you cannot change these values When an instrument parameter is set via one of the entry widgets text box button or pull down menu the widget will turn amber to indicate changing state If the requested setting is successful the widge
18. longer wavelengths so the actual flux per raw spectral pixel will be larger This is difficult to represent on plots like those shown in Figure 5 unlike the case of the grating efficiency curves 2 3 Filters 2 3 1 Science Camera Filters Science filters are mounted in 8 position filter wheels in the f 3 beams of the blue and red channel cameras The filter is located between the camera primary mirror and field flattener lens that doubles as the CCD Dewar entrance window Filter change times are 2 to 8sec The properties of the current MODS filter complement and a description of their intended use is given in Table 3 with transmission curves in Figure 6 and Figure 7 Imaging filter 13 MODS Instrument Manual effective instrumental band pass parameters are summarized in Table 4 A description of the derivation of the filter parameters is given in Appendix B Table 3 MODS Filters Notes 1 The FilterID is the name used to select the filter in the MODS instrument control system For example red filter r sdss Or blue filter nd1 5 Note that all FilterIDs are case insensitive and have no spaces The filter ID 1s stored in the FILTNAME keyword in the image FITS headers along with a FILTINFO keyword that includes additional information e g full name manufacturer etc 2 The Clear filters in each channel are designed to ensure proper camera focus balancing between unfiltered and filtered configurations The cameras cannot
19. readily detected at 3200 3 Atmospheric Transmission The calculated atmospheric transmittance from 3000 10500A calculated for the altitude and typical atmospheric conditions of Mt Graham for airmass 1 2 El 60 is shown in Figure 40 1 8 O Q IT Cc amp 6 E Uu 2 E Es x 4 al cee 0 H O z 300 400 500 600 700 800 900 1000 Wavelength nm Figure 40 Model atmospheric transmittance for Mt Graham at airmass 1 2 At short wavelengths transmittance is dominated by continuous Rayleigh scattering opacity from molecules and aerosols that increases towards the UV Blue ward of 3400 the O4 Huggins bands become an important source of telluric absorption features Red ward of 60004 the O2 A and B absorption bands at 7694 and 6867 respectively are the sharpest telluric features with H2O water vapor bands growing in importance into the near IR especially between 9000 and 10000 3 2 Atmospheric Emission Thermal emission from the night sky dome and telescope is negligible for MODS The dominant sources of atmospheric 1 e background emission will be from airglow lines arising in the upper atmosphere light pollution from nearby population centers and moonlight The calculated night sky brightness during new moon for the typical atmospheric 42 MODS Instrument Manual OSU MODS 201 1 003 Version 1 2 conditions and typical of Mt Graham for airmass 1 2 elevation 60 1s shown in F
20. star OH emission lines are the dominant source of diffuse background in SDSS 1 and z images during dark sky conditions Because the MODS red CCDs are 40um thick deep depletion devices they fringe less than typical thinned CCDs and it has not been a limiting factor in taking deep images in the 1 and z bands 43 MODS Instrument Manual 3 3 Moonlight and Twilight Impacts Moonlight and twilight also affect MODS performance Both appear as a faint blue reflected solar spectrum filling the slit Figure 42 shows a composite of blue and red spectra taken during morning twilight of a bright supernova E Figure 42 Spectra of SN2010jl taken during morning twilight Top Blue Channel Bottom Red Channel Unlike the Near IR 0 9 2 5um you generally cannot observe very far into astronomical twilight with MODS unless the target is very bright Moonlight also causes problems for the AGw unit bright background can make it hard to find and lock on guide stars and can also make accurate measurement of the Shack Hartmann spots in the WFS difficult In general you should avoid observing within 30 of the moon 3 4 Differential Atmospheric Refraction Light entering the atmosphere is refracted in the vertical direction relative to the horizon with greater refraction at bluer wavelengths Figure 43 shows a plot of this differential atmospheric refraction relative to 6200A the guide wavelength of MODS for Mt Graham Differential Atmo
21. then resumed the image FITS headers will record the cumulative open shutter time in the EXPTIME keyword and the elapsed open closed shutter time as the DARKTIME keyword The latter is a bit of a mis nomer but is useful for assessing the expected cosmic ray accumulation on an image when the shutter 1s closed the dark CCD is still collecting charge from cosmic ray hits and the small number of hot pixels on the detectors While the detector 1s being read out you cannot stop or abort the image acquisition but must instead ride out the CCD read out and write to disk steps If you click on Abort while the CCD is reading out it should catch the abort request and terminate the exposure sequence after the image 1s readout and stored Aborting an imaging sequence can sometimes get messy leaving the exposure control boxes in an apparently stuck state If this happens type red reset or blue reset in the Command Entry box see below to reset the state of the exposure controls 52 MODS Instrument Manual OSU MODS 201 1 003 Version 1 2 Interactive Command Entry Beneath the channel control panels is an entry box labeled Command where data taking system commands may be entered and executed any instrument function including all scripting commands may be executed by hand with this box Command Status Below the command entry box is a status display that will show any command messages emitted by the data takin
22. use of modsAlign 1s common for acquiring spectrophotometric standard stars in the 5 arcsecond wide slit Detailed worked examples for both alignment modes are given in the MODS Observing Scripts manual where it is used in the context of the acqMODS target acquisition script engine the primary way that observers will interact with MODS and modsAlign 59 MODS Instrument Manual 5 MODS Calibration This section describes the basic calibration procedures for MODS Standard facility scripts are provided to acquire the calibration data described below Ask the support astronomer for the current set also see the LBTO Wiki s PartnerObserving sections for the most up to date notes 5 1 Calibration Plan A summary of the basic calibration data needed to reduce MODS data is given in Table 12 Table 12 Basic Instrument Calibration Data Imaging once per run once per run Grating Se once per run once per run one per run Prism Spectroscopy 5 2 Bias Zero Images MODS science CCD biases are very stable and only need to be obtained once per night for each of the major CCD region of interest readout modes used during that night Five 5 biases provide sufficient signal to noise for most applications when median combined At present 2D Biases are not required to reduce the full frame 8Kx3K images as the pre scan columns remove most of the bias structure without significant residual 2D bias structure Imaging 3Kx3K and Pri
23. virtue of giving an exact conversion between broadband flux densities in frequency F and wavelength F units V ye EE m C The width of a filter 1s poorly defined for a finite bandpass but we will follow the successful practice of HST instruments and adopt an effective width defined as 61 2 2In2 oA where o 1s the effective dimensionless Gaussian width of the filter MICE jor and Ais the mean wavelength of the bandpass defined as dA T nA _ f OT m4 7 A exp da TF o fo A Pi 74 MODS Instrument Manual OSU MODS 201 1 003 Version 1 2 This unconventional definition of the mean filter wavelength in terms of the first logarithmic moment of the average throughput has the property that the corresponding mean frequency is c A giving a value that lies between the more conventional frequency and wavelength first moments Finally the Full Width at Half Maximum FWHM of the filter is evaluated numerically by measuring the rectangular bandpass width from the average throughput curves Q T When evaluating these integrals numerically for the different filters we truncated the integration where the measured transmission fell below 0 001 at the wings of the measured transmission curves This avoids introducing spurious shifts 1n the bandpass parameters because of leaks at long or short wavelengths far from the nominal filter center Figure 49 shows these parameters plotted for two MODS filters in di
24. with LUCI AGw 1s a larger guide acquire camera field of view 50x50 and 2x larger pixels an off center WFS pickoff and guider hotspot and a smaller off axis guide star patrol field Overall guiding accuracy with the MODS AGw unit and the current generation LBT GCS software is measured to be 30milliarcsec RMS except at very low elevations 25 where the guide star images are smeared out due to differential atmospheric refraction and increased astigmatism coming from the adaptive secondary mirrors The MODS AGw unit is tightly coupled mechanically to the MODS focal plane sharing a mounting surface with the sensor that determines the location of the slit mask in the science field of the instrument focal plane Measurements made during commissioning demonstrated 29 MODS Instrument Manual that there is essentially no differential flexure between the guide camera and science slits after accounting for the effects of differential atmospheric refraction 3 4 The guide stage is very stable and repeatable During a telescope offset the guide probe is commanded to follow the offset After repeated offsets e g as would be done when nodding along a slit the RMS offset repeatability was measured to be 35milliarcseconds independent of telescope elevation down to 30 the final measurements were near 25 the current lower elevation limit of the telescope This is within the general error envelope of guiding error in the system and rou
25. 086 8t 7 9 6 2 074 cvO 7vVLZ 8127907 Lev 8969 10 5x10 nav 6000 7000 8000 9000 10000 5000 Wavelength Angstroms Figure 54 Argon lamp spectrum taken with the Red Grating G760L Xenon Krypton Lamps MODS1 Red Channel G760L 061 666 9x 007 66 6 9x 0689 2916 ex 99v SVv06 9X 669 8668 1 Z 8068 M 087 9 78 Jy 80 8 8098 14 L6L60Tv8 9X 6601 8668 IM 99800618 Jy CLOG cLL8 JM SPOS 0000 CECB G98 6Sv c 897 LOPS t697 1 9 Lv 78 8 1 L979 L09Z 4 9160 88 X Egee c9868 tv 686 0788 I 19 e x LO 1 5x10 10 nav 6000 7000 8000 9000 10000 5000 Wavelength Angstroms Figure 55 Xenon Krypton lamp spectra taken with the Red Grating G670L 78 MODS Instrument Manual OSU MODS 201 1 003 Version 1 2 References l Z 10 Telescope Specifications for the LBT UA 98 01 LBT 002s004g MODSI Laboratory Acceptance Test Report OSU MODS 2009 001 Version 1 3 2 2010 July 13 MODSI AGw Unit Commissioning Report OSU MODS 2010 003 Version 1 4 3 2011 Aug 18 Definition of the Flexible Image Transport System FITS FITS Standard Version 3 0 2008 July 10 http fits gsfc nasa gov 1aufwg MODS AGw Unit Guide Camera Filters OSU MODS 2010 002 Version 1 1 2 2010 January 12 MODS Observing Scripts OSU MODS 2011 002 Version 1 1 2011 Dec 10 LBTO Coordinate System Description LBT 002s105b D Miller 2010 Jan 22 An Image Motion Compensation System
26. 60 e y 5 50 E 50 o e Filter 40 ui iT ii Direct Mode 30 30 Dichroic Mode 20 20 10 10 0 Q 500 550 600 650 700 750 650 700 750 800 850 900 Wavelength nm Wavelength nm MODS1 SDSS z Filter 100 80 80 70 60 50 40 30 20 10 0 600 850 900 950 1000 Wavelength nm Filter Direct Mode Dichroic Moda Efficiency Figure 6 MODSI Imaging Filters Blue shows the filter only transmission in parallel light magenta the effective filter bandpass including instrumental response with no dichroic direct and red with the dichroic Schott UGS Filter MODS1 GG496 Order Separation Filter 100 F Filter aD an 5 Direct Mode Dichroic Mode Filter 70 Direct Mode Dichroic Mode 60 50 40 30 20 10 Efficiency Efficiency 300 350 400 450 500 550 600 500 600 700 800 900 1000 Wavelength nm Wavelength nm Figure 7 MODS blocking filters Left GG495 red 2 order blocker Right UG5 red blocker 15 MODS Instrument Manual Table 4 MODS Imaging Filter Parameters Pivot Effective ior du Wavelength Filter Mode Ap A SDSS u Direct 3589 4 ne E 8 480 Dichroic 3592 9 408 8 3584 5 470 Dichroic 4773 8 1017 3 4735 3 1460 Dichroic 6323 4 878 7 6301 3 1270 Dichroic 7651 0 1014 4 7626 6 1500 Dichroic 8952 9 8936 7 1100 2 3 2 AGw Guide Camera Filters The AGw unit has a 4 position filter in the guide channel the WFS
27. DIS seco eetem a teet at emiten ees anite ds em ERR 54 4 3 Where do the MODS dta go asset mmo Cat eceeica de tcd areata 56 44 GmnodsDisp Raw Data DIS Play siesta etra ERE EEEO in Er 58 4 5 modsAlign Interactive Mask Alignment Tool sese 58 5 MODS 3lbEFAUOD i eredi eret Eee ve ERE Ere Wee ES cei AEE E E EES at 60 5 1 od ore IE Foe ed Lal NE RD NO TUR UNIO ERE IECREOTEERPUROEPURHONEN 60 3 2 Blas Zor VMAS i ccs tsi elena Pig uda ob pa imm igo itp ids 60 5 3 Dirk Trines S EU NUN 60 V PACF eR RUPEE DET TENTER 61 Sl Injasine TIAS oean e case numis uumues aspici enu umaiie ique tdm tea dubie 6l 5 4 2 Grating Spectroscopy Pixel Flats eese eee eeeeeee eene 62 54 Spectral lite FIIS ae n n Ro CoS d tnu 63 2d wash sv MR ee pe miei toe eti are em a ee LuM 63 2 5 Wavelength Calibration Comparison Lamps eeeessssssseeeeeeeeeeeennnnnn 64 5 DiSperstonooluboDS cudstetuisedtecistad e DOG d pou apAbun d esu tas fao A 64 Joa Calibration Lamp Files siira E EEE acabas ott p EER 65 3 5 9 Prsni Mode Waveleneth C aliDratlOtl s so aenea T O 65 5 6 Spectrophotomettic Standard Stars inc nii o re Gebet ed arre t Pre ore Gebet edo ed 65 DNI otandard Calibration SCHDIS 55 nesetodmisot atecssstte taste mtu ete mise invicem ita te ete ei 67 Appendix A MODS FITS Headers ee teen ratio eoa eS Neuro e Ren eere EXE P e inten EYE no aet eer ehem ese 68 Appendix B Filter Bandpass Parameter
28. Gratings and Prisms Each channel has a grating turret containing an imaging flat mirror and up to 3 dispersers The current complement of flats and dispersers 1s listed in Table 6 and their detailed properties are given in Table 7 At present there are 2 dispersers per channel with space for a future third disperser in each channel Table 6 MODS Imaging Flats and Dispersers Channel Grating ID Description 0 6 Slit 1 The Grating ID is stored as GRATNAME in image FITS headers along with a GRATINFO keyword that includes additional information e g full name manufacturer etc 2 Grating resolutions quoted are measured at the reference wavelength 4000 for Blue 7600 for Red in a 0 6 reference slit extended source resolution Prism resolutions are quoted for the full range of wavelengths blue to red 3 Prisms have variable dispersion that decreases from blue to red see Figure 12 Table 7 MODS1 Disperser Properties Lines Blaze Nominal Linear Spectral Type mm FUSES EE Range Dispersion ETS Grating G670L 250 4 25 25 1 5800 10000 0 8A pix 5700 G400L E RECEN 3200 5800 0 5A pix 5200 s 1 Disperser ID coding G Grating P Prism nominal central wavelength in nm L Low dispersion M Medium dispersion 2 Nominal wavelength range for dual mode operation with the dichroic 3 Nominal linear dispersion at the nominal central wavelength of the spectrum MODS Instrument Manual
29. IENCE Camera Filters uerit rim Rr Ogame N O n CHR 13 2 2 AUW Guide Camera Piers s neo DR o ston tere nr efe EC DR tss 16 24 NACI ONG occus eat imet bpm ed T ten eeiam piri a es 17 2 5 Granas Sano Wl PS eee Nees bet enn Or RR Eee mE Fe en Onn res Ty io odes tT chute Dicti terre 18 DON AREMECUOM GLAMIS Sicut poros tc recausdeddadasenen a sovsdediadeteeeseuvacneaes 19 2 02 JDoUDIC DOS a ic 0 Lc ep ence ene Ree me Pee ee eee RY ae ire dd Leiter 19 2 6 CCDE eU REM c 20 2 0 L Basic PLO PCIe S uatoseneute Get doaed icit bane Gast O pu Bede 20 2 0 2 Exposute Overheads cesset attetkin dete mid et mu pe antenna dde tus t adulte 21 20 3 Quant tm E H CIEN y aec otn ene Eta HS b etium et waseceeeteeeieass 27 DOA Dco SAUL AMON eee sero set ems SEED UI T bots de ouis TEN 22 240 5 3CTOSSST OI tg een tench acea eid bett ON butt enia tese bacis err E 24 2 EMA K ciconia eto Cat monta dict tas Mobi dL cca 25 2 A Permanent Facility Sht MaSKES ee eorr eee to th et ett need eget reos 25 2 1 2 C Un MaSK Sosire UNI iue SU E ei E 26 2 71 9 Multi Slit Mask Field Of View i i at seta b mad tuta i aduiadus 27 2 8 CUO EO UN Me Tr Lr 28 2 9 Acquisition Guiding amp Wavefront Sensing AGw Unit eeeeeeeeees 29 29k Aide Star Patrol BICC ci eee p tue metes ies ten aestate titu tice iid 30 2 9 2 Guiding and WFS Star Brightness Limits eeeeeeessssssssseeseeeeeeeeeenn 3l 2 10 Image Motion Compensation System IMCS
30. MODS Instrument Manual Document Number OSU MODS 2011 003 Version 1 2 4 OHIO SAIE Date 2012 February 7 UNIVERSITY Prepared by R W Pogge The Ohio State University MODS Instrument Manual Distribution List Institution Company Number of Copies Richard Pogge The Ohio State University 1 file Chris Kochanek The Ohio State University 1 PDF Mark Wagner LBTO 1 PDF Dave Thompson LBTO 1 PDF Olga Kuhn LBTO PDF Document Change Record CC MODS Instrument Manual OSU MODS 201 1 003 Version 1 2 Contents 1 PRICE OCU CUION esnie ee EE EE ESE E I a 6 1 1 SCOPE PU RM NN 6 1 2 Citing and Acknowledging MODS ccccccccccccceeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeees 6 1 3 OnE MaI doeet ebat bedient capones donated emacs awa CE E UM meret 6 kA Acronyuis and ADOPE VIA ONS cueste Dass quet Heo estes tust iun oae burst ap us aset edidi ud 7 2 Instrument C BaracterIstl6S isisscesciadsasaccarareutsnccncdecansvaatiadsdsescatetandenssacscarseasstandatiassasenaute 8 2 1 l stument C Omi CULATIONS oi oec eoi DuC Euri T ERR EN do SV atee Suv deett abd 11 2 2 Tnstrumental TNOU put are p HE tu ER EUER Eoi eb m p AER 11 22h masne LDroup DU cue rotto auttm tae es 11 2 2 2 Grating Mode Spectral Throughput eeeeessssssssssseseeeeeeeeeeeennnnnnn 12 22 9 Prism Mode Spectral Throughput e ERREUR pERAEM REUS 13 2 3 PULSES e de dulled Malacca 13 2 9 1 SC
31. OSU MODS 2011 003 Version 1 2 2 5 1 Reflection Gratings MODS currently has a set of red and blue gratings giving R 2000 A o in first order with a 0 6 slit Space 1s reserved in the grating turret for a future set of higher resolution gratings up to Rx8000 with a 0 6 slit The grating blaze curves measured in Littrow configuration by Richardson Grating Lab are shown in Figure 10 for the useful wavelength ranges in MODS The gratings are tilted to give the optimal wavelength coverage for the full spectral range of MODS At present we do not plan on providing alternative tilt angles MODS1 G400L Grating MR162 2 7 1 MODS1 G670L Grating MR122 5 5 1 80 90 Efficiency 96 Efficiency 96 30 30 20 20 tr 4 L h 4 4 ee ee ee ee ee ee ee ee ee ee ee ee ee ee 300 350 400 450 500 550 600 500 550 600 650 700 750 800 850 900 950 1000 Wavelength nm Wavelength nm Figure 10 MODS1 Grating Blaze Functions Left G400L Right G670L When used in red only mode the red G670L grating must be used with a GG495 filter in the red camera to block light from 2 order from contaminating the 1 order spectra In dual channel mode the dichroic acts as an order blocking filter and the clear filter is used in the red camera Because there is still a small amount 2 of blue light reflected by the dichroic in to the red ch
32. T E ce SR GCS Requested guide probe PCS X mm GCS Requested guide probe PCS Y mm GCS Actual guide probe PCS X mm GCS Actual guide probe PCX Y mm PCS Predicted guide star X position mm PCS Predicted guide star Y position mm GCS Measured guide star X position mm GCS Measured guide star Y position mm GCS cumulative X guide update position mm GCS cumulative Y guide update position mm Instrument Environmental Monitoring Data These keywords list the temperatures pressures and voltages measured in the CCD detector head electronics box at the start of the exposure DEWPRES DEWTEMP CCDTEMP HEBTEMP HSTEMP CTEMP IN CTEMPOUT TEDPOWER LEDPOWER IGPOWER HEB3V HEBFANV HEBI5V HEBSV HEB24V BOGHTRV Ed TOU ESQ ES SER 1 3 24 Die 90 PENE ON Y ORBE ON Y 16 78 09 Duro T Eo t 4 93 Bore L299 86 04 16 TS E MES E MER ES cC CR Dewar Pressure torr Dewar LN2 Reservoir Temperature deg C CCD Mount Temperature deg C HEB Air Temperature deg C HEB Post Heat Sink Air Temperature deg C HEB Coolant Input Temperature deg C HEB Coolant Output Temperature deg C HEB Thermoelectric Device Power State HEB External LED Power State Dewar Vacuum Ion Gauge Power State HEB 3V Power Supply VDC HEB Fan Power Supply VDC HEB 15V Power Supply VDC HEB 5V Power Supply VDC HEB 24V Power Supply VDC Dewar boil off gas heater voltage 71 MODS
33. agnitude system The estimated total source 11 MODS Instrument Manual counts Sx in ADU for a source of brightness mx in the SDSS X band X u g r 1 z filter in AB magnitude for exposure time texp 1S log Sy log 0 4m logt where S o is the zero point in ADU for mx 0 8 This zero point includes the combined telescope and instrumental throughput for a source observed at 1 2 airmasses The photometric zero points for the five MODS imaging filters see 2 3 1 are listed in Table 2 Imaging Zero PointsTable 2 Table 2 Imaging Zero Points Blue Red As general guidance an r 15 S star will just begin saturating the central pixels on the Red CCD after 30 seconds of integration in 0 6 arcsec seeing 2 2 2 Grating Mode Spectral Throughput The total efficiency instrument and telescope of MODS in grating mode is shown in Figure 4 normalized to the efficiency at 1 2 airmasses elevation 60 The efficiency curves do not include strong telluric absorption features e g Figure 40 MODS1 Grating Mode MODS LBT at 1 2 Airmasses ES Direct ees Dichroic Efficiency 6000 000 8000 9000 10000 Wavelength Ang 4000 2000 Figure 4 MODS1 Grating mode efficiencies Solid lines are direct mode blue or red only dashed lines are dichroic dual mode Efficiency includes instrument telescope and atmosphere at 1 2 airmasses These values depend on the cleanliness of the primary and secondary
34. ame but map into only 650 pixels An example prism spectrum is shown in Figure 31 The short span of prism mode spectra makes it possible to stack multi object mask slits 3 deep horizontally Figure 31 Raw MODS Red Prism Spectrum with the LS5x60 slit 4Kx3K ROI readout mode 36 MODS Instrument Manual OSU MODS 2011 003 Version 1 2 2 12 2 Bias and Flat Field Structure The MODS CCDs are divided into 4 quadrants that are read out simultaneously using the four on chip read out amplifiers located at each corner of the array The output from each of these amplifiers is then split into two output channels one for odd pixels and one for even pixels for a total of 8 output channels Each of the 8 output channels has its own DC bias level and conversion gain The difference in bias and gain for each quadrant makes each of the four quadrants distinctly visible in the raw images Figure 32 shows examples of blue and red CCD full frame bias images in which the 4 quadrants are clearly visible Figure 32 MODSI Bias Frames Top Blue CCD Bottom Red CCD The effect of the separate output channels for the even and odd pixels within a quadrant is seen in the raw images as vertical striping when viewed zoomed in close to see individual pixels An example of this even odd effect 1s shown in Figure 33 which is a high contrast zoom into the boundary between the four quadrants at the center of the red bias image shown in Figure 32 Figure 33 The E
35. amounts along the parallactic angle If the deflection 1s large enough light will begin to fall out of the slit particularly at blue wavelengths see e g Filippenko 1982 8 430 PA 0 HA 1 to 2 West 82430 PA 76 HA 1 to 2 West 1 1 5 5 w a eo e S 0 n oman S 0 z E 5 5 EB 0 5 1 5 0 Be 1 Ax arcsec Ax arcsec Figure 45 Differential atmospheric refraction from HA 1 to 2 slit along PA 0 and PA 76 the typical parallactic angle for this HA interval Colors run from deep red 9500A to purple 3500A in 1000A intervals The circle marks the start of the tracks at HA 1 The guide reference wavelength is 6200A Figure 45 shows calculations of differential atmospheric refraction deflection tracks relative to a 1 slit for a source at 5 30 observed for 1 hour from HA 1 to 2 west of the meridian 45 MODS Instrument Manual The first plot shows the tracks with the slit oriented along PA 0 the second with the slit along PA 76 the median parallactic angle during this hour angle interval In the case of the slit oriented along PA 0 the bluest wavelengths 3500 and below start to fall out of the l arcsec slit about half way through the track but in the slit aligned with the median parallactic angle for this interval PA 76 there will be no light lost even at the bluest wavelengths MODS does not have an Atmospheric Dispersion Corrector so choosing a slit orientation that mini
36. and exposure times appropriate for all modes These will be posted on the MODS website as we develop the standard star library Where appropriate these scripts include the proper motion data that the LBT telescope needs for some of the faster moving white dwarfs and subdwarfs in the group We will be expanding this list of recommended standard stars as we get suggestions from observers and from additional searches of the literature particularly the ESO and STScI lists Stars with high quality high density flux calibrations from the HST CALSPEC database are proving to be particularly useful but need some editing to censor the flux data around the telluric O2 and H2O absorption bands not seen by the Hubble Space Telescope A finer flux point grid than the typical IRAF 50A tables is required to properly correct for the wiggles in the dichroic transmission The Oke 1990 flux tables with 2 sampling 7 in the far red from CALSPEC work very well for most applications A set of tested appropriately edited fine grained flux tables 1s being compiled and should be available on the MODS calibration web pages More will appear as we accumulate observations of more standard stars and qualify the tables 5 7 Standard Calibration Scripts A set of standard scripts for routine calibrations like biases flat fields comparison lamps and spectrophotometric standard stars are currently kept on the LBT control room observing machines in the home MODSeng modsSc
37. and in progress status messages amber is warning messages red is error messages The messages are time date tagged and use the instrument messaging protocol syntax employed by MODS and other OSU built instruments Details are discussed in the MODS engineering manuals and should not be of general concern to observers though they are of concern to the LBTO support astronomers and MODS instrument team members 4 1 4 Utilities Screen This screen shows some MODS engineering functions with locked out controls including instrument power management controls Regular observers should treat this console as read only controls are locked by default and leave unlocking and operating instrument utilities 53 MODS Instrument Manual through this screen to qualified LBTO support personnel or MODS instrument team members You could inadvertently disable the instrument losing valuable observing time if you do more than just look 4 2 MODS Observing Scripts The most efficient way to use MODS is with its command scripting interface MODS scripts are plain ASCII text files containing lists of instrument and telescope commands to be executed in order from the start to the end of the file All functions of the MODS Control Panel are available via the scripting interface Scripts help maximize observing efficiency by automating routine observing tasks telescope pointing target acquisition instrument configuration and data acquisition Scripts take
38. ane and passes through the same optics as the science light onto a germanium quad cells mounted off axis just above each channel s science CCD A sketch of the IMCS metrology laser path for the Red channel is shown in Figure 24 the blue channel laser path is analogous red alignment v9 Scale 0 06 PLB 19 Mar 02 Figure 24 IMCS IR laser beam path for the MODS red channel Error signals from the quad cells are measured every second during an exposure with an average of three measurements used to compute the compensating tip tilt corrections for the collimator The red and blue channels run independently as each has different flexure modes Because the IMCS IR laser beam shares the same optical path as the science beam from slit to CCD the IMCS quad cells are only illuminated when the shutter is open so compensation can only occur during an exposure After a new telescope pointing and after reconfiguration of the instrument e g switching from imaging to grating spectroscopy mode the IMCS 32 MODS Instrument Manual OSU MODS 2011 003 Version 1 2 needs to be run briefly to zero the alignment of the optics so that the first science exposure starts in the properly aligned configuration Because multiple measurement cycles are required to average out instrument seeing the IR laser beam path is 10 meters long the IMCS is only engaged during open shutter exposures of 10 seconds or longer On average the IMCS nulls image motion to 0
39. annel you may optionally deploy the GG495 filter to provide for additional suppression of 2 order Note that 2 order from the blue G400L grating is in the UV not transmitted by the atmosphere so no order blocking filter is required 2 5 2 Double Pass Prisms MODS has two double pass prisms with immersed reflection coatings to provide a very low resolution R 100 500 mode The blue prism is made of Fused Silica glass with an immersed aluminum coating the red prism is made of TIH6 glass with an immersed silver coating The MODSI red prism and the double pass ray tracing is shown in Figure 11 Unlike gratings prisms have a very strong wavelength dependent resolution The measured resolution curves for the MODSI prisms are shown in Figure 12 The dispersion in the prisms varies as a low order polynomial in wavelength higher at bluer wavelengths lower at redder wavelengths The nominal prism parameters listed in Table 7 were measured at the mid point of the spectral ranges Also unlike the gratings the prisms do not require order blocking filters but future band limiting filters for special experiments may be installed in the instrument Because prism spectra map onto a small number of pixels in dispersion 600 vs 5000 pixels for the gratings it is in principle possible to horizontally slits on the multislit masks to increase the number of objects observed 19 MODS Instrument Manual Figure 11 MODSI red prism lef
40. approximate time the query was received and should be within a second or two of the actual time the shutter 1s opened This shutter lag 1s measured and described in SX AGw Stage Configuration These keywords list the state of the AGw Acquisition Guide and Wavefront Sensing camera stage system AGWXGP AGWYGP AGWFOCUS AGWFILT AGWENAME AGWFINFO AGWXS AGWYS AGWE S AGWE SO m eub 0 EDO qEr t Edmund 525nm Long Pass Yellow 50x5mm 87 o 47 3G cod 845 000 2 483 2037 818 go 00 i mm mm AGW Guide Probe X focal plane position AGW Guide Probe Y focal plane position AGW Guide Probe relative focus mm Guide Camera Filter Wheel position Guide Camera Filter Name Guide Camera Filter Descript X actuator position mm Y sctuaror position mm EoOus actusgbot positon Focus Zero Point mm AGW Stage AGW Stage AGW Stage AGW Stage mm GCS and PCS Guide Star Position Data These keywords give data from the GCS Guiding and Collimation System and PCS Pointing Control System regarding where the GCS sent the guide probe and the measured guide star positions These are mostly used for engineering commissioning work GPREQ_X GPREQ_Y GPACT_X GPACT_Y GSPRED_X GSPRED_Y GSMEAS X GSMEAS Y GSDELTAX GSDELTAY 4 mle 30 4 db 4 sup Os dia L24 T2 JUI 961 929 960 926 v9 929 818 2006 541 543 Tu Mm CN Ac M
41. are seeing limited low to medium dispersion spectrographs working in the 3200 to 10000A range with a 6x6 arcminute field of view MODS can be used for imaging long slit and multi object spectroscopy Multi object spectroscopy 1s accomplished with user designed laser machined slit masks loaded into a 24 position mask cassette There are two identical MODS spectrographs MODSI is on the LBT and began science operations in September 2011 MODS2 will arrive in the autumn of 2012 They are mounted at the direct Gregorian foci of the LBT as shown in Figure 1 Figure 1 MODSI on the LBT Left Direct Gregorian focal station After light passes through a common slit mask and field lens a dichroic splits light into red and blue optimized spectrograph channels Each channel has its own collimator dispersers camera filters field flattener FF lens and detector Figure 2 The beam selector can also direct light into the red or blue channels alone extending wavelength coverage into the dichroic cross over region 5700A for one or other channel alone Dichroic or Red Fold 1 Disperser Blue Corrector Mirror Figure 2 MODS Optical Layout MODS Instrument Manual OSU MODS 201 1 003 Version 1 2 MODS was designed to deliver high throughput over a wavelength range of 3200 to 100004 moderate spectral resolution R A AA 10 10 with a 0 6 wide slit and imaging performance over a 4x4 field without serious compromise of the LBT del
42. arget and guide star from one of the many preset catalogs of objects by pressing the Catalog button This launches the catalog browser This is most useful when using MODS during technical nights When MODS acquisition scripts upload targets or a script executes an offset the parameters will appear in this window This window does not track the current telescope parameters Instrument Configuration The next subpanel outlined by the green box provides controls for selecting the slit mask and setting the configuration of the two channels of the instrument e g dual grating mode Slit mask selection occurs promptly the current mask if any is extracted from the focal plane into the storage cassette the storage cassette translates to the position of the requested mask then that mask is extracted from the cassette and deployed in the focal plane Mask insert retract takes about 10 seconds whereas cassette motion can take up to 15 seconds for the longest move between positions 1 and 24 Because configuring the instrument channels requires a lot of motions it can take 20 40 seconds depending on the starting point When you make a dichroic and channel selection the Commit button will turn amber You must click on the Commit button to actually send the configuration command The Clear button lets you clear the selection Instrument Channel Control Panels The next two subpanels left and right are the cont
43. ask Alignment Tool modsAlign is a standalone Python program that will interactively lead you through the process of aligning targets with the slit mask It launches a named ds9 mage display and uses the PyRAF interface to IRAF to compute the offset and or instrument rotational offset needed to align the mask with the targets and executes the offset 58 MODS Instrument Manual OSU MODS 201 1 003 Version 1 2 modsAlign works with pairs of images An image of the target field through the slit mask thru mask image An image of the target field with the slit mask retracted field image There are two alignment modes long slit and multi slit For long slit mask alignment modsAlign gets the identity of the facility slit mask from the image FITS header and uses that information to setup the program For multi slit mask alignment you give modsAlign the name of the MMS file used to create the mask This tells it where the alignment star holes the 4x4 holes centered on field stars are located in the mask All multi object masks must have a minimum 2 alignment star holes for modsAlign to work but 3 4 usually give better results modsAlign guides the user through the steps needed to align your target with the mask computes the target offset and then sends the offset to the telescope modsAlign can also work with a single thru slit image if the slit is wide enough to have the star image fully within the aperture This single image
44. ation Left Image with central pixels just above full well showing charge bleeding along columns Right Severely saturated star image near a quadrant boundary showing charge induced horizontal banding artifacts If 1mages are very strongly saturated far above the full well threshold charge induced readout artifacts can dominate and ruin an image An extreme example is shown in Figure 17 which shows the result of taking a 300s SDSS z filter images with an R 8 4mag star in the upper left quadrant and on the quadrant boundary 25 MODS Instrument Manual Figure 17 Extremely saturated star image showing severe bleeding and inter quadrant readout artifacts Left The star located in 1 quadrant Right The same star on the quadrant boundary The general rule is that one should avoid saturating bright stars calibration lamp lines and bright emission line or continuum objects In general the effects will result in damaged images that cannot be recovered The effects do not generally persist into subsequent images in general After very severe saturation like in Figure 17 it is a good idea to take a couple of highly binned e g 8x8 bias images to make sure any residual charge from such punishment of the CCD is cleared out 2 6 5 Cross Talk There is no measurable cross talk between the 4 quadrants of the CCDs Representative cross talk test images for the blue and red CCDs are shown in Figure 18 Figure 18 Cross Talk T
45. be used without a filter in the beam without substantial refocusing 3 The GG495 order blocker filter in the red channel is also often used with the dichroic to provide for additional rejection of 2 order and blue light leaks through the dichroic Imaging filters are 86x86mm square format and may be up to 8mm thick Spectroscopic filters are 128x86mm rectangular format up to 8mm thick The imaging filters were made as a set with the same optical thickness and require essentially no refocusing when changed There are currently no plans to support the installation of custom user filters during the initial phases of MODSI and MODS2 deployment Custom filters especially medium and narrow band interference filters must be designed to take into account use in a relatively fast f 3 0 beam and the thickness restrictions imposed by the filter wheel The OSU instrument team will consider collaborations to design new filters especially for MODS 14 MODS Instrument Manual OSU MODS 201 1 003 Version 1 2 MODS1 SDSS uFilter MODS1 SDSS g Filter m 100 Filter Filter 90 Direct Mode Direct Mode BO Dichroic Mode Dichroic Mode _ r 2R F Z 40 60 e d e p 3 2 30 5 i 40 20 30 20 10 10 o 0 300 350 400 375 425 475 525 575 Wavelength nm Wavelength nm MODS1 SDSS r Filter MODS1 SDSS i Filter 100 100 80 90 Filter 80 Direct Mode 80 _ 70 Dichroic Mode 70 amp g0
46. bruary 2010 Shown left to right are Front Row Paul Byard optical designer Tom O Brien lead mechanical engineer Mark Derwent mechanical engineer Ross Zhelem optical engineer Ray Gonzalez software engineer Back Row Pat Osmer original project PI and project astronomer Brad Peterson Astronomy Dept Chair Ed Teiga electronics technician Dave Steinbrecher senior instrument maker Chris Colarosa student engineering assistant Josh Rosenbeck student engineering assistant Dave Brewer senior instrument maker Bruce Atwood detector scientist Paul Martini project astronomer Jerry Mason software engineer Way Back Left Rick Pogge project scientist and project PI Not Present Dan Pappalardo electronics engineer Past Team Members Darren DePoy project manager astronomer and interim PI Philip Covington electronics engineer S Ralph Belville design engineer retired Brandyn Ward student electronics assistant Andy Krygier student engineering assistant Justin Randles students engineering assistant Jennifer Marshall graduate student Jason Eastman graduate student 80
47. care of the fine details of operating MODS and choreograph interactions with the LBT control systems Properly applied observing scripts will save observers time that might otherwise be lost to errors resulting from trying to remember all of the instrument and observing setup steps late at night or after a long hiatus from observing with the LBT There are five types of MODS scripts distinguished by their filename extensions 1 Target Acquisition acq scripts that point the LBT to a new spectroscopic target setup the AGw for guiding and active optics and take through slit and field acquisition images for alignment of the target with the slit mask 2 Observing obs scripts that acquire science images of a spectroscopic target once it has been aligned with the slit mask 3 Imaging img scripts a hybrid of acq and obs scripts for direct imaging observations that do not require alignment with a slit mask 4 Calibration cal scripts that acquire bias flat field and wavelength calibration data Instrument Procedure pro scripts that perform instrument setup shutdown and housekeeping tasks Scripts are executed using special script engines programs run in Linux terminal shells that read and process the script files and then execute the script commands in a prescribed sequence There are two script engines for MODS acqMODS executes target acquisition acq scripts execMODS executes observing obs and img
48. channel is unfiltered The filter parameters are listed in Table 5 with transmission curves shown in Figure 8 Table 5 AGw Guide Camera Filter Parameters p On Clear Clear Fused Silica Fused Silica 6210 3670 5790 Default acquisition amp Default acquisition amp guiding F525LP Red long pass 6930 3210 6660 Moon suppression amp red guiding Bessel B Filter 4310 4290 Deep blue guiding NDI 0 Neutral Density 1 0 6210 3670 5790 Bright target acquisition Notes 1 The AGw Filter ID is used to select the filter with the AGWFILT command and appears as the AGWFNAME keyword in image FITS headers along with AGWFINFO giving additional filter information The filter parameters are derived from the laboratory measured transmission curves multiplied by the measured AGw guide camera CCD quantum efficiency curves The pivot wavelength Ap 1s the effective wavelength of guiding for purposes of estimating the effects of differential atmospheric refraction see Appendix B for a definition of Ap The Clear filter 1s the default guiding filter recommended for all dual channel and routine observations The effective guide wavelength 1s good for most applications For unusually blue targets in the blue only modes red channel idle the B Bessel filter may be used but you take a 1 to 1 5 magnitude penalty for guiding at B The F525LP filter can be used to guide in cases of unusually red sources where it i
49. d to the Germany amp Italy archives AE E Within a few minutes multiple copies of each image will be distributed across a network of RAID6 data arrays in the observatory archive machines The images will stay on the newdata disk until noon the following day when they are deleted to make room for the next night s data Images copied to the Repository disk will be available read only for a month or two stored in subdirectories organized by date For example data from UTC 2011 Dec 24 will be stored in Repository 20111224 Guider and WFS images taken during that same night will be stored in a GCS subdirectory of this same folder The Repository disk 1s kept organized by the archive software older data are automatically deleted to make room for the newest data Both newdata and Repository are available read only to observers logged into the observing workstations at the LBT The PARTNER PROPID and P1 NAME FITS header keywords are used to assign ownership of the data primarily by the PARTNER keyword The PARTNER 1s defined by the observatory to be one or more of these reserved values LBTO LBT Observatory Staff LBTB LBTB Germany Partner Observing INAF INAF Italy Partner Observing OSURC Ohio State SU and Research Corporation Partner Observing A Arizona Partner Observing for regular science operations Additional PARTNER IDs e g COMMISSIONING and CALIBRATION are used for technical observing and special app
50. e filter wheel 1s located between the field flattener and the camera primary mirror FIELD FLATTENER DETECTOR r Aspheric Corrector Lens Spherical Mirror 500 cm Figure 3 Left Cross sectional view of a MODS camera Right MODS1 red corrector lens The MODS science detectors are e2v CCD231 68 8Kx3K monolithic CCDs with 151m pixels The blue CCDs are thin 16um backside illuminated standard silicon devices with a broadband AR coating providing excellent blue response The red CCDs are 40um thick deep depletion silicon CCDs with a proprietary extended red AR coating providing greatly improved sensitivity beyond 8000A and much reduced fringing compared to thinned CCDs MODS does not have an Atmospheric Dispersion Corrector as this was prohibitively expensive to build for the full 6x6 arcmin FoV We note that most other large telescope optical spectrographs GMOS DEIMOS ESI and VMOS also do not have atmospheric dispersion correctors This means that observers wishing to work at the furthest blue wavelengths of MODS need to pay attention to effects of differential atmospheric refraction see 3 4 MODS Instrument Manual Each MODS has its own internal calibration system with an integrating sphere and pupil projector with a selection of Pen Ray wavelength calibration lamps and continuum lamps for spectral and imaging flat fields MODS has its own integrated Acquisition Guide and Wavefront Sensor AGw Unit that
51. e corners of the field when the instrument 1s used for direct imaging When the seeing is very good MODS can deliver this image quality on its science cameras especially in the central 2 An example of this is shown in Figure 34 where we present images taken under conditions of exceptionally good 0 36 seeing along with the radial profile plot of one of the unsaturated stars in the image Note also the complex diffraction spikes that become visible around the very bright saturated star during good seeing In less good seeing this pattern is smeared out it can also be smeared out by rotation of the swing arm relative to the detector on long exposures m Lick Mongo on mods1data J B i x Nm Stone Q Transform Check 6 Radius Arcsec Figure 34 Example of image quality during excellent 0 36 seeing Images of bright stars will show diffraction spikes from the secondary mirror swing arm supports that give these images the appearance of having a pair of long ears an artifact 38 MODS Instrument Manual OSU MODS 201 1 003 Version 1 2 nicknamed Evil Space Bunnies during MODS1 instrument commissioning An example is shown in Figure 35 Figure 35 Image of bright stars showing the characteristic secondary mirror swing arm diffraction spike pattern known as the Evil Space Bunnies The position angle of the bunny ears depends on the rotator angle as the position of the secondary mirror swing arm is
52. erience with Prism mode observations we will update the manual and webpages with specific recommendations for improving prism mode wavelength calibration 5 6 Spectrophotometric Standard Stars To derive response curves for grating and prism spectroscopy we recommend using common flux standard stars Because MODS works out to 10 000A we have adopted the Massey amp Gronwall 1990 red extension of the KPNO standards as our recommended spectrophotometric reference stars augmented by stars from the HST white dwarf primary 65 MODS Instrument Manual standards and CALSPEC database that have good spectrophotometry in the near IR Table 13lists the recommended stars to date Table 13 Recommended MODS Spectrophotometric Standard Stars Des Sp Type scs pA Feige66 12 37 23 5 425 03 59 9 Feige67 12 41 51 8 H17 31 19 8 HZ44 132335 3 36 07 59 5 BD 33 2642 15 51 59 9 32 56 54 8 Wolf 1346 20 34 21 9 25 03 49 7 12000 mag masiy Notes l RA amp Dec are FK5 coordinates Equinox J2000 Epoch 2000 from Simbad 2 pmRA amp pmDec are proper motions in mas yr FK5 Epoch 2000 from Simbad 3 BD 28 4211 has a faint red companion 2 8 arcsec away and is only recommended for use in the blue channel in very good seeing 4 GI9I B2B is the northern most of the two bright stars in the field These are all hot stars a mix of white dwarfs subdwarf O stars and a few bright B stars Finding charts template acquisition
53. erture as in the blue channel ghost image Figure 37 but with enhanced contrast to show the much fainter red channel dichroic ghost The dichroic ghost image is located immediately above the parent image of an object by 145 pixels on the detector as expected from the thickness of the dichroic and the 35 angle of incidence as shown in the schematic ray trace of the dichroic ghost in the left hand panel of Figure 39 40 MODS Instrument Manual OSU MODS 201 1 003 Version 1 2 Figure 39 Ray tracings showing the origins of the dichroic ghosts left and the field lens ghosts right Field lens ghost images are located along a line connecting the center of science field and the bright parent object A schematic ray tracing showing the origin of the radial field lens ghosts is shown in Figure 39 right panel In general ghost images are only visible for the very brightest stars because they are of order 10 4 to 10 5 of the brightness of the parent image They are usually seen faintly in multi object wavelength calibration frames for very saturated comparison lines Most people will rarely if ever see a detectable ghost image in MODS data 4 MODS Instrument Manual 3 Observing in the Near UV to Near IR MODS operates from 3200A to 1 0um but for most practical observations the UV cutoff is around 34004 unless observing very bright blue sources e g most of the white dwarf and subdwarf O star spectrophotometric standards are
54. ess graphically Below the exposure progress bars is a status 51 MODS Instrument Manual window that will display messages as exposure configuration or execution proceeds Finally below the status window are controls and status indicators for that channel s IMCS Exposure Control The GO button starts exposures of the requested exposure time and number of exposures During an integration the GO button will become an Abort button and the Stop and Pause buttons will become active Hs shutter Closed Image GO Exposure euet Readout During integrations you can take one of three actions using the labeled buttons 1 Abort Abort the integration close the shutter if open and discard the image 2 Stop Stop the integration close the shutter if open and then read out and save the image The actual open shutter time 1s recorded in the FITS header EXPTIME keyword 3 Pause Pause the integration closing the shutter if open The Pause button will be relabeled Resume and turn amber and then wait indefinitely for one of three actions to occur a The observer clicks the Resume button to resume the integration On resuming an exposure it turns back into a gray Pause button b Theobserver clicks the Stop button This ends the exposure reads out the CCD and writes the data to disk c The observer clicks the Abort button ending the exposure and discarding the data For exposures that have been paused for some time
55. est images for the blue left and red right CCDs The red crosses mark the positions where cross talk would appear if there were cross talk 24 MODS Instrument Manual OSU MODS 201 1 003 Version 1 2 While there are no excess counts there is a small deficit of counts at a mean level of 1 ADU in the regions of most significant charge bleeding The level of these artifacts 1s at or below the detector noise when measured formally but because the pattern is coherent over many 10s of pixels it 1s discernible to the eye 2 7 Slit Masks Each MODS has a 24 position slit mask storage cassette that can be loaded by instrument support personnel during the afternoon The first 12 mask slots are reserved for the 9 fixed facility long slit masks and up to 3 test masks The bottom 12 mask slots are reserved for user designed masks There are three types of slit masks Permanent Facility Slit Masks These include segmented long slits imaging stops and calibration masks that are always available in each MODS instrument They are stored in the first 9 slots in the mask storage cassette and may not be removed except under unusual circumstances User Designed Slit Masks Up to 12 custom user designed slit masks may be used per night Observers create these masks using the MMS program and submit them a couple of weeks in advance of the observing run to LBTO for cutting and transport to the telescope Engineering Masks These are masks used during inst
56. f00 B00 800 1000 300 400 500 amp Qu 700 BOQ gag 1000 Wavelength nm Wavelength nm Figure 14 Measured QE of the MODS1 Blue left and Red right CCDs 2 6 4 Detector Saturation There are two relevant saturation thresholds for the MODS CCDs ADC Saturation when the 16 bit ADC converts run out of bits at 65535 ADU Full Well Saturation when the pixels fill with electrons at about 200 000e 80 000A DU and charge begins to spill into surrounding pixels At ADC saturation saturated objects will have flat topped radial profiles that flat line at 65 535 A DU in raw images An example of a star with the central few pixels above ADC saturation is shown in Figure 15 Note that the bunny ears artifact in brightest stars is due to diffraction from the secondary mirror swing arm support see 2 12 3 27 MODS Instrument Manual OSU MODS 201 1 003 Version 1 2 Figure 15 Star image showing ADC saturation Left image Right radial intensity profile Above the full well saturation threshold the central pixels of will begin to bleed vertically along columns as charge spills out of the affected pixels With very saturated images well above the full well threshold the excess charge begins to cause problems with the readout amplifiers resulting in strong vertical banding and data values of 0 0 in the image pixels Both types of full well saturation artifacts are shown in Figure 16 Figure 16 Images of star above full well satur
57. fixed with respect to the telescope structure In the corners of the full 6x6 field of view however the images degrade due to growing astigmatism from the off axis collimator mirrors Examples of images from the upper right and lower left corners taken during 0 9 seeing are shown in Figure 36 i to i Figure 36 Images from the upper right left panel and lower left right panel of the MODS full field of view showing image quality degradation due to growing astigmatism far off axis 39 MODS Instrument Manual 2 12 4 Ghost Images The MODS optical system two sources of ghost images from the field lens located behind the slit and from the dichroic All ghost images have an intensity of a few x10 to 4x107 Field lens ghost image are present in all modes while dichroic ghosts only appear when MODS is in dual beam mode Dichroic ghosts are fainter in the red channel than in blue Dichroic Ghost Field Lens Ghost Baseline 1s Image 300s Image Figure 37 Example ghost images for the Blue Channel Left parent image in a 1s exposure with 37 thousand ADU in the peak pixel Right 300s saturation image of the same ghost mask pinhole at the same scale as left showing the field lens and dichroic ghosts Total counts in the peak pixel of the saturated pinhole image are 140 million ADU Dichroic Ghost Field Lens Ghost Baseline 1s Image 300s Image Figure 38 Red Channel ghost images This figure uses the same ap
58. for the Multi Object Double Spectrograph Marshall J L O Brien Thomas P Atwood Bruce Byard Paul L DePoy D L Derwent Mark Eastman Jason D Gonzalez Raymond Pappalardo Daniel P Pogge Richard W 2006 SPIE 6269 51 2006S PIE 6269E 51M Optical Refractive Index of Air Dependence on Pressure Temperature and Composition Owens J C 1967 Ap Opt 6 51 1967ApOpt 6 510 The Importance of Atmospheric Differential Refraction in Spectrophotometry Filippenko A V 1982 PASP 94 715 1982PASP 94 715F 11 Astrophysical Quantities T Edition Cox A N Ed 1999 AIP Press Faint spectrophotometric standard stars Oke J B 1990 AJ 99 1621 1990AJ 99 16210 The Kitt Peak spectrophotometric standards Extension to 1 micron Massey P amp Gronwall C 1990 ApJ 358 344 1990A pJ 358 344M CCD photometry of Abell clusters I Magnitudes and redshifts for 84 brightest cluster galaxies Schneider D P Gunn J E amp Hoessel J G 1983 ApJ 264 337 1983 ApJ 264 3378 Synthetic Photometry and the Calibration of the Hubble Space Telescope Koornneef J Bohlin R Buser R Horne K amp Turnshek D 1986 Highlights of Astronomy 7 833 1986H1A 7 833K 79 MODS Instrument Manual The MODS Instrument Team j The MODS instrument team with MODSI in the high bay instrument assembly lab on the Ohio State University main campus in Columbus Ohio Fe
59. fter a MODS FITS image MODS data server s staging disk has been processed for archiving it 1s then copied across the network to the LBTO new data archive staging disk named appropriately enough newdata The transfer from the MODS data server to the LBTO newdata disk usually takes around 1 second for unbinned 8Kx3K images Data written to newdata are read only but may be copied onto the observing workstation disks for further analysis 56 MODS Instrument Manual OSU MODS 201 1 003 Version 1 2 The newdata disk 1s where new MODS images first become available to the observers on the observing workstations This disk is where modsDisp 4 4 will watch for new raw images to display and where modsAlign 4 5 will get thru slit and field images for mask alignment Step 3 newdata to LBTO archive Repository and beyond Each image that appears on newdata will be immediately ingested by the LBTO data archive software triggering a sequence of events that typically take no more than a couple minutes to complete These steps include 1n approximately this order 1 Image FITS header keywords are read and the archive database is updated The image is copied to Repository UTDate for access on subsequent days The image 1s gzip compressed and filed on the archive disk no user access The image copied to the Tucson archive machine The Tucson archive repeats steps 1 through 4 above 6 The Tucson archive sends copies as neede
60. g system s data staging disks See 4 3 for more details A sample FITS header is given in Appendix A 2 12 1 Image Format The MODS CCDs are rectangular 8192 pixels wide by 3088 pixels high The number of pixels readout depends on the instrument mode 8Kx3K full frame for grating spectra 4Kx3K for prism spectra 3Kx3K actually 2900x2900 for imaging mode and MOS target acquisition and 1Kx1K for long slit target acquisition A graphical summary is in Figure 27 Grating 8Kx3K Figure 27 MODS Primary CCD readout modes Direct images are oriented North Up East Left when the instrument is rotated to PA 0 The imaging cameras are slightly rotated with respect to the science field 0 6 and 0 05 for the red and blue cameras respectively A special imaging mask acts as a field stop Examples of raw MODS direct images are shown in Figure 28 LJ 9 Y Figure 28 Raw Blue left and Red right MODS direct images In the spectral modes dispersion is mapped onto the long x axis of the detector Blue channel spectra run blue to red from left to right Figure 29 whereas red channel spectra run red to blue from left to right Figure 30 35 MODS Instrument Manual PT 1 Wu H iA wi PUE Figure 30 MODSI Red long slit grating spectrum red only mode Image is bias corrected only Note the strong OH Meinel bands that dominate the night sky emission spectrum Raw prism spectra are oriented the s
61. g system The interactive commands for MODS are described in the MODS Observing Scripts manual Any command that can appear in a script can be typed into the Command box for execution This 1s also true of a wide range of low level engineering commands documented elsewhere Other Controls Refresh ABORT View Log The Refresh button at the lower left of this control panel will refresh the dashboard querying the instrument control system for updates A full refresh takes about 10 12 seconds The View Log button will open a runtime communications log window showing all recent data taking system traffic from this dashboard This is generally only useful for engineering work The Abort button is a general panic button It will stop exposures in both channels and wait for further instructions We are reserving this space for future functions especially for binocular MODS operation 4 1 3 Housekeeping Screen The housekeeping screen shows MODS engineering and housekeeping status e g power state temperature pressures etc in the system It 1s primarily useful when looking at the initial state of the instrument and 1s generally only used by the support astronomer or telescope operator It also has a communication log monitor that shows all data taking system traffic passing through the GUI The text in this screen is color coded black is outgoing commands green is command complete messages blue is comm
62. ghly 10 of the width of the instrument s 0 6 arcsecond wide design reference slit 2 9 1 Guide Star Patrol Field A schematic of the MODS AGw patrol field is shown in Figure 22 This figure also shows the XY coordinate axes in the DET XY rotator invariant common focal plane coordinates system Offsets made in DETXY coordinates are used to move targets in acquisition images into the slit and offsets along the Y axis of DETXY coordinates will dither the target along the slit MODS Science Field Patrol Field Figure 22 The MODS AGw WFS patrol field blue plotted on top of the 6x6 arcmin science field The small white square is the guide probe FoV green is the WES pickoff FoV and red is the shadow of the guide probe The 1lbt View program is available to help select guide stars see the MODS website for information on 1btView Because the guide probe is located above the f 15 focal plane its shadow is larger than the size of the guide probe itself The detailed shadow is fairly complex as shown in Figure 23 30 MODS Instrument Manual OSU MODS 201 1 003 Version 1 2 id ite at ee n ki s Figure 23 MODS AGw Guide Probe shadow depicted using a sum of two images with and without the probe in the field The red lines delimit the guider and WFS fields Figure 22 with the large red box outlining a conservative zone of avoidance for the guide probe head and a sensor cable loop at left The large red box is
63. h wavelengths in units of Angstroms The line lists are in 2 column ASCII text format the default format for IRAF and only those lines positively identified in MODS comparison lamp spectra The line wavelengths in the calibration tables are taken from the NIST Handbook of Basic Atomic Spectroscopic Data A set of representative line plots is given in Appendix C 5 5 3 Prism Mode Wavelength Calibration For prism mode the lower resolutions R 100 500 with subsequent greater line blending makes wavelength calibration very challenging We recommend using the 0 3 arcsec slit for long slit prism comparison lamps and single rather than multiple lamp comparison spectra to minimize line blending A second effect is that with the larger spectral pixels from the prisms and the small but non trivial leakage through the dichroic beyond the nominal dichroic cross over there are significant out of band artifacts in the comparison lamp spectra primarily from very bright saturated emission lines that are slightly out of focus because they are from so far out of range This can be seen in the spectra of the Krypton lamp taken with a 0 3 arcsec pinhole slit during lab testing in Figure 48 The out of band red emission lines appear at the far right I I T I T I I I I T T I Krypton 3500 4000 4500 BOO 5200 6000 Wavelength Angstroms Figure 48 Prism spectra of the Krypton comparison lamp and extracted spectral scan AS we gain more exp
64. hat time for slit illumination correction spectra or twilight imaging flats We therefore do not recommend taking slit less flats of the twilight sky in any mode standard scripts for taking grating pixel flats are provided at the observatory and on the MODS webpage The basic procedure is to configure the instrument for grating spectroscopy dual mode or either red or blue only mode as required but instead of using one of the slit masks insert the imaging mask into the focal plane The internal calibration lamps are used for illumination Five 5 flats are taken in each mode with the goal of getting slit less lamp spectra with typical peak signal levels of 30 35 000 ADU to stay well within the linear regime Bias subtracted frames are stacked to remove any cosmic rays then the color term 1s divided out to leave just the pixel to pixel variations The resulting image has a global mean value of 1 0 but each quadrant will have slightly different levels reflecting the different conversion gains for each quadrant and the even odd pixel differences within each quadrant As with imaging flats pixel flats should be taken with the telescope stationary and pointed at the Zenith Dome lights need to be turned OFF to avoid contaminating the spectra with light leaking into the instrument the enclosure lights are bright metal halide lamps We suggest taking pixel flats at least nightly or every other night Pixel flats are used for all target spect
65. he field center separated by 3 arcsec wide struts The struts are required to maintain the structural integrity of the spherical mask shells Figure 19 shows drawings of the LS5x60x1 0 and LS60x5 slit masks as examples of the layout of the segmented long slits and the spectrophotometric fat slit Figure 19 Examples of facility long slit masks Left LS5x60x1 0 note the struts Right LS60x5 2 7 2 Custom Masks The MODS Mask Simulator MMS program is provided to design custom slit masks It is a modified version of the LMS program for LUCI and works much the same way as LMS The MMS webpage describes the important differences with LMS Instructions for submitting masks for manufacture are given in the MMS manual available on the MODS webpage A major important difference is that MODS masks require at least two alignment star holes for aligning the mask in XY offset and rotation These are 4x4 square apertures centered on stars with good astrometry We use these stars with the modsAlign program 4 5 Slit masks are laser machined in spherical mask blanks made of 150um thick electroformed NiColoy a proprietary electro deposited Nickel Cobalt alloy produced by the mask blank vendor NiCoForm Inc of Rochester NY Machining is done at URIC at the University of 26 MODS Instrument Manual OSU MODS 201 1 003 Version 1 2 Arizona The cut masks are then mounted in special handling cells and transported to the mountain where they
66. hrough two output chains even and odd each of which have slightly different gain and DC bias levels This makes the four quadrants distinct in raw images and on close examination there is visible even odd vertical striping due to the two channel readout per quadrant See 2 11 for more details on MODS data and 5 for the recommended calibration procedures to correct for them 2 6 2 Exposure Overheads Typical CCD exposure overhead times for unbinned are summarized in Table 9 The overheads listed in Table 9 include all overheads associated with image acquisition pre exposure erase cycles post exposure readout time disk write time and instrument telescope telemetry queries prior to start of the exposure Overheads should be treated as guidance only for observing planning variations at the 2 5 second level are typical mostly due to unpredictable communication latencies in the system 21 MODS Instrument Manual Table 9 Typical CCD Exposure Overhead Times SKx3K Grating Spectroscopy Full Frame 4Kx3K Prism Spectroscopy 3Kx3K Direct Imaging IKx1K Long Slit acquisition imaging 2 6 3 Quantum Efficiency The measured quantum efficiencies of the blue and red CCDs for MODSI are shown in Figure 14 These are the laboratory measurements provided by e2v MODSI Blue CCD e2v CCD231 68 5 N06272 23 1 MODSI Red CCD e2v CCD231 68 S N06271 9 1 100 l Jf Do c QE 3 QE je Tong pcdes tai eta 300 400 500 600
67. igure 41 with the main emission lines labeled 1 2n T B OQ _ 6x10 o D wo o amp E ES E E Y ala 54x10 D e so e I SE I v O O 10O S KEEN o Sh eee cuc oy t he E 3d 1 o Ou mr OF 10 O N oo di Ie XECIS E amp E GE GEO E T T T O O OO O O O O O QO Q E ra O I B C zi Nal xi 0 p ee deu e e pe Deeds c rede Te o jp p pp e de E 300 400 500 600 700 800 900 1000 Wavelength nm Figure 41 Model new moon night sky emission spectrum for Mt Graham at airmass 1 2 The night sky emission spectrum is dominated by the OH rotational vibrational Meinel bands for A gt 7000A At bluer wavelengths night sky auroral lines principally O 1 and N 1 and upper atmosphere Nai D emission are important augmented by emission lines and continuum from street lamps in surrounding population centers particularly Nal D and Hg 44358 emission lines Night sky lines vary on 5 15 minute timescales throughout the night For very faint targets at red wavelengths e g trying to observe faint high redshift objects long ward of 7000 where the OH lines are important you may wish to dither along the slit Depending on the sizes of your objects typical dither steps of 10 20 along the Y axis along the slit can be used with the facility segmented long slit masks Overheads associated with dithering are typically 5 10 seconds dominated by waiting for the guide probe to move and the GCS to re lock on the guide
68. ivered imaging performance lt 0 6 in the original specification At the behest of the LBT appointed Optical Spectrograph Working Group the available field of MODS was increased to a 6x6 extended field with a penalty of reduced image quality outside the inner 4x4 The primary contribution to reduced image quality outside a 5 6 arcminute diameter circle is astigmatism from the off axis paraboloid collimator mirrors plus any uncorrected field aberrations in the LBT f 15 direct Gregorian focal plane proper In practical terms the MODS sweet spot for imaging is the same FoV as LUCI The baseline configuration of MODS has large 420x320mm reflection gratings for Rx2000 spectroscopy and smaller 240x220mm double pass prisms with immersed reflection coatings for low R 100 500 spectroscopy A set of imaging flats rounds out the complement of dispersing optics A fourth unassigned disperser cell in each channel is reserved for a future large grating e g for higher dispersion The basic design of MODS should permit operation up to R 8000 in a 0 6 slit if such a grating can be manufactured Decentered fast f 3 Maksutov Schmidt cameras with spherical primary mirrors and aspheric corrector lenses are used to reform an image on the CCDs Figure 3 These help provide the high throughput of MODS because there are no obscurations anywhere in the MODS beam below the slit plane A field flattener lens doubles as the CCD dewar vacuum window Th
69. lative Lamp Brightness 0 2 0 0 2 3 4 5 6 7 8 9 10 VFlat Intensity Figure 21 VFlat calibration lamp output curve Lamp brightness is scaled to VFlat 10 The red line is the curve for the SDSS 1 band the green line for the SDSS g band To view the calibration system the AGw guide stage has to be retracted to its home position the instrument dark hatch closed and the calibration optics inserted into the beam a process that requires about 30 seconds up to 40 seconds if the guide probe is near the science field center Special CALMODE and OBSMODE commands are provided to configure the instrument for taking internal calibrations CALMODE or observing the sky OBSMODE taking care of the fine configuration details for you Good combinations of exposure times filters and compatible lamp combinations have been found for flat field and wavelength calibration for all modes imaging and spectroscopic See 5 for the recommended calibration procedures Copies of the standard calibration template scripts are available on the MODS website as well as in public folders on the LBT control room observing workstations 2 9 Acquisition Guiding amp Wavefront Sensing AGw Unit The front of MODS extends into the back of the LBT primary mirror cell and cannot use the facility direct Gregorian AGw unit Instead it has its own integrated AGw unit that uses the same guider and off axis WFS cameras as the AIP units used by LUCI The main differences
70. lications The PROPID and PI NAME are used differently by different partners and are user definable or at least defined within a partner group for example the OSURC partner block has internal conventions for how to assign PROPID and P1 NAME values for its observing queue 57 MODS Instrument Manual 4 4 modsDisp Raw Data Display The modsDisp program will display the latest raw MODS images on dedicated ds9 display windows It is run by typing modsDisp in an xterminal window While multiple instances of modsDisp may be run on the network only one instance per workstation is allowed The modsDisp program will open one dedicated ds9 window for each active MODS channel As each new image is written to newdata modsDisp will open and display it in the appropriate ds9 window and a print a brief summary of header information in the xterminal window modsDisp will typically catch a new image on newdata within 1 3 seconds The usual practice is to start modsDisp on the same workstation as you are running the MODS Control Panel GUI I usually put the MODS GUI on the right hand monitor with an xterm for running the scripting engines and run modsDisp on the left monitor Other instances of modsDisp are run on the second astronomer and support astronomer s workstations respectively This lets all users watch the progress of MODS data taking and is a convenient way to have the latest raw filenames available for cut and paste into other
71. lowing flexible resumption of observing once the problem has been corrected Imaging 1mg scripts are a hybrid of acq and obs scripts executing target acquisition and science data taking in a single script but with a pause between acquisition and first science images to allow the WES collimation correction loop to converge See the MODS Observing Scripts manual for details of how to create and execute scripts handle errors and access the full scripting command set 55 MODS Instrument Manual 4 3 Where do the MODS data go Step 1 CCD Controller to MODS Data Server A MODS CCD image is first read off the CCD into memory on the CCD control computer e g MI RC for the MODSI Red Channel M1 BC for the MODSI Blue Channel From memory it is written onto a transfer disk shared between the DOS based CCD control computer and the MODS data server machine a Linux workstation named mods1data for MODSI and mods2data for MODS2 Once on the transfer disk a dedicated instance of the Caliban data transfer daemon running on the data server copies it from the transfer disk and writes it onto the 1home data staging disk in FITS format Data from the red and blue channels are written to the same data server staging disk Once written its FITS header 1s checked and augmented with additional archive and engineering keywords its header is scanned and the running data log in Logs 1s updated and the image 1s ready to be copied to the LBTO data a
72. me CLOSED Instrument Dark Hatch IN j Calibration Tower Position KR Calibration Lamps 2 5 Variable Intensity Flat Lamp Intensity 6 Slit Mask Cassette position IN Slit Mask Position hob5so0xl 0 Slit Mask Name 1 0 arcsec Segmented Long Slit Slit Mask Description 2 7 IDuetsedo Turret Positron Dual i Dichroic Beam Selector position ID OSTRO 575pnmobrochrodc NL Dichroic Beam Selector Description This block lists the instrument configuration of the channel used Blue for the collimator mirror grating filter and camera focus values at the start of the exposure CHANNEL COLFOCUS COLTTFA COLTTFB COLTTFC GRATING GRATNAME GRATINFO GRATTILT GRCENLAM GRORDER FILTER FILTNAME FILTINFO BLUE Channel name Blue or Red 18368 Collimator Focus A B C microns 17086 7 Collimator TTF actuator A microns 17925 f Collimator TTF actuator B microns 20092 Collimator TTF actuator C microns 3 Grating Turret Position P450L Name of the Disperser at GRATING FuSi tAl Double Pass Prism Nrl Description of the Disperser LOL50 Grating Tale pmoerons t Grating nominal central wavelength Ang O0 Grating order used 1 Camera Filter Wheel Position Clear r Camera Filter Name Fused Silica Clear 1298 96x0m0m Nrl 7 Camera Filter Deseription 69 MODS Instrument Manual CAMFOCUS 3380 Camera Primary Mirror Focus microns Note that during an exposure the COL TTF
73. mirrors The sharp cutoff around 5600A in the dichroic mode curves is the dichroic cross over wavelength 12 MODS Instrument Manual OSU MODS 201 1 003 Version 1 2 2 4 The blue downturn in the red direct mode efficiency curves it the cut off in the GG495 order blocking filter 82 3 1 2 2 3 Prism Mode Spectral Throughput The total efficiency instrument and telescope of MODS in double pass prism mode is shown in Figure 5 normalized to efficiency at 1 2 airmasses elevation 60 The efficiency curves do not include strong telluric absorption features The wavelength coverage was artificially cutoff at 5000 for the red and 6000A for the blue channel but coverage does extend beyond those wavelengths if at somewhat degrading efficiency Unlike the red grading the red prism does not require an order blocking filter MODS 1 Prism Mode MODS LBT at 1 2 Airmasses Direct MM UH Dichroic Efficiency 4000 9000 6000 7000 8000 9000 10000 Wavelength Ang Figure 5 MODS1 Prism mode efficiencies Solid lines are direct mode blue or red only dashed lines are dichroic dual mode Efficiency includes instrument telescope and atmosphere at 1 2 airmasses The spectral efficiencies depicted in Figure 5 are linearized following the usual reduction procedures for flux standard stars Because the prism resolution is a strongly decreasing function of wavelength see 2 5 the size of a raw spectral pixel in wavelength is larger at
74. mizes slit losses due to differential atmospheric refraction especially at blue wavelengths is essential Observing planning tools are provided on the MODS website to help determine a good slit orientation to use to minimize slit losses Note that the best slit position angle is not just the parallactic angle at mid exposure but in long integrations it will be a combination of the refraction expected track over the integration time and the choice of slit width that determines the slit PA that minimizes slit losses Sometimes the best slit PA can have no good guide stars available within the patrol field this is sometimes a problem in sparse star fields at high galactic latitude You may have to use the 180 rotated slit position reported by the planning tools Ultimately it will require a judgment call on the part of the astronomer planning the observation and this is why we do not offer automated ways of setting the slit PA just tools to help you compute and visualize the effects Avoiding light losses due to differential refraction 1s why we recommend that you observe all spectrophotometric standard stars with the wide 5 slit LS60x5 to ensure accurate flux calibrations at the blue UV extremes of the spectrum 46 MODS Instrument Manual OSU MODS 201 1 003 Version 1 2 4 Observing with MODS Basic startup and operation of MODS is performed by LBTO support astronomers When logged into one of the observing workstations at the LBT
75. ncer clocks PIXITIME 1 2000E 6 Pixel integration time in seconds CCDXBIN Lo OODX axise Binning Factor CCDYBIN LJ CCD Y axis Binning Factor OVERSCNX 0 OVERSCNY 0 READOUT ARLBRL Amplifiers used in readout DISPAXIS 1 Spectral Dispersion Axis PIXSCALE 0 120 Unbinned pixel scale arcsec per pixel PIXSIZE 154 0 Unbin ned prxel sive microns Observatory Information This block gives the LBT standard information about the LBT MGIO observing site and the telescope focal plane ORIGIN MGIO LBT Location where the data was generated OBSERVAT MGIO LBT Observatory Site TELESCOP LBT SX 7 Telescope and Focus FOCSCALE 1 67 Focal Plane Scale arcsec per mm LATITUDE 22 10191 y Site bhatrtude Taeg N LONGITUD 109 88906 Site Longitude deg W ELEVATIO 3221 0 Site Elevation meters Observer Partner Project Information This block lists the observer partner and project IDs used by the LBTO data archive and additional information about LBTO personnel during the observing run These data are set either by observing scripts or in the Setup screen of the MODS instrument control panel 84 1 1 OBSERVER Pogge amp Skillman Observer s 68 MODS Instrument Manual OSU MODS 201 1 003 Version 1 2 PARTNER PROP LD PI NAME SUPPORT TELOPS OSURC LBT Project Partner s OSU HIIAbund Observing Proposal ID Pogge Project PI Name s Pedani Shin Steven Allan
76. ngstroms Figure 51 Argon lamp spectrum taken with the Blue Grating G400L 76 Version 1 2 OSU MODS 2011 003 MODS Instrument Manual Xenon Krypton Lamps MODS1 Blue Channel G400L 0916 0788 14 81968 6179S 1M gee 2988 4 V 682 0299 1M CSL ECEPTOLS 916v OX pe6z vgy 0610 708v 9x 8IS8L v v 9X i 9 egzz L 9v ox 80 0 769v 9X 9926 Yc9v OX LovGe cO0GY 4 00069 9t v IM 6S LcL97 r IW G6 G 6LET I v696 EZ cP IM 100 80 60 40 20 Nav 4500 5000 5500 6000 Wavelength Angstroms 4000 3500 Figure 52 Xenon Krypton lamp spectra taken with the Blue Grating G400L Neon Lamp MODS1 Red Channel G760L 6 2x10 99 9 0828 LESE v6598 07 v9 vE98 86S G6r8 0809 77 8 85c 00 8 t rvO vvSZ LVZZ S684 969 0cr 2178 88v7 L866 EL HZ Lely c 0Z 497 6069 0 vY0 7179 9 0 8 99 6086 8699 L8e8 9089 8 cV VEEN 086v 9929 216e L129 9290 EPLI sop 9600 6966 6609 coves tros 6 8V cS8S 08Frc cOv9 1 5x10 10 5x10 NAY 6000 7000 8000 9000 10000 5000 Wavelength Angstroms Figure 53 Neon lamp spectrum taken with the Red Grating G760L T MODS Instrument Manual Argon Lamp MODS1 Red Channel G760L 98 7896 66v vcc6 496 2cL6 tv6 7998 evi 1688 8v9 vov8 0 Lc 80v8 ceS v9c8 LLESLEI8 69 018 98 vL08 7 GL 9008 9 1 862 10 vc4Z 901 S 97 o X HE rT 699 Y 165 699 087
77. not of much interest to observers 72 MODS Instrument Manual OSU MODS 201 1 003 TTEMP201 1 614 TTEMP202 0 939 TTEMP203 0 969 TTEMP204 0 568 TTEMP205 0 731 TTEMP206 0 672 TTEMP207 0 051 TTEMP208 0 499 TTEMP209 1072 TTEMP210 dis OV TTEMP 301 0 540 TTEMP 302 0 476 TTEMP303 1 351 TTEMP304 0 734 TTEMP305 1 028 TTEMP306 0 361 TTEMP307 0 282 TTEMP308 0 548 TTEMP309 1 080 TTEMP310 0 320 LBT Weather Data Tae Se OS OS Te SR NG EE RS OS OSE TE TM ELM Channel 201 Temperature deg C Channel 202 Temperature deg C Channel 203 Temperature deg C Channel 204 Temperature deg C Channel 205 Temperature deg C Channel 206 Temperature deg C Channel 207 Temperature deg C Channel 208 Temperature deg C Channel 209 Temperature deg C Channel 210 Temperature deg C Channel 301 Temperature deg C Channel 302 Temperature deg C Channel 303 Temperature deg C Channel 304 Temperature deg C Channel 305 Temperature deg C Channel 306 Temperature deg C Channel 307 Temperature deg C Channel 308 Temperature deg C Channel 309 Temperature deg C Channel 310 Temperature deg C Version 1 2 These keywords report weather conditions at the start of the exposure as read from the LBT weather station located on the roof At present we only report ambient temperature pressure relative humidity and dewpoint temperature LBTWLINK Up LBTPRES 6925 70 LEITBEME D LBEBTHUM 20000 LBTDWPT SA OO
78. ombination of the general stability of the instrument and the image motion compensation system IMCS obviates the need for in place flats Slit flats can be used with at most a small shift in wavelength to remove fringing at the far red end of the Red Channel range for very red targets but because the fringe amplitude is at most 2 peak to trough the fringe pattern is only visible 1n data at very high Signal to Noise Ratios 5 4 1 Imaging Flats Imaging flats taken with the variable intensity flat field VFlat lamp work well for most filters except the UV e g u_sdss where one of the QTH lamps should be used to ensure sufficient counts However the internal flats show a 2 top to bottom gradient from scattered light internal to the calibration illumination system Twilight sky flats can be used as illumination correction frames to correct for this gradient and because most of the pixel to pixel flat fielding can be done with the internal lamp flats only 1 or 2 well exposed sky flats per filter are required to create the necessary illumination correction frames This eliminates the general difficulty of obtaining twilight sky flats at the LBT In general imaging flats are very stable on run timescales because the filters are way out of focus right in front of the CCD field flattener lenses on the dewars they are rarely removed or handled and so are in a protected and thus relatively clean environment Typical flats should be exposed
79. ormat The second 4 digit number is the image counter running 0001 through 9999 At present these need to be set correctly at the start of each night and getting them correct 1s the joint responsibility of the observers and LBTO support astronomers 48 MODS Instrument Manual OSU MODS 201 1 003 Version 1 2 4 1 2 MODS Dashboard The main control panel screen is the Dashboard In a full 2 MODS system there will be two dashboards one each for MODSI and MODS2 Figure 47 shows the MODSI dashboard r y MODS Control Panel Ox Session Help MODS1 Dashboard Calibration and AGw Unit Telescope Preset Left Side b Target RA Dec Catalog Mode Calibration Observing Restore AGw Probe GuideStar RA Dec Clear Calibration Lamps Hatch Open Closed 8 Ne Hg ar xe kr Rotator o deg Position gt Mode Acquire Send Preset E OFF CoordSys DETXY MoveType Relative Clear Absorb AGw X 91 000 Y 243 500 Foc 10 000 Filter ND1 0 gt Home Offset AX 0 AY 0 PA 0 deg Send Offset Slit Mask SieveMask In Out Dichroic Dual Blue Channel Imaging Red Channel Imaging Commit Blue Channel 4 Red Channel Configuration Configuration Mode Imaging Grating Prism Commit Clear Mode Imaging Grating Prism Commit Clear Filler g sdss Grating ID Flat CenLam Filler r sdss Grating ID Flat CenLam m i
80. ormation FITS Image File Names AU MODS1 3 b Observer s Pogge Stoll Skillman VT Root Name 20110810 Get Date Partner s OSURC Blue CCD modsib 20110810 fits Refresh MODS1 ProplD OSU BrightSNe 0001 A PI Name Stoll Stanek Pogge Red CCD modsir 20110810 0001 fits pply EM Support Kuhn MODS2 TelOps Gonzalez Huerta Root Name yyyymmdd Housekeeping Comment OSURC Partner Block Queue Observing Blue CCD mods2b yyyymmdd fite n Clear Reload Apply Save Red CCD modser yyyymmdd fils Huq Utilities Helresh View Log Figure 46 MODS Control Panel GUI Setup Screen The icons for navigating the GUI are in the vertical box on the left side of the GUI Clicking the mouse on one of these icons will take you to that control screen The left half of the Setup screen has entry boxes for the Observer and Project information This sets the default info written into the FITS headers for tracking observing and support personnel and project information All of the parameters in this block are saved in files owned by the individual partner observing accounts so if LBTB observers login and start the GUI they will see the last LBTB default The right half shows the filenames for the next FITS images to be written by the instrument At present these are user settable with filename patterns like modslb 20110810 0001 fits inodslr z01108Sl10 0001 frts etc The first number 20110810 1s the UTC date in standard CCY YMMDD f
81. poor results 1f used without a proper twilight sky correction Only one or at most two twilight flats are needed with signal 10K ADU to ensure a good illumination correction the high signal to noise data for good pixel to pixel correction comes from the internal lamp flats We are still in the process of developing scripts for taking twilight sky spectral and imaging flats 5 5 Wavelength Calibration Comparison Lamps We recommend taking wavelength calibration lamps through the 0 6 arcsec long slit mask for grating comparison lamp spectra and the 0 3 arcsec long slit mask for the prism comparison lamp spectra For the grating 0 6 arcsec 1s the design reference slit and you will have 80 100 reasonably bright and unblended lines to work with in the red maybe 50 in the blue which covers the range of interest For the prism the 0 3 arcsec slit 1s the smallest slit that will be imaged cleanly by the camera optics and gives the least blended spectra see below Wavelength calibrations are very stable particularly the high order terms that represent the details of the optical design including optical aberrations but you can see small shifts of order 1 2 pixels at most due to the residual global image motion in the IMCS system see 2 10 This small shift is readily measured and removed using night sky lines Only one 1 exposure per lamp or lamp combination 1s needed the exposures are very short 1 5 seconds and there are many 10
82. programs e g modsAlign IRAF etc A few important caveats to keep in mind as you use modsDisp and its related ds9 windows 1 The ds9 displays are look only RAF cannot interact with these images This is by design IRAF s does not like to share its image display and interaction pipes To avoid confusing and potentially crippling resource conflicts by asking IRAF to share if you want to run imexamine or other IRAF tasks on raw images you need to open a separate IRAF session with its own ds9 display for this purpose We recommend using a different workstation than the one running the MODS Control Panel GUI 2 modsDisp works by watching the newdata disk for new arrivals Sometimes it gets out of sync and crashes back to the Linux prompt with a spray of unhelpful Python error traceback In these cases restart nodsDisp and start again Note that you do not have to kill and restart the ds9 windows it originally launched Ctrl C typed in the modsDisp window will kill a hung program and let you start over 3 While images are displayed on the ds9 windows you can pan zoom mess with the color map etc but once a new image arrives your changes will be undone it tries to always reset to a known default configuration Except for the most cursory examination open a separate IRAF IDL whatever instance to look at the data offline modsDisp is written in Python and will likely be improved and extended over time 4 5 modsAlign Interactive M
83. ps Observing Mode Open the instrument hatch retract the calibration tower and turn off any calibration lamps that might be turned on If the Restore AGw Probe box 1s checked it moves the AGw guide probe back to the previously stored position Calibration lamps are turned on or off with the push buttons the LED icon on the left part of the button is green when the lamp is on the gray when the lamp is off and red for faults The entry box next to the VFlat button sets the intensity of the variable intensity flat field lamp 1 to 10 see Figure 21 for the VFlat lamp intensity curves The OFF button turns off all calibration lamps You can also manually open close the hatch insert retract the calibration projector change the AGw guide filter or home stow the AGw guide probe Telescope Preset Control Panel At the top right subpanel has controls for manually sending coordinates for a target and guide star to the telescope for a target preset changing the preset mode or sending an offset in celestial RADEC or slit XY plane DETXY coordinates Telescope Preset Left Side Target RA Dec Catalog GuideStar RA Dec Clear Rotator 0 deg Position Mode Acquire Send Preset Offset Pointing CoordSys DETXY MoveType Relative Clear Absorb Offset AX o AY o PA o deg Send Offset 50 MODS Instrument Manual OSU MODS 201 1 003 Version 1 2 You can also select a t
84. ra objects and standard stars Any real wavelength dependent wiggles in the CCD wavelength response e g due to the dichroic transmission will be calibrated out later using observations of spectrophotometric standard stars Taking pixel flats in the prism mode has proven problematic because the large spectral pixels lead to a wide range of spectral intensities and a risk of saturation at the low dispersion red 62 MODS Instrument Manual OSU MODS 201 1 003 Version 1 2 end the continuum lamps are all very red 1n color We do not recommend taking prism pixel flats at this time 5 4 3 Spectral Slit Flats We recommend acquiring a few 3 each lamp flats in the individual long slits in the red for use as fringe corrector frames in the far red end of the spectrum Blue slit flats by contrast do not appear to be useful but scripts with useful lamp filter and exposure time combinations are provided The MODS Red Channel CCD is a thick 40um deep depletion device so fringing 1s generally small 2 396 max peak to trough amplitude beyond 8500A compared to typical CCDs and should not constitute a major correction for most faint targets although it will become an issue when the signal to noise is in the high 10s in that part of the spectrum For MOS masks the spectral slit flats in both red and blue channels are essential for helping create high quality traces of the slits for MOS reductions By comparing the flat field spectra slit to
85. rchive The step of copying the raw 1mage from the transfer disk onto the data server s staging disk 1s between 4 and 12 seconds depending on the size of the image this 1s a known bottleneck in the system we hope to improve with future hardware upgrades we will qualify first with MODS2 Post processing of the 1mage header and logging take less than a second This final transfer and process step 1s asynchronous if a sequence of images is being acquired the next image in the sequence will be started as soon as the last file 1s written to the inter machine transfer disk and the final transfer and process step will occur while the next image starts IMPORTANT NOTE It is at the data transfer step between the CCD control computer and the MODS data server that the data transfer queue can stall The symptom is that the Last File counter will fail to increment see GUI screenshots in 4 1 2 and images will stop appearing in the modsDisp windows despite the fact that the Next File counter is incrementing If the difference between LastFile and NextFile grows larger the data transfer queue has stalled If you notice that the transfer queue has stalled type the command Ilctosrlsh in the Command window on the MODS dashboard GUI This should restart flush the FITS data transfer queue and you ll start seeing the Last Image counter increment and images appearing on the disk Step 2 Data Server to the LBTO Archive Staging Disk newdata A
86. re of all of these details for you as some of the steps can be quite involved and unforgiving if a crucial step 1s forgotten especially late at night while suffering from the effects of high altitude and sleep deprivation 4 MODS Control Panel Most MODS functions are controlled via the MODS Control Panel a graphical user interface GUI that 1s run on one of the observing workstations Only one instance of the MODS GUI may be running at a time The data taking system should stop you from launching more than one instance of the GUI but please be aware of this restriction Your LBTO support astronomer will show you how to start the GUI and its associated programs The MODS Control Panel is a multi layered GUI with different control screens that may be selected by clicking on the icons in the GUI s sidebar When launched it starts with a small splash screen as it configures itself and builds all of the control panels and then shows you the main Setup screen The following subsections describe each of the main control panel screens Generally observers will only use the Setup and Dashboard screens spending most of their time on the Dashboard 47 MODS Instrument Manual 4 1 1 Setup Screen A screenshot of a typical MODS GUI setup screen is shown in Figure 46 Yy MODS Control Panel o x Session Help E g OHIO LARGE xc DNE MODS Instrument Setup BINOCULAR a UNIVERSITY TELESCOPE mm Observer Project Inf
87. rect non dichroic mode the relatively symmetric SDSS r filter and the asymmetric SDSS u filter MODST SDSS Elin Direct Mods MODS1 SDSS u Filter Direct Mode Q T Percent Q T Percent 0 id 1 5200 5400 5600 5800 6000 6200 6400 6600 6800 7000 7200 7400 QD 0 3200 3400 3600 3800 4000 Wavelength Angstroms Wavelength Angstroms Figure 49 Filter bandpass parameters plotted for left SDSS r and right SDSS u in direct no dichroic mode The dotted vertical line near the solid line marking Ap is the average wavelength X as defined above Note that the effective width oA is generally narrower than the FWHM The effective width is used to estimate the flux in the band by multiplying the observed flux at the pivot wavelength by the effective width The effective width is equivalent to the width of a perfectly square sided bandpass centered on Ap with unit transmission 75 MODS Instrument Manual Appendix C Wavelength Calibration Lamps Mercury Lamp MODS1 Blue Channel G400L 10 LO ce eo LO e co e LO m lt t lt Ke 6 D e lt 4 2 0 rr le ee Typ T a 3000 3500 4000 4500 5000 5500 6000 Wavelength Angstroms Figure 50 Hg Ar lamp spectrum taken with the Blue Grating G400L Argon Lamp MODS1 Blue Channel G400L m P O N lt S Or e 20 O 10 LO Ss e S5 t S285 lt 4044 418 1522 323 4510 733 5495 874 5558 702 5606 733 5650 704 3500 4000 4500 5000 5500 6000 Wavelength A
88. ripts directory with a parallel set of copies available on the MODS webpage The LBT copies of the standard scripts are read only but you can copy them into your partner observing account if you need to modify them We recommend only making copies of scripts you intend to modify as we will sometimes change key parameters in response to changes in the instrument e g changes in calibration lamp brightness after replacing a burned out lamp 67 MODS Instrument Manual Appendix A MODS FITS Headers Below is a sample MODS science image FITS header divided into the various blocks of data The example is drawn from various MODS1 image headers MODS science images are standard FITS format with single header and data units Basic FITS Data FITS images are written as 16 bit integers scaled using the standard BzERO and BSCALE keywords to get all 16 bits of raw ADC data encoded SIMPLE dr 3 BITPIX 16 NAXIS 2 y NAXIS1L 8288 NAXIS2 S088 BSCALE 1 PHYSICAL INTEGER BSCALE BZERO BZERO 32768 f BUNIT ADU units of physical values LBT Detector Information This block gives the readout configuration binning overscan amplifiers etc and the physical size and image scale of unbinned pixels DETECTOR e2y OCD231 68 Blue CCD I Detector name DETSIZE 1 8288 1 3088 Unbinned size of detector full array NAMPS 4 Number of amplifiers in the detector GAINDL 3 Pixel integration time in seque
89. rol panels for the blue and red spectrograph channels outlined by the blue and red boxes respectively Blue Channel Configuration Configuration Mode Imaging I Grating I Prism Commit Mode Imaging Grating Prism Commit Clear Filter g _sdss A Grating ID Flat CenLam Filter r_sdss Grating ID Flat CenLam Focus 3220 um TTF A 17378 B 17804 C 19922 um Focus 1200 um TTF A 15880 B 14860 C 12644 um Exposure Control Exposure Control Name VFlat Lamp Name VFlat Lamp Type Object s ExpTime 8 sec Images 1 Type Object ExpTime 2 sec Images 1 Binning X 1 Y 1 24 CCD Readout 8Kx3K gt Binning X 1 Y 4 ad CCD Readout 2Kx2K NextFile mods1b 20110810 0035 LastFile mods1b 20110810 0034 NextFile mods1r 20110810 0017 LastFile mods1r 20110810 0016 Shutter Closed Image Shutter Closed Image Pause GO Exposure otop Pause GO Exposure Slop Readout Readout IMCS Idle Use IMCS Lock On IMCS Idle Y Use IMCS Lock On These have controls for setting the channel configuration disperser and filter with informational displays about the camera focus and collimator tip tilt focus actuator values and the exposure and CCD readout mode controls 1mage type exposure time number of exposures binning ROI etc To the right of the big GO button are progress bars that will show exposure and readout progr
90. rument testing and evaluation lab and telescope including focus and alignment pinhole masks and ghost image masks 2 7 1 Permanent Facility Slit Masks The first 9 slots in each MODS slit mask cassette are reserved for the permanent set of facility slit masks Table 10 summarizes the current set of facility slit masks Table 10 MODS Permanent Facility Slit Masks l eem MaskID Position Description Dark Mask Dark Mask closed blind 0 3 wide 5x60 segmented long slit 0 6 wide 5x60 segmented long slit LS5x60x08 6 0 8 wide 5x60 segmented long slit 1 wide 5x60 segmented long slit LS5x60x12 8 1 2 wide 5x60 segmented long slit LS60xs 9 60x5 spectrophotometric fat slit 25 MODS Instrument Manual Notes 1 MaskID is the name used to select the mask in MODS observing scripts e g slitmask 1s5x60x0 6 MaskIDs are case insensitive and have no spaces in them It 1s stored in the MASKNAME keyword in image FITS headers along with the MASKINFO keyword with additional descriptive 2 The dark mask blocks the view out of the instrument telescope focal plane to measure internal scattered light and to provide protection for the post slit field lens when the instrument is put to sleep The imaging field stop mask ensures proper baffling at the edges of the FoV 4 Segmented long slit masks consist of a line of five 5 60 arcsec long slits centered at 0 63 and 126 arcsec from t
91. s ccce eee e eee e eee eee e eee eene eee eee eee eeeeeeoous 74 Appendix C Wavelength Calibration Lamps eee eee e eee eee ee eee eee eese eeeooos 76 IR GIERENCES ED T T D T I 79 The MODS Instrument Teani iini v DE ehe vt nex eae 80 MODS Instrument Manual OSU MODS 201 1 003 Version 1 2 List of Tables Table 1 MODS Instrument Confieuratiofis teni ee tuvo stu a etum ecu eeu eua DeU dd 11 Table 2 masine Zro Gis ie acs cetacean date pea nee etude eo serra Hut eate erate 12 Table MODS FEIET eee ee eee Semen one E uti cU ecco Me osea dca 14 Table 4 MODS Imagine Filter Parameter S neioii ae RR ORE rence 16 Table 5 AGw Guide Camera Filter Parameters eese 16 Table 6 MODS Imaging Flats and DISDetSets io ETE ari ERE NEP EE noes 18 Table 7 MODSI Disperser Properties cere repa neroni t SEE E PU IH pR D IU RE a E redu ERRE 18 Table8 MODS Science CG D Properties esci a ed FoptU aite Enti dnd ree t perg Ir REESE 21 Table S Typical OCD Exposure Ov erhead TIMO S o oo a teal ee eos ee 22 Table 10 MODS Permanent Facility Slit Masks eeeeeeeeessssesseeseeeeenenennnn 25 Table 11 MODS Internal Calibration Lamps eessseeeeeeeernnnneennnn nennen 28 Table 12 Basic Instrument Calibration Data eeeeeseseeesssssseeseeeeeeeeeeeeennnnn nnne 60 Table 13 Recommended MODS Spectrophotometric Standard Stars
92. s script that acquires the science data Imaging observations don t require detailed alignment with a slit mask so a single hybrid imaging 1mg script is used to point the telescope and take images Consider a long slit grating spectral observation of a z 6 5 QSO You would create two scripts for this target l ji1151 acq target acquisition script thru slit and field images 2 j1151 0bs long slit spectral observation script At the telescope you would execute the observation following these steps 1 Point the telescope and take thru slit and field images after locking on the guide star acqMODS j1151 acq 2 When acqMODS pauses use the modsAlign program 4 5 to align the target with the slit using the two acquisition images in the newdata directory mocsA lon cuomedslrs0ll10929 O02 es TLES mod lr ZO bOO 7S O02 2 2k s mark the slit and target then accept and execute the computed offset 3 Resume acqMODS from its paused state and it will take a confirmatory thru slit image to make sure the target is where you want it on the slit 4 Reconfigure MODS for spectroscopy and start the science integrations execMODS jl1l1i5l obs And so on Both long slit and multi slit spectroscopy observations follow a basic 3 step sequence of actions 1 acquire the target 2 align the targets with the slit 3 start taking science data If errors occur the acquisition and observing scripting system is fully reentrant al
93. s not possible to orient along the mean parallactic angle and it can also help a little in bright moonlight 16 MODS Instrument Manual OSU MODS 2011 003 Version 1 2 MODS AGw Clear Filter MODS AGw F525LP Long Pass Filter Clear Raw F525LP Raw Clear x CCD F525LP x CCD Transmission Transmission 300 400 500 600 700 800 900 1000 1100 300 400 500 600 700 800 900 1000 1100 Wavelength nm Wavelength nm MODS AGw B_Bessel Filter MODS AGw ND1 0 Filter ND1 0 Raw ND1 0 x CCD 0 8 Transmission Transmission 0 6 0 4 0 2 0 0 300 350 400 450 550 600 Wavelength nm Wavelength nm B Bessel Raw B Bessel x CCD 500 Figure 8 MODS1 AGw Camera Filters 2 4 Dichroic The MODS dichroic is located below the slit mask and field lens and is the last element of the common focal plane optics The dichroic passes blue light and reflects red light with the 50 cross over wavelength at 5650A The dichroic efficiency curves for the blue transmission and red reflection channels are shown in Figure 9 MODS 1 Dichroic 100 Efficiency 96 Cc o 300 400 500 600 700 800 900 1000 Wavelength nm Figure 9 MODS1 Dichroic Transmission Curves The wiggles in efficiency are real and a source of additional complication for flux calibrating MODS using flux standard stars See 5 6 for the recommended flux calibration procedure 17 MODS Instrument Manual 2 5
94. s of lines so even single CR hits are not a problem you don t risk losing too many lines to cosmic ray hits Short exposures plus long readout times in full frame mode mean you could waste a lot of time taking a lot of extra comparison lamp data that doesn t give you any benefit The MODS calibration lamp system s integrating sphere and projection optics are very efficient and the standard scripts take comparison lamp spectra through the ND1 5 filter to avoid badly saturating the spectra Unlike flat fields and bias frames it 1s possible to take comparison lamp spectra during the afternoon with instrument dark and full dome lights on but the telescope must be stationary and pointed at the Zenith 5 5 1 Dispersion Solutions For the grating modes using the IRAF identify task as a prototype we recommend performing a 5 order polynomial fit to the data This gives an excellent representation of the high order terms in the wavelength calibration function whereas a 4 order fit leaves significant systematic residuals and 6 order polynomial does not measurably improve the fit 64 MODS Instrument Manual OSU MODS 201 1 003 Version 1 2 In round numbers the linear dispersion terms are 0 5A pixel in the blue grating 0 8A pixel in the red grating for unbinned pixels 5 5 2 Calibration Lamp Files On the MODS webpage we provide annotated plots of MODS 1D comparison lamp spectra taken in the grating and prism modes Lines are identified wit
95. seessssssssssseseeeeeeenenennnnnnn 32 24 Camera MUNET ose dere phe tpe a E E tte ester uut cuti duet 33 2 1 Shutter Shading PuUnCuOMn uso ome pert b vette pisi ta 34 LPR SSDUU TE d9 sestcosatantet tei tates a mtubate Mot ee 34 212 ILOIDCSDI C 34 ZIEL gGhnascdobmabosussntt susto onn cess sou T 35 2 12 2 Bias and Flat Ficld StEUC LUE 2m a Pe DEVI ERR TU USD EEHEERRERAA 37 2 12 59 Ime c OUAlibVsasutecsetieseietn emu nere risa E pn tester End eduteauiadu 38 DN GROS AMDOCS ct cs ras aeo bett on Medea utut Ca e sos dito equos 40 3 Observing in the Near UV to Near IR eee eee eere eere eere esee esee aas 42 3 1 Atmosphere Transmission eane iun rE peo a E E UE eae aes 42 5 2 ANIMOS PACC E MISSO oe cose e E RI Ne rcm poH aid pese citi URE USE DM adt ius 42 MODS Instrument Manual a Mo onlightand Twilight Dipaelsonssenivecasotietisede et EH EDI eter Pieter 44 3 4 Differential Atmospheric Refraction eeeeessssssssseeeeeeeeeeeeee enne ennt 44 4 Observe wIthoMODISS ceccasasescccsnseteteacdacdsteslavecisuissccbvateseseiats ee Ranee aa EAEE 47 4 1 IAT SC Ome OLED TING leraren AN ide bets 47 Biol Setup SrCe inresa saut basia teda tds ia dtiu beds tad 48 d1 2 MODS DasShDOSEU oi icr u ete eI eo Em tira utem ente etian dien 49 41 3 MOUSE KEG DMO CRE CU o semi Men cd ete dodu oc msi Maitt otav dera a M dE DNE dU 53 AN ESCAS Eee SEU UU eee ehageue net a earner aa 53 42 MODS JODSGIrVID9 SCEI
96. ser cut slits are extremely clean and parallel lt 1 width variations on small and large scales MOS Mask Inter Calibration Corrections Because it 1s impractical read a waste of good telescope time to put a standard star down the slit of every slit on an MOS mask twilight sky flats can be used to determine inter slit calibration corrections for a mask Lamp flats also work well for this purpose especially if there are a lot of masks to calibrate as taking twilight spectra of all MOS masks for a run would be impractical The flat field lamp or twilight sky observations through the MOS mask give you the relative throughput of each slits If all slits were the same design size this should be a small correction because of the 63 MODS Instrument Manual generally high precision of the laser cutting machine provided the masks are kept clean of debris that can block the slits The only instance in which observation of a standard star through one of the slits in an MOS mask is indicated 1s when accurate absolute calibrations are required for the science If you need a good relative calibration the regular wide slit standard star spectra will be sufficient combined with the inter slit gray shift found from the lamp or twilight spectra through the mask Imaging Sky Flats Imaging programs will need to take at least one set of twilight sky flats during the course of a run as the 2 gradient due to the internal calibration lamps will produce
97. slit you can determine any gray shift between the slits due to differences in the slit throughput cut width differences changes in image quality especially at the far edges of the field etc Because spectral slit flats especially for the multitude of MOS masks are not time critical they are primarily for spectral trace finding and illumination correction it is a good idea to execute these during cloudy nights 1f so unfortunate or spread them out over your afternoon or post handoff hours but remember to have the telescope pointed at the Zenith with the enclosure lights turned off when you take them 5 4 4 Twilight Sky Flats Twilight sky evening or dawn flats are often used for 1 slit illumination corrections 2 multi slit inter calibration corrections or 3 imaging mode sky flats Slit Illumination Corrections Spectroscopic programs that need to use the entire long slit e g observations of a galaxy or nebula that fills most of the science FoV will likely require at least 1 or 2 twilight spectral flats in red and blue to help perform a high precision illumination correction There is a slight 2 gradient from top to bottom due to the internal lamp illumination system that such twilight flats will remove We have achieved good lt 0 5 sky subtractions in the central 1 arcminute segment of the facility long slit masks for single targets without using twilight sky flats because the slow gradient is free of structure the la
98. sm 4Kx3K mode images still require separate bias frames until we get overscan working for sub frame readout on the CCDs a stubborn bug we haven t managed to root out yet and it remains to be seen if sub frame readout introduces significant residual 2D bias structure in the images Standard scripts are provided on the mountain for acquiring bias 1mages as part of the calibration procedure The dome enclosure should be dark enclosure lights off while taking bias frames 5 3 Dark Frames Measured dark rates on both the red and blue channel CCDs are 0 5 e pixel hour see Table 8 so we have determined that taking explicit dark frames 1s not an indicated calibration step 60 MODS Instrument Manual OSU MODS 201 1 003 Version 1 2 with MODS and would only add noise in most cases No scripts are provided for taking dark frames 5 4 Flat Fields All internal flat fields should be taken with the telescope stationary and pointed at the Zenith They should not be taken while the telescope is moving The instrument must be dark closed hatch in Calibration Mode and with all dome enclosure lights turned OFF We still have some unresolved light leaks in the instrument near the mounting point with the instrument rotator that should be fixed when MODS is next off the telescope Summer 2012 but our leak mitigation measures may not be suitable in all cases We have no indication that flats need to be taken at position on a target as the c
99. son LBT Support Scientist s LBT Telescope Operator s MEUS TA i Ty Exposure Information This block lists information about the exposure including the image type object name exposure and dark time and the raw filename and associated acquisition identifiers GROUP LMAGE LY OBJECT EXPTIME DARKTIME LEDFLASH FILENAME UNIQNAME ACOTAG O0 Group identifier for related images TOBJEGE J Type of observation Dual Prism IMCS Test PA 45 El 90 Name of object 5 0 Exposure time seconds 7 85 Cumulative Dark Time seconds 0 0000E 0 Time to flash lab LEDs seconds mods1b 20110113 0049 Filename assigned by the data taking system EDLU EIZMOu le Unique filename if filename is invalid MODSIB 20110113061700 7 Unique Acquisritronm ID Tag The UNIQNAME keyword is protection against accidentally overwriting image files If an image with the same name as this file 1s found in the raw data directory it writes the FITS file with UNIQNAME instead Instrument Configuration This block lists the instrument vobs18 MODSI Blue Channel the state of the dark hatch calibration tower calibration lamps the hatch is closed the calibration tower is in and the Krypton lamp is lit and gives information about the slit mask and the dichroic beam selector state INSTRUME HATCH CALIB CALLAMPS VELAT SLITMASK MASKPOS MASKNAME MASKINFO DICHROIC DICHNAME DICHINFO MODS1B Instrument Na
100. spheric Refraction on Mt Graham Refraction Relative to A620nm arcsec 1 0 1 5 2 0 2 5 3 0 Airmass Figure 43 Differential atmospheric refraction relative to 6200A as a function of airmass at Mt Graham Different wavelengths are deflected along a great circle running from the zenith through the target The celestial position angle of this arc is the parallactic angle and varies with time 44 MODS Instrument Manual OSU MODS 2011 003 Version 1 2 Figure 44 shows the geometry of the parallactic angle along with plots for Mt Graham using the Owens 1976 atmosphere model Parallactic Angle for Mt Graham LBT ry E a a E a ae Latitude 30 Z 5 20 Pry ryt Cg o Parallactic Position Angle degrees ce eo on o EE E NR EN eg dg ul 3ac4 3 4 3 34351 HA 4 W 0 1 2 3 4 5 6 7 8 Hour Angle West of Meridian hours Figure 44 Left definition of the parallactic angle n CNP is the Celestial North Pole Z is the Zenith S is the source O is the observer and W is the west compass point Right Parallactic angle as a function of hour angle west of the meridian for Mt Graham for declinations 6 30 to 80 Curves are truncated at El 20 The effect on a slit spectrograph 1s that wavelengths near the guiding wavelength will stay at the same location in the slit as the telescope guides but bluer and to a lesser extent redder wavelengths will deflect by different
101. sulting in reduced spectral resolution 1mages are smeared over more pixels Flux and wavelength calibration precision will also be reduced in the corners of the field Mask alignment stars should be kept within the 5 circle as precise centroid measurements of alignment boxes and stars 1s degraded by image aberrations outside this reducing the Zi MODS Instrument Manual precision of position and rotation offsets computed Note that this 1s a conservative limit not a hard limit and if you are taking spectra of brighter objects where the alignment with the slits will be obvious in short thru slit acquisition images e g multi object masks of star fields or bright HII regions in galaxies this restriction may be relaxed without penalty except that additional mask alignment iterations may be required to fully align targets with the science slits 2 8 Calibration Unit Each MODS has its own internal calibration system consisting of an integrating sphere and projection optics located in a deployable calibration tower located above the slit plane A selection of Pen Ray wavelength calibration lamps and continuum lamps for spectral and imaging flat fields are mounted in the integrating sphere The lamps and their uses are summarized in Table 11 The projection optics includes a mask that will produce a representation of the telescope secondary mirror obscuration at the correct location in the MODS pupil plane for the spectrograph Table 11
102. t and beam geometry right 600 500 Red TIH6 Ag Prism 400 300 Resolution 4 54 200 Blue FuSi Al Prism 100 300 400 500 600 700 BOO 900 1900 1100 Wavelength nm Figure 12 Measured MODS double pass prism resolution curves 2 6 CCD Detectors The MODS detectors are e2v Technologies Ltd CCD23 1 68 monolithic backside illuminated 8192x3088 15um pixel CCDs operated with OSU MkIX detector controllers The Blue CCD is made on standard 16um thick 100 2 cm silicon coated with the e2v Astro BB broadband coating The Red CCD is made on 40um thick deep depletion gt 1500 Q cm silicon with the e2v Astro ER1 extended red coating A photograph of one of the detector packages is shown in Figure 13 with a sketch of the readout geometry 2 6 1 Basic Properties The basic properties of the CCDs measured at operating temperature 100 C are summarized in Table 8 The conversion gain readout noise linearity and dark current are average values for the four quadrants measured on telescope under normal operating conditions 20 MODS Instrument Manual OSU MODS 2011 003 Version 1 2 N N Figure 13 Left MODS e2v 8Kx3K CCD package Right Schematic of the 4 amp readout geometry Table 8 MODS Science CCD Properties Notes 1 Measured on telescope during 2010 2011 commissioning average of 4 quadrants 2 Measured in the LBT instrument lab at 28 C and on telescope down to 5 C Each quadrant is read t
103. t will revert to its normal state Ifa fault occurs the widget will turn bright red and stay red until the fault condition has been cleared Occasionally an instrument setting will be stuck amber meaning that the request has apparently not completed for some reason A common cause of a stuck control 1s a lost completion message The way to clear a stuck control is to press the Refresh button at the lower left corner of the dashboard and then see if the stuck control reverts to its normal state You may have to repeat the setting as needed 49 MODS Instrument Manual Calibration and AGw Unit The top left subpanel contains the controls for the common focal plane suite the instrument dark hatch calibration system and AGw unit Calibration and AGw Unit Mode Calibration Observing Restore AGw Probe Calibration Lamps Hatch Open Glosed ye 3 Hg a ar axe a xr NN a OFF Calib In Out QTH1 arH2 8 vrai 2 lI AGw X 81 000 Y 243 500 Foc 0 000 Filter IND1 0 Home This lets you open the instrument to the sky Observing Mode or put in the calibration system Calibration Mode Calibration Mode Close the instrument hatch stow the AGw guide probe and save its location before being stowed and then insert the calibration projector At this point you are ready to observe internal calibration sources flat field and comparison lam
104. ting Spectroscopy R 2000 0 6 arcsec slit Low Dispersion Prism Spectroscopy R 500 150 Slits Laser cut spherical slit masks up to 24 per MODS 9 fixed 15 user Calibration Internal pseudo pupil projector and integrating sphere Pen Ray wavelength calibration lamps Hg Ne Ar Xe Kr Quartz Halogen and variable intensity incandescent continuum lamps CCD Detectors e2v CCD231 68 3072x8192 15um Pixels Flexure Compensation Real time closed loop IR laser metrology system sending collimator mirror tip tilt corrections during exposures Acquisition amp Guiding 50x50 arcsec FoV CCD camera 4 filters Active Optics Off axis Shack Hartman wavefront sensor Dichroic Blue transmit Red reflect design 5700A cross over wavelength Collimators 3450mm focal length off axis paraboloids Red Ag Blue Al Cameras f 3 decentered Maksutov Schmidt Red BK7 Blue Fused Silica Minimum Exposure Time second Dimensions 4 0x2 5 meters Mass 3079kg 10 MODS Instrument Manual OSU MODS 201 1 003 Version 1 2 2 Instrument Configurations There are three basic operating modes imaging grating spectroscopy and prism spectroscopy summarized in Table 1 Within in each mode MODS may be configured for dual channel red blue blue only and red only operation The instrument configuration also sets the basic CCD sub frame region of interest ROI readout for that mode Table 1 MODS Instrument Configurations Resolution Wavelengths CCD Mode
105. tion that opens along the long slit axis This gives no shading along the dispersion axis and only minimal shading along the slit axis FIELD FLATTENER Aspheric Corrector Spherical Lens Mirror Figure 25 Schematic of the MODS camera shutter shown in the open position The open close time results in a minimum exposure time of 0 45 seconds In general though we have adopted a minimum working exposure time of 1 0 seconds but a 0 5 second exposure time can be requested if necessary e g for a prism mode calibration 33 MODS Instrument Manual 2 11 1 Shutter Shading Function The finite shutter opening time and the asymmetric geometry of the shutter behind the off axis Maksutov Schmidt corrector lens means that there 1s a correspondingly asymmetric shutter shading function along the slit but no shading in the dispersion detector X axis MODS1 Blue Camera MODS1 Red Camera 98 98 Shutter Shading cO 6 Shutter Shading QD Mo a dou ais 3 8 0 500 1000 1500 2000 2500 0 500 1000 1500 2000 2500 Y pixels Y pixels Figure 26 Mean 1 second vertical shutter shading functions for blue left and red right cameras At most the shutter shading function manifests as a 2 gradient from bottom to top in a 1 second integration This gradient will decline linearly with increasing exposure time It 1s sufficiently small that for typical spectroscopic exposure times even standard stars it may be safely ignored
106. to deliver an average of 30 000 ADU per pixel about the middle of the dynamic range to ensure that the flats are always well in the linear regime of the CCD response A minimum of 5 flats is needed in order to eliminate cosmic rays Fixed pattern noise is more obvious in the UV imaging flats mostly picking up annealing artifacts in the thin blue CCDs There are no lamp and filter combinations that permit simultaneous acquisition of blue and red flats so we have found that they need to be acquired serially A series of standard scripts are provided at the telescope to acquire basic flat field images and we recommend acquiring imaging flats for the red and blue only modes without the dichroic this also eliminates some artifacts due to low level ghost images from the dichroic in very bright flats Internal lamp flats should only be taken with the telescope stationary and pointed at the Zenith and with the imaging mask in place The imaging mask acts as part of the fore optics baffling for the instrument and reduced stray light entering the system Procedures for taking twilight flats are still being worked out as this requires help from the telescope operator and there are differences of opinion regarding whether the telescope should track but dither or be 6l MODS Instrument Manual fixed and allowed to drift We remain agnostic about either method and leave that to the preferences of the observer 5 4 2 Grating Spectroscopy Pixel Flats
107. umentation Program TSIP with additional funds from the Ohio Board of Regents and the Ohio State University Office of Research Please also send links astro ph ADS etc of papers using MODS data to Rick Pogge pogge astronomy ohio state edu so we can track the scientific use of MODS for ourselves and our funding agencies 1 3 Online Materials There is a large amount of supplementary online data and documentation for MODS To avoid having many easily broken web links scattered throughout this document wherever we refer to the MODS Webpage the URL is www astronomy ohio state edu MODS When in doubt or if there are discrepancies between this manual and the data on the MODS webpage consider the webpage the most up to date and therefore definitive source The LBTO wiki has a section devoted to partner observing pages including MODS wiki lbto org twiki bin view PartnerObserving with additional technical documents under the Instrumentation pages All of these web pages are works in progress so check them regularly for updates MODS Instrument Manual ADC ADU AGw AR CCD DPOSS FITS FoV FWHM GCS GPS GUI HA ID IMCS IMPv2 IR ISS LBT LED LMS LN LUCI MMS MODS MOS ND NIR NTP OSU PA PCS QE OTH RMS ROI SDSS TCS URIC UTC UV WES OSU MODS 201 1 003 Version 1 2 1 4 Acronyms and Abbreviations Analog to Digital Converter also Atmospheric Dispersion Corrector ADC units aka counts
108. upply and return temperatures and pressures at the start of the observation GSPRES 2140 f Glycol Supply Pressure osir g GSTEMP 0 2 Glycol Supply Temperature deg C GRPRES 24 7 Glycol Return Pressure psi g GRTEMP 0 0 Glycol Return Temperature deg C Telescope Telemetry These keywords list the primary and secondary mirror collimation parameters at the start of the exposure MIPOSX 0 504 Primary Mirror X Position mm MIPOSY si Z Primary Mirror Y Position mrm MIPOSZA 0 226 Primary Mirror Z Position mm MIROTX 30 721 Primary Mirror RX Rotation arcsec MIROTY 4 298 Primary Mirror RY Rotation arcsec MIROTZ 0 000 Primary Mirror RZ Rotation arcsec MICTEMP 4 50 Primary Mirror Temperature deg C MIATEMP 0 67 Primary Mirror Ambient Air Temp deg C M2POSK 4 010 Secondary Mirror X Position mm M2POSY V ZIT 4 Secondary Mirror Y Position mm M2POSZ 0 000 Secondary Mirror Z Position mm M2ROTX 148 650 Secondary Mirror RX Rotation arcsec M2ROTY 181 800 Secondary Mirror RY Rotation arcsec M2ROTZ 0 000 Secondary Mirror RZ Rotation arcsec M2CTEMP 4 50 Secondary Mirror Temperature deg C These keywords report the readings from temperature sensors located on the telescope structure Measurements are taken at the start of the exposure These data are important for helping track and derive the temperature corrections for the telescope collimation model but
109. ven Odd Effect here from the same red CCD bias frame shown in Figure 32 MODS Instrument Manual The difference in the bias level is larger than the pixel to pixel variations due to readout noise which is why the even odd pattern artifact is so prominent It is also however quite stable during readout so the overscan columns in the images can be used to remove the differences in bias levels while flat fields remove the remaining differences in gain between the output channels This readout scheme is unique to MODS and in practical terms it means that raw MODS images cannot be bias corrected using the usual IRAF or IDL tasks which assume that all pixels within a quadrant are read through a single output channel For the current suggested methods please see the MODS webpage for data reduction notes Once you get past the particular requirements of MODS for bias and flat field correction the 2D data are ready for use by any of the established packages for working with long slit and multi object spectra e g IRAF twodspec and onedspec packages for long slit spectroscopy or adaptations of the Carnegie COSMOS package for MOS spectroscopy 2 12 3 Image Quality As noted in 32 MODS produces its best images inside a 4x4 central sweet spot with degrading image performance into the 6 extended field of view see Figure 20 This has implications for the location of slits in MOS masks 82 7 3 and for overall image quality at th
110. x keywords will change as the IMCS steers the collimator to compensate for instrument flexure but COLFOCUS will remain roughly constant tip tip corrections are made relative to the collimator focus vertex IMCS IR Laser Status These keywords list the status of the IR laser used by the Image Motion Compensation System IMCS IRLASER ON i IMCS IR Laser AC Power On or Off IRBEAM ENABLED IMCS IR Laser Beam Enabled or Disabled IRPSET 1 0 IMCS IR Laser Beam Power Set Point mW IRPOUT 1 1 IMCS IR Laser Beam Power Output mW IRTEMP 25 4 IMCS IR Laser Head Temperature deg C IRTSET 25 5 IMCS IR Laser Head Temp Set Point deg C Target and Guide Star Coordinates These keywords list the target and guide star coordinate information uploaded to the TCS by the preset used to point the telescope for this observation OBJNAME M1 Target Name OBJRA 1054344 30 00 0 Target RA OBJDEC 52 26 Opes 9 Target DEC OBJRADEC FK5 Target Coordinate System OBJEQUIN J2000 Target Coordinate System Equinox OBJPMRA 0 00 Target RA proper motion mas per yr OBJPMDEC 0 00 Z Target Dec proper motion mas per yr OBJEPOCH 2000 00 Target Epoch GUINAME gstar Guide Star Name GUIRA Uxue34 20 449 Guide Star RA GUIDEC 21 57 2294190 Guide Star DEC GUIRADEC FK5 Guide Star Coordinate System GUIEQUIN J2000 Guide Star Coord System Equinox GUIPMRA 0 00 Guide Star RA proper motion
111. you will run a number of custom programs while observing with MODS in particular 1 The MODS Control Panel GUI 2 The modsDisp raw image display agent and newdata disk watcher 3 The MODS scripting engines acqMODS and execMODS 4 The modsAlign interactive mask alignment program uses PyRAF and ds9 A typical observing run requires two people one on a workstation dedicated to operating the MODS instruments with the control panel and related programs and a second person on a separate workstation using IRAF IDL or another data analysis package to examine the incoming data manage an observing queue etc The LBTO support astronomer will usually be logged into a third workstation with access to engineering programs to monitor MODS health and to take actions if there are problems Your LBTO support astronomer is responsible for technical instrument startup including starting the various servers and agents needed to coordinate the activities of MODS and for helping deal with problems As an on site observer you will only run the four programs described above The recommended way to operate MODS for routine observing is with MODS observing scripts 84 2 While MODS may be operated by hand using the MODS control panel this requires attention to the many details of the inner workings of the instrument and the choreography between the instrument telescope and IMCS that is needed for efficient observing The MODS script engines take ca

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