A Taste of ESPRESSO or How to Use The San Pedro Martir REOSC
Contents
1. He Ar etc To remove the mirror from the light path pull outward on the knob which rotates the mirror The spectrograph has a continuously adjustable slit with a minimum slit width of 30um and a maximum of 900um Presuming that you are using the f 7 5 secondary the plate scale 1 33x102 um The optimum slit width matched to 2 pixel resolution is close to 2 arcseconds 150um see Table 11 in the appendix We present some typical slit sizes in microns and arcseconds in Table 3 3 of 22 The slit viewer observes an 18 mm diameter field around the slit An intensified CCD camera that can be used to view the slit is currently being tested out This replaces the eyepiece and should allow the observer to both acquire objects and guide on the slit the ability to guide on the slit is dependent upon the slit size used As it exists now the camera has no difficulty reaching tenth magnitude objects as faint as I have seen tried and should be able to image at least as faint as twelfth magnitude for useful guiding The slit length is 30 mm 6 7 but it can be shortened using a series of masks or deckers The masks typically come in pairs and there are 10 masks all together Each pair consists of a central mask and a double mask positioned outside the central mask This permits the observer to make two non overlapping exposures on the same image subject to the inter order spacing The widths of the masks are given Table 4 Pulling the m2. cise This starts up the cycle that continuously clears the CCD obs n This takes an exposure of n deciseconds duration N B DO NOT type cise during an observation Doing so will cause the control program to lock up and the only apparent solution is to re boot the computer pcoln Plot column n of the current image prown Plot row n of the current image qshow This displays an image on the image display N B If you will be saving full frame images to disk there is only enough space on the disk for about 50 images so plan ahead Also while the software makes no mention of it the Heurikon only has enough RAM to hold 6 7 15 of 22 images in memory at one time 7 General Caveats Contents Next Section Prev Section a light leak with the dome lights on or the slit open during the day you will notice a gradient in the bias frames This should go away pretty much as soon as the lights go out It may take several minutes and you should probably do a cisc on CCD UMass to flush the chip wells The bias should be very flat typically at about 310 ADU On the CCD Mil try taking a series of bias frames to acheive the same objective since there doesn t appear to be a cisc equivalent b dome slit openings It is not possible to have the whole slit open There are 3 possible configurations 2 doors down which cuts off the lower 40 degrees roughly 2 doors up which cuts off about 30 degrees from the zenith and 1 up and 1
3. with either of its two camera detector setups This translates into a 2 pixel velocity resolution of roughly 17 km s A note on the choice of name The echelle unlike the newer instruments at San Pedro has not been graced with a name other than to call it the echelle at least as far as the author is aware While writing this manual I had a free moment to ponder this terrible state of affairs and propose to call the echelle ESPRESSO for ESPectrografo Reosc Echelle de Sierra San pedro martir Observatorio or if you prefer Echelle SPectrograph from REosc for the Sierra San pedro martir Observatory Thus this is the ESPRESSO User s Guide and if you like you can tell your friends that you are up on the mountain using the ESPRESSO Machine 1 1 General Information The layout of the main optical elements of the spectrograph is shown in Figure 2 of 22 1 and the instrument controls in Figure 2 The main echelle grating is ruled on a 254mmx128mm area and has 79 lines mm The blaze angle is 63 433 and the incidence angle is 71 The beam is dispersed into an angle of 2 63 The dispersion depends upon the order and ranges from 4A mm to 16A mm The values differ slightly depending upon which of the two available cameras you are using see the sections on the two cameras ucl or reosc The two grating cross dispersers available are listed in Table 1 The 900 lines mm cross disperser B was most useful when the detector was a photo
4. 1024x1024 CCD detector either the CCD Mil or the CCD Tek We shall refer to this system as the UCL camera This is the current default configuration for the echelle The second camera is the original REOSC camera which is used with a 4 of 22 512x512 CCD run by a system put together by Photometrics CCD UMass We shall call this combined camera and CCD setup the REOSC camera 1 3 Efficiency and Response Be aware that there is a very wide range in response with wavelength of the system as a whole including the CCD and it is likely that some regions of your spectra will be saturated while others are barely exposed The blaze functions for the first order of the cross dispersers are shown in the top panel of Figure 4 along with the typical blaze function along each of the echelle orders bottom panel To help point out somewhat more the wide dynamic range of the system as a whole Figure 5 shows first the panel 1 and the the product of the QE and the cross disperser A blaze function panel 2 To illustrate how these might combine with some typical input spectra panel 3 shows black body spectra for objects of temperatures 3200 K an approximation of a tungsten flat lamp and 5800 K roughly solar and panel 4 shows what the relative output intensities would be Note that the total relative intensities are better than you will actually get since we have not included transmission and reflection loss from all of the rest of the v
5. gives details about running the CCD UMass control system IA UNAM Technical Report 97 March 1992 Instructivo para Observadores en el CCD MIL by M Pe a and S Torres Peimbert This gives details for running the CCD Mil and some technical specifications see also the elmil manual by L Gutierrez IA UNAM Technical Report 102 March 1992 Ajuste de Ganancia a una Camera CCD by F Barbosa This gives CGAIN to gain factor conversions for the various CCD s available and seems to be based upon the information supplied by Photometrics Literature Filippenko A 1982 The Importance of Atmospheric Differential Refraction in Spectrophotometry PASP 94 715 Massey P Strobel K Barnes J amp Anderson E 1988 Spectrophotometric Standards ApJ 328 315 Atlases Th Ar Atlas by G Koenigsberger G Canalizo and D Pe a Made with this echelle using the UCL camera and the CCD Mil This atlas been submitted to the IA UNAM Technical Reports 20 of 22 Th Ar Atlas by J A Lopez and M Moreno Made with this echelle using the UCL camera and the CCD Tek He Ar Atlas by J Echevarria Made with this echelle using the UCL camera and the CCD Mil UCLES Spectrum of the Thorium Argon Hollow Cathode Lamp I 79 grooves mm echelle grating and IPCS detector by M Bessell and M Pettini from the Anglo Australian Observatory A useful map of the Thorium Argon line spectrum done with an echelle similar to this one A CC
6. 6 383 meaning that the A D conversion factor is 10 6 e ADU at a gain of 1 The readout noise is 5 71 12 of 22 electrons The normal bias level at 110 C is 275 ADU at gain factor of 1 See the Photometrics specifications for this chip as well as IA UNAM Technical Reports 97 and 102 for more information The constants for conversion from gain factor f to CGAIN are C_1 15 and C_2 11 for this chip Note though that as it is implemented in the elmil control program C_2 0 so that a gain factor of 1 is entered as CGAIN 0 and a gain factor of 5 is entered as CGAIN 60 See Table 9 A warning to users who are observing faint objects this chip has charge transfer problems which you will most likely see on one edge of your orders The CCD Mil chip is run using the elmil program on the Sun workstations or using a photometrics control program on a PC The elmil program is pretty much self explanatory except for the section on setting the gain factor and is documented by L Gutierrez in the user s manual see also IA UNAM Technical Report 97 though the system documented there is not being used The elmil control program does not seem to have facility for a continual chip flush between exposures You may wish to take a few biases to clear the wells after a bright exposure 6 2 The CCD Tek The CCD Tek system is also supplied by Photometrics and is constructed around a Tektronix TK1024AB CCD with a Metachrome II coating to
7. 90 C to 130 C with typical temperatures being 101 C and 110 C If the dewar is filled at the beginning of the night the charge will normally last all night Once the temperature of the CCD system stabilizes the temperature probably won t vary by more than than 0 1 C over the course of the night The gain conversion factors for this CCD are C_1 30 and C_2 0 6 3 1 Basic Operation The computer control for the CCD UMass system is a Heurikon Systems computer running a system V version of the Unix operating system The CCD control software is called ccd_oan and is a FORTH interpreter written in C Usually you will find the Heurikon up and running but just in case we now tell you how to re boot the computer see IA UNAM Technical Report 61 for more details 1 Turn on the power to the computer The switch is on the front of the box that says Photometrics 3000 Turn on the HDS terminal this is the computer console 2 When the gt prompt appears type bw lt return gt This begins the boot strap process 3 After several messages the computer will say Standalone boot At the prompt type lt return gt 4 Several more messages will scroll by and then the computer will prompt you with type return to start at 0x100 You type lt return gt 5 Finally the basic unix system will be up and running in single user mode and the system will say Welcome to UNIX System 7a and display a prompt At th
8. A orders 33 through 20 the percentage of coverage steadily decrease until in order 20 10 of 22 only about 5 8 of the free spectral range is on the chip According to J Echevarria it is possible to cover the entire range from 3 000A to 11 000A in three or four overlapping frames Orders 74 through 41 40 through 27 and 26 through 20 To observe redder than Halpha it is necessary to insert a filter to cut out the overlap from the second and third orders 4 2 Positioning the Spectrum To determine what portion of the spectrum is imaged onto the CCD you can rotate the cross disperser using the rotating micrometer on the side of the echelle housing See Table 5 for a list of which wavelength regions are imaged onto the CCD at which micrometer angles Figure 6 illustrates where the orders lie on the camera focal plane and the box shows the size of the CCD Note that you get the full free spectral range for all orders bluer than order 34 5 The REOSC Camera Contents Next Section Prev Section 5 1 REOSC Camera Specifications The REOSC camera has a focal length of 204 mm and is constructed of a spherical plate mirror with a double lens afocal correcting plate and a field lens It has a flat field of 8 x 5 2 with an f 1 4 focal ratio The blur circle is less than 15um for every wavelength in the whole field The central wavelengths of the orders are given in Table 8 Because of the short focal length of the camera it is necessary to
9. A Taste of ESPRESSO or How to Use The San Pedro Martir REOSC Echelle Spectrograph S Levine Observatorio Astron mico Nacional IA UNAM Ensenada B C Mex levine bufadora astrosen unam mx and D Chakrabarty California Institute of Technology Pasadena CA USA ITA UNAM TECHNICAL REPORT MU 94 04 submitted 24 August 1994 1 0a revised 4 October 1994 1 0b revised 20 April 1995 version 1 0b Contents 1 Spectrograph Characteristics 1 1 General Information 1 2 The Cameras 1 3 Efficiency and Response 2 How to Adjust the Spectrograph for Your Needs 2 1 Focusing the Spectrograph 2 2 Changing the Wavelength Region 2 3 Rotating the Slit 3 Basic Procedures for Taking Spectra lof 22 3 1 How To 3 2 Taking Standard Star Spectra 4 The UCL Camera 4 1 UCL Camera Specifications 4 2 Positioning the Spectrum 5 The REOSC Camera 5 1 REOSC Camera Specifications 5 2 Positioning the Spectrum 5 3 Moving the Focal Plane Stage 6 Detectors 6 1 The CCD Mil 6 2 The CCD Tek 6 3 The CCD UMass 7 General Caveats 8 Appendix Some Basic Optics 9 References and Other Useful Documents 10 Pictures of the Spectrum List of Tables List of Figures 1 Spectrograph Characteristics Contents Next Section The Echelle Spectrograph at San Pedro Martir SPM was constructed in the late 1970 s by the French Optical firm of REOSC The echelle is capable of a resolution of R 18 000 at 5 000A
10. D Atlas of Comparison Spectra Thorium Argon Hollow Cathode 3 180A 9540A by D Willmarth from Kitt Peak National Observatory Another useful Thorium Argon line atlas Spectrograph Characteristics Any good optics textbook for basic diffraction gratings Jenkins and White Born and Wolf whatever your favorite is Gray D 1976 The Observation and Analysis of Stellar Photospheres chapter 3 Schroeder D 1970 Design Considerations for Astronomical Echelle Spectrographs PASP 82 1253 The next four references go into detail regarding how to properly compute the intensity profile that emerges from an echelle They are interesting reading and go into much more detail than we have done here Schroeder D amp Hilliard R 1980 Echelle Efficiencies theory and experiment Applied Optics 19 2833 Bottema M 1981 Echelle Efficiencies theory and experiment comment Applied Optics 20 528 Schroeder D 1981 Echelle Efficiencies theory and experiment author s reply to comment Applied Optics 20 530 Engman S amp Lindblom P 1982 Blaze Characteristics of Echelle Gratings Applied Optics 21 4356 Other Useful Sources 21 of 22 IRAF noao astutil gratings utility and help page The Manual for the Boller amp Chivens Spectrograph 10 Pictures of the Spectrum Contents Prev Section To aid in oriention we provide an echelle spectrogram of the Thorium Argon lamp and one of the Helium Argon lam
11. ages Bias frames are simple and easy to take and do not depend upon the settings of the echelle Note however that the dome should be dark as the whole system is not totally light tight and if you take a bias with bright lights on in the dome you will notice a gradient in the bias images this has been seen with both the CCD Mil and the CCD UMass There is also some scattered light in the system When taking flat field images you have several options You can use the internally mounted flat lamp which is I believe a tungsten bulb so the effective temperature of the spectrum is around 3200 K see Figure 5 or you can take dome flats which seem to be slightly bluer but take much longer to reach the same saturation level they are fainter by roughly a factor of 10 Sky flats have difficulty with solar and atmospheric lines in the spectrum Note that the response of the system and the QE of the CCD and the temperature of the typical flat image lamp all conspire to cause a very wide range in intensity with wavelength and you will very likely need exposures of long and short duration to properly fill the wells in the blue and red respectively The flats are typically used for two things correcting for the response of the CCD and correcting for the blaze functions of the gratings They should be taken with the same instrumental setup up that your data are taken with To take a flat field using the internal lamp 1 Go up to the inst
12. arious optical elements How long do you need to integrate to reach a given Signal to Noise ratio S N for a given magnitude S N lambda sqrt e pixel in echelle dispersion direction sqrt gamma photons above the atmosphere s cm A x epsilon transmission of atmosphere telescope amp echelle x A collecting area pi r cm x Delta x A pixel x QE e photon x t integration time in seconds We can re write gamma in terms of V magnitudes lambda 5556A as gamma V 948 x 104 V 2 5 photons cm s A where the 948 is the number of photons from a star of magnitude V 0 Actually this is the monochromatic flux at 5556A but for a rough estimate it will be close enough See Mihalas amp Binney 1981 Galactic Astronomy p 62 So S N ambda sqrt 948 x 104 V 2 5 x epsilon A Delta x QE t 5 of 22 For the 2 meter telescope at SPM at 5 000A A 31416cm2 With the UCL camera and CCD Mil chip Delta_x 0 14A pixel We ll assume that the CCD QE is about 15 and that the combined transmission epsilon 6 this presumes 90 transmission or reflectance at every optical element and through the atmosphere and 40 at each of the gratings When combined with the QE of the CCD this gives a throughput of just under 1 So the S N ambda will be S N ambda sqrt 39 000 x 10 V 2 5 t x e pixel in echelle dispersion direction This is the S N you should get after you sum together th
13. ask slide all the way out beyond mask position 10 permits the user to view the whole slit length or at least the full field of view of the slit viewer eyepiece Which mask you use is dependent upon the wavelength region you wish to observe The further towards the blue you wish to observe the more closely spaced the orders become thus limiting the length of the slit that you can effectively use without getting overlap of the orders The cross disperser and camera optics magnify the image of the length of the slit see the appendix just as the echelle grating and camera optics magnify the slit width hence the derived optimum slit width In Figure 3 the distance between the order centers is plotted and overlaid upon that are dashed lines showing the length of the slit as it actually appears on the detector For the orders NOT to overlap the inter order spacing must be greated than the image length So with the 2 mm mask 7 all the orders redward of order 50 lambda gt 4400A should not overlap Of course you will want to leave some space between the orders so effectively this means that you should not go all the way to order 50 At 4200A the maximum slit length for no overlap is 1 mm 13 3 decker number 8 see the Figure accompanying Table 4 and also Figure 3 1 2 The Cameras There are now two cameras available for use with the echelle The first has been recently constructed at University College London and is paired with a
14. ating holder by hand Fine adjustment is done using the left thumb screw The right hand thumb screw should be tightened before doing fine adjustment Using the vernier you can read the grating tilt angle to one minute of arc Maps of grating angles blaze wavelengths and visible orders are given in each of the sections on the two possible camera setups 2 3 Rotating the Slit To rotate the echelle slit the whole instrument is rotated on the telescope mounting There are two locking knobs and a third knob that causes the mounting platen to rotate There is an angle indicator next to this knob Check the value when you begin Usually the spectrograph is mounted with the slit pointing in an east west direction There are two reasons you might wish to rotate the slit The first and obvious one is to orient the slit in a particular direction on an extended object The 7 of 22 second reason is that you may wish to minimize the differential refraction due to the atmosphere see paper by Filippenko A 1982 PASP vol 94 p 715 These two objectives are not usually compatible and those of you doing intensity measurements are advised to read Filippenko s paper and see if this will make a big difference for you 3 Basic Procedures for Taking Spectra Contents Next Sectionl Prev Section 3 1 How To In the course of taking data with the echelle spectrograph you will need to know how to take flat and bias frames and arc lamp and object im
15. blocks out the optical path from the telescope 3 Set the desired slit width and choice of mask 4 Take ann second exposure Typically for the Thorium Argon lamp a 60 second exposure is long enough to give well defined lines Now go find your objects For fainter objects you may well find it necessary to take several shorter exposures and co add the images later since the number of cosmic rays in a 15 minute exposure is fairly high To take a spectrum of an astronomical object 1 Turn off all the lamps 2 Pull the knob on the lamp housing OUT 3 Set the desired slit width and choice of mask 4 Take ann second exposure Depending upon the wavelength region you are working in you may need to insert a long pass filter to block out overlap from the second and third orders 9 of 22 of the cross disperser We use the first order of the cross disperser to separate the echelle orders Second and third order images also show up on the image plane For stellar spectra this is not a problem shortward of about 6 500 7 000A since the atmosphere effectively blocks transmission of much of the radiation short of 3 000A However if you wish to observe further towards the red you will want to have a long pass filter to remove the second and third order spectra For the comparison arcs technically this is a problem for all the echelle orders since you don t have the atmosphere to cut out the violet though the first order is substantially
16. brighter than the second and third orders While we do not currently have an optical quality long pass filter some tests have already been made with a photographic wratten filter placed in a temporary mounting behind the slit assembly 3 2 Taking Standard Star Spectra For absolute flux calibration you will need standard star spectra The spectrophotometric standards given by P Massey K Strobel J Barnes and E Anderson 1988 ApJ 328 315 are available on line in IRAF along with several other sets of spectrophotometric standards When in IRAF type page onedstds README to see a listing of the standards Copies of the finder charts for the Massey et al standards should be in the control room 4 The UCL Camera Contents Next Sectionl Prev Section 4 1 UCL Camera Specifications The UCL camera was recently constructed at University College London to replace the original REOSC camera First observations with the new camera were done in the fall of 1993 The camera has a focal length of 215 6 mm This camera is used with the CCD Mil see section 6 1 and the CCD Tek see section 6 2 This camera setup is capable of observing between 3 000A and 11 000A though not all at once For orders 74 through 34 the blue end from about 3 000A to about 6 700A including Halpha in order 34 Hbeta in order 46 Hgamma in order 52 and Hdelta the CCD can be set to cover the complete free spectral range For wavelengths longer than about 6 700
17. down It takes about 10 minutes to change between door configurations c high humidity If the humidity climbs above about 70 condensation begins to form on the face of the fiber bundle that is cemented to the CCD UMass This is a time variable phenomenon that can be very noticeable The face should be cleaned every few hours when this happens Also be aware that under current observatory operating procedure the telescope will not be opened if the humidity reaches 85 d tape drive troubles The 9 track on the Heurikon is very finicky about loading tapes The only apparent solution short of a serial link or new tape drive is persistence The Exabytes on the Suns need to be cleaned on a regular basis e warning about the clock settings 4 on the HEURIKON The clock on the Heurikon can be set by the user when logged in as root Except for the short period between old and new change over dates to daylight savings time the clock should be displaying Pacific time The Heurikon actually stores time in UT internally If the time gets re set improperly it will affect the UT that gets written into your FITS image headers by wfits when it writes the images to tape ii on the SUN The date and time can be set only by the root user The times written into your FITS images by elmil and or IRAF will use the system time so make sure it is right Be aware that the clocks on the computers can drift by up to several seconds per day There is a WWV
18. e pixels dispersed in the spatial direction For those who cannot do this divide this number by the number of pixels in the cross disperser direction let us estimate it at 4 for ease of illustration making the S N pixel 1 2xS N ambda The example above is probably a bit optimistic Remember too that the S N is also going to depend upon the wavelength and the spectral structure of the object under study You are advised to take several spectra of various magnitude objects to get an empirical value for the constant term in the S N relations which are summarized below a S N given V and t b t given S N and V c V limit given t and S N a S N sqrt const x 10 V 2 5 t b t S N x 104 V 2 5 const c V 2 5 log10 S N const x t 2 How to Adjust the Spectrograph for Your Needs Contents Next Section Prev Section First don t forget to focus the telescope 2 1 Focusing the Spectrograph In addition to focusing the telescope it is also necessary to make sure that the spectrograph is focused The spectrograph is focused by moving the collimator using the micrometer on the bottom of the instrument housing The telescope should be pointing at or near the zenith for focusing This procedure can be done fairly rapidly with two people and should be done everytime the dewar is removed and replaced We found that once done in the early evening the focus would remain ok for the rest of the night The smallest line
19. e zeroed at any position This makes it easy to change back and forth between stage positions When you are done moving the stage don t forget to tighten the locking nuts 6 Detectors Contents Next Section Prev Section A note before we begin the gain factor and CGAIN are NOT the same thing The gain factor f is just that the ratio of the number of electrons ADU relative to the number of electrons ADU when the A D converter s range equals the full well depth Hence a gain of 1 implies that the A D converter should saturate when the CCD s wells fill A gain factor of 2 implies that the A D converter saturates at half the well depth etc The CGAIN that many of the control programs use is a holdover from the Photometrics control systems CGAIN is related to the gain typically by a formula like CGAIN C_1 x f 1 C_2 where the constants C_1 and C_2 vary depending upon the CCD and controller C_1 is usually either 15 or 30 and C_2 either 0 or 11 see IA UNAM Technical Report 102 6 1 The CCD Mil The CCD Mil system features a Thompson THX31156 CCD with a Metachrome II coating to improve blue response It has 1024x1024 pixels each 19umx19um This chip is most sensitive in the red its quantum efficiency at 7 000A is about 40 and at 4 000A is about 15 See Figure 5 panel 1 for the quantum efficiency curve According to Photometrics the well depth is 173 000 electrons and the A D converter has a range from 0 to 2 14 1 1
20. ember that the spectrum is actually the product of two diffraction gratings and hence both will affect the final intensity See D Gray 1976 The Observation and Analysis of Stellar Photospheres chapter 3 for more on this as well as the references noted with it in the references section Finally it is often pointed out that aspects other than the diffraction limit of the telescope optics dictate the optimal slit width The telescope diffraction 18 of 22 limit is roughly 1 22xlambda D where D is the diameter of the objective for the 2 meter telescope at 5 000A this is 1 20 The internal optics of the spectrograph will magnify the image of the slit on the detector plane where the resolution is fixed by the pixel size so the optimal slit is that slit which lets in as much light as possible while the slit image is no wider than one pixel roughly If we have a pure monochromatic beam of light illuminating the entrance aperture which has a width w mm then the angular size of the image of that aperture on the detector is obtained from the derivative of the grating equation with respect to alpha taken over a finite but small angle Delta_beta cos alpha cos beta x Delta_alpha To convert to linear measures on the detector Delta x and on the telescope focal plane w respectively Delta_beta Delta_x mm f_cam mm and Delta_alpha arctan w f_col w mm f_col mm Putting it all together w mm cos beta c
21. graphic plate With a CCD and its much smaller detector surface area the 300 lines mm grating A is more useful With this it is possible to fit between 8 near Halpha and 16 near 4 000A orders onto the chip with the REOSC UMass setup or between 12 and 30 orders onto the UCL Mil setup The current default setup uses cross disperser A with the UCL camera and the CCD Mil detector There are half a dozen comparison lamps available see Table 2 The lamp housing has three permanently mounted lamps and one hole that permits the easy interchange of additional lamps Currently mounted in the permanent sockets are lamps of Argon Cesium and Rubidium In the open hole a Thorium Argon lamp with its own voltage supply can be mounted The flat lamp mounting socket also fits in the same hole and Neon and Helium Argon bulbs can be fitted into this socket On the underside of the lamp housing a sliding diffuser is mounted To insert the diffuser push up in It should click into position This is used with the flat lamp to help ensure even illumination of the slit The most commonly used comparison lamps currently are the Th Ar and the He Ar Each of the permanently mounted lamps has a switch on the lamp housing There is a mirror in the housing that can be rotated to select which lamp is reflected onto the slit assembly The mirror can be pulled out and should be when using light sources that mount in the open socket the flat lamp the Th Ar
22. guide the light from the focal plane to the CCD using a fiber optic bundle The only CCD equipped with the necessary fiber bundle is the CCD UMass see section 6 3 The CCD is not quite large enough to encompass the whole free spectral range of most of the orders so provision is made for moving the CCD mounting stage Details of how to do this are given below 5 2 Positioning the Spectrum To determine what portion of the spectrum is imaged onto the CCD you can rotate the cross disperser using the rotating micrometer on the side of the echelle housing See Table 7 for a list of which wavelength regions are imaged onto the CCD at which micrometer angles Figure 7 illustrates where the orders lie on the camera focal plane and the box shows the size of the CCD 5 3 Moving the Focal Plane Stage 11 of 22 Because CCD UMass is not big enough to capture overlapping orders it is possible to move the stage that the CCD is mounted on roughly parallel to the echelle dispersion direction This is accomplished using the micrometer mounted on the left hand side of the dewar when facing the instrument from the dewar side Move so that the micrometer is directly in front of you To move the stage you first loosen the two brass colored bolts on the left and right sides just below the stage Now you can move the stage by turning the micrometer This micrometer is calibrated in inches not meters Attached to the micrometer is a digital readout that can b
23. improve the blue response It has 1024x1024 pixels each 24umx24um The quantum efficiency is between 30 and 40 between 2 500A and 5 000A and then climbs to 65 at 6 000A before declining to 50 longward of 8 000A and finally 30 between 9 000A and 10 000A See Figure 5 panel 1 for the quantum efficiency curve The full well depth is 319 000 electrons and the A D converter resolution is 216 from 0 to 65 535 ADU s giving a unit gain of 4 88 e ADU The bias level at gain of 1 is 944 ADU and the tested dark current at the operating temperature of 100 C is 0 76 e pixel hour See Table 10 This system is being documented by J A Lopez 6 3 The CCD UMass The CCD UMass system has been assembled and packaged by Photometrics 13 of 22 and the whole package is referred to as a Photometrics 3000 system The detector in the CCD UMass camera is a Ford Aerospace PM 512 CCD this division of Ford Aerospace has since been sold to the Loral Corporation It actually has 516x516 pixels each 20umx20um though the user can only use 512x512 The CCD wells are 250 000 electrons deep and the A D converter has a dynamic range from 0 to 2414 1 16 383 ADU s Readout noise is between 7 and 12 electrons pixel rms and the unit gain with CGAIN 0 is 15 e ADU IA UNAM Technical Report 102 A fiber optic bundle is cemented to the face of the chip This bundle is made up of 5um fibers in roughly hexagonal sheaves The operating temperature range 1s
24. is point you should type init 2 This will tell the system to initialize multi user mode 6 The computer will next ask if you wish it to check the file system You should answer y lt return gt unless you know for sure that the file system 14 of 22 is ok and uncorrupted The checking should take no more than 3 to 5 minutes 7 Presuming all has gone well the next prompt you receive will be a standard UNIX login prompt login Log on as ccd no password Congrats the computer is now up and ready to run the CCD control program 8 Type ccd_oan to start up the FORTH interpreter that controls the CCD Since you know how to start the computer you should also know how to shut it off in case the power goes out for example Shut down is a much simpler procedure 1 If you are in ccd_oan type quit to exit 2 At the UNIX prompt type su lt return gt This will log you in as the root user You will now have the access necessary to shut down the computer gracefully At the UNIX prompt type sync lt return gt Now type it again 4 You should now turn off the power on the computer box labeled Photometrics 3000 then the power to the HDS terminal and the Cabel graphics terminal and finally the power on the tape drive Oo CCD control commands are listed in the Photometrics 3000 Users Manual but for convenience we list a few of the most commonly used ones bias This takes a bias frame
25. os alpha x f_col f_cam x Delta x mm For the echelle and camera combinations we have we find the theoretical optimal slit sizes given in Table 11 The focal length of the collimator is 720 mm Note that the optimal slit width is close to what the typical seeing is at the 2 meter 9 References and Other Useful Documents Contents Next Section Prev Section Bitacoras There are separate log books for the various instruments as well as the telescope itself Check all of them since various observers have recorded their runs in different logs Manuals Manual for the Italian Guider the offset autoguider Manual for the CCD Mil elmil control program by L Gutierrez 19 of 22 Manual for the CCD Tek by J A Lopez Photometrics 3000 Manuals The UNIX Manual has a listing of the available CCD control commands The User s Manual has information about the actual hardware This is for the CCD UMass setup IA UNAM Technical Reports IA UNAM Technical Report 35 Espectro de Calibracion del Helio y Argon en Alta Dispersion en el Intervalo Espectral lambda lambda 3470 5525 A y Programas de extraccion de ordenes para el Echelle Mepsicron by F Diego J Echevarria and M Alvarez This report details the wavelength coverage in each of the orders It is quite useful in helping the user to get oriented IA UNAM Technical Report 61 Instructivo Elemental del Sistema CCD Photometrics 3000 by I Cruz Gonzalez and L Carrasco This
26. p For more detailed information please refer to the various atlases now available Both the Thorium Argon and the Helium Argon echelle spectrograms were taken using cross disperser A with the UCL camera and the CCD Mil detector The wavelength increases from top to bottom and left to right The lowest order and longest wavelength is thus at the bottom right and the highest order and shortest wavelength at the top left With each spectrum the cross disperser rotation angle is given this is the angle read off of the rotating micrometer and the range in wavelength and order Several orders in each echelle spectrogram are labeled either on the left or right edge Thorium Argon echelle spectrogram courtesy of W Schuster Helium Argon echelle spectrogram courtesy of J Bohigas This manual converted from TeX to HTML by S Levine Comments questions offers to write this page etc to S Levine Regresar 22 of 22
27. receiver up in the 16 of 22 telescope console room 8 Appendix Some Basic Optics Contents Next Sectionl Prev Section If the set ups change or you just feel like checking the numbers given in the tables we provide for your convenience a few of the more important equations relating to gratings A few definitions first e f cam is the focal length of the camera e f col is the focal length of the spectrograph collimator e gis the number of grooves per millimeter e mis the spectral order and is an integer e xis the position on the detector measured with respect to the position of the blaze wavelength tan psi x f_cam e alpha is the angle of incidence with respect to the grating normal e beta 0is the angle of diffraction of the blaze wavelength of the order beta_0 2 theta_B alpha beta is the angle of diffraction with respect to the grating normal beta beta_O psi e theta B is the blaze angle of the grating e lambda is the wavelength in angstroms e psiis the angle of diffraction away from the angle of diffraction of the blaze wavelength ie psi is an angle away from beta_0 We define the blaze angle to be positive and any angle on the same side of the grating normal is also positive Any angle on the opposite side of the grating normal is negative see Figure 8 In the terms defined above the grating equation is written lambda A 107 A mm sin alpha sin beta g m The blaze
28. rument Put the flat lamp into the lamp holder Remember to push in the diffuser the slide is found on the underside of 8 of 22 the lamp housing and pull out the rotating mirror The diffuser helps to ensure even illumination of the slit 2 Push the knob on top of the lamp housing IN This reflects the lamp light down through the spectrograph slit and blocks out the optical path from the telescope Set the desired slit width and choice of mask 4 Take an n second exposure where n is long enough to mostly fill but not saturate the image Oo For images of the comparison lamps you are presented with a plethora of possibilities but the most commonly used lamps currently are the Thorium Argon lamp and the Helium Argon lamp G Koenigsberger G Canalizo and D Pefia have recently compiled a Th Ar atlas using this echelle In addition there are two high resolution line atlases are available for the Th Ar lamp one from the AAT and one from KPNO and J Echevarria has compiled a He Ar lamp atlas using this echelle To take a spectrum of a comparison lamp 1 Choose the comparison lamp you wish to use and turn it on Remember to rotate the mirror to the proper setting or to pull it out if you are using one of the bulbs that mounts in the same holder as the flat lamp and if necessary to push in the diffuser 2 Push the knob on top of the lamp housing IN This reflects the lamp light down through the spectrograph slit and
29. wavelength of an order m is given by the grating equation when psi 0 so beta beta_0 2theta_B alpha The position on the detector with respect to the position of the blaze wavelength of any wavelength in the order m can be found by solving the grating equation for psi and thence x and works out to be 17 of 22 gm lambda x mm f_cam tan arcsin sin alpha 2theta_B alpha 1047 A mm The linear dispersion can be derived by differentiating the grating equation with respect to beta and using the chain rule to work out the derivative with respect to x dlambda dx A mm 107 A mm cos beta g m f_cam The free spectral range FSR of each order m is given by FSR lambda_blaze m For this echelle for the main grating as pointed out previously g 79 lines mm theta_B 63 433 and alpha 71 so lambda_blaze A 2 244615 x 1045 m and dlambda_blaze dx A mm 7 10291 x 1044 m x f_cam mm where f_cam 215 6 mm for the UCL camera and f_cam 204 mm for the REOSC camera The intensity in each order relative to that of the blaze wavelength can be described very roughly by I beta is proportional to sin A A where A m pi cos theta_B sin theta_B tan alpha beta 2 Again inverting the grating equation we can write beta arcsin g m lambda 10 7 A mm sin alpha and hence compute I as a function of wavelength Within the echelle rem
30. widths FWHM you will realistically be able to achieve are about 2 5 pixels 6 of 22 To focus 1 Turn on an arc lamp push in the knob on top of the lamp housing and set up a small slit say about 40um the key point here is really only that the slit be small enough that the CCD pixel size not the slit width limits the resolution see the appendix and Table 11 2 Unlock the collimator by loosening the three set screws spaced around the side of the housing These should only be finger tight 3 The collimator should now be free to move It is moved by turning the knob of the micrometer found on the bottom of the instrument housing This micrometer can be difficult to turn 4 Take an image noting the micrometer setting 5 Turn the micrometer 0 5 or 1 0 turns and take another image 6 Repeat 4 and 5 until you have acheived a satisfactory focus Be aware that the optimum focus settings may be slightly different for the red and the blue ends of the spectrum 7 Tighten the set screws by hand when you have a good focus 2 2 Changing the Wavelength Region To access different wavelength regions you will need to rotate the cross disperser On your images you will see that this causes motion perpendicular to the dispersion direction of the orders The cross disperser angle is set with a circular micrometer on the lower side of the instrument housing Course adjustment is done by loosening the right thumb screw and turning the gr
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