Characteristics and Use of the CCD Slit Camera on the Italian B&C
Contents
1. The s stands for slit Each time you save an image the sequence number is incremented You can change the sequence number by editing the field adjacent to the Save button Focusing the Telescope The resident astronomer should have focused the slit camera on the slit on the engineering night that precedes each run Once this is done you can use the slit camera to focus the telescope quickly and accurately 1 Find a star between say magnitude 8 and magnitude 15 and place it in the field of the slit camera 20 2 If you have not already done so select the Display in IRAF option which will as its name suggests display subsequent slit camera images both in the image panel and in IRAF Make sure that IRAF is running on the computer on which you started the high level control program These steps will allow you to use imexam in IRAF to give a quantitative measure of the image quality which will make focusing much more precise 3 Take an image by clicking on the Expose button The image should ap pear both in the image panel and in IRAF Examine the star using imexam in IRAF For this to work you have to run imexam without a file name ar gument so that imexam knows to use the image currently being displayed Adjust the exposure time if necessary so that the star is well exposed but not close to saturation 4 Click on the Start Movie button The camera will repetitively take images Agai2. 182 Blue 141 149 15 6 163 169 17 5 18 1 Table 4 Saturation Limits Filter Saturation Limit mag Clear 6 5 Red 4 3 Green 5 1 Blue 4 9 14 Figure 3 A 0 11 second exposure of a saturated 2 magnitude star centered on the slit Tests show that it is possible to accurately center saturated stars 15 4 Procedures Here Ill give procedures for operating the CCD slit camera The CCD camera is sensitive and offers capabilities for making your observing more efficient but this comes at a small cost in complexity I recommend you follow the procedures here to begin with until you develop a feel for the camera this should not take long and then experiment to discover how to use it to best suits your needs Before giving the procedures I d like to mention two things that are key to happy observing 1 Make sure you set the correct rotator angle If you have the wrong rotation angle none of the controls that interact with the telescope will work correctly For example you ll try to move the tele scope to put the slit over your object and it will move in the wrong direction Also the reference mark will be in the wrong place which will doom any attempt to use it to put the slit over a faint object The initial value of the rotator angle is 0 which is correct for the most com mon orientation of the spectrograph with the slit east west and the science CCD to the north If the rotator angle display doe
3. 25 at 7000 0 17 at 8000 0 03 at 9000 Filters Clear Red 6450 A 500 A Green 5300 A 1000 A Blue 4500 A 1200 A a With the slit aligned east west and the science CCD to the north P There is a significant tail of hotter pixels 2 3 Filters The filter wheel is a SBIG ST 8CFW The filters are SBIG clear red green and blue parfocal filters and one unused slot The red green and blue filters were designed for color imaging and do not correspond to any standard astronomical filters The clear filter is used most often as it gives the best sensitivity The central wavelengths and widths of the filters are given in Table 1 More information on the filter wheel is given in the SBIG catalog and on the filters in an SBIG application note 2 4 Control Software The control software allows you to take single images and movies Images are displayed by the software itself and can optionally be displayed in IRAF where they can be examined with imexam and can be saved as FITS files The control software interacts with the telescope You can move the telescope in the frame of the slit camera even when the spectrograph is rotated You can place the slit on or at least near an object by simply clicking on the object and then clicking on a button If a suitable star is visible you can use it to guide automatically The camera control software is split into two programs with the low level part running on an embe
4. places the reference mark a small cyan cross on the image display The offset of the reference mark from the slit mark is given by the associated fields The Move Reference to ROI button moves the telescope so that the object in the region of interest is under or at least near to the reference mark The reference mark and its associated controls are used when placing the slit over a faint object see 4 6 for more information about this 10 K B amp C Slit Camera Expose Start Movie Bias and Dark Exposure Time 1 0 Filter dear i Display in IRAF Save as 0001 s fit CCD Set Point 0 0 CCD Temp Air Temp 127 85 z z Normalz Rotator Angle 0 Slit PA 90 270 Ea xX 1 X Ea Move Slit to ROI Start Guiding Mark Reference 0 0 N 0 0 E _Move Reference to ROI Figure 1 The Control Program Window 11 3 Performance 3 1 Sensitivity The CCD camera is significantly more sensitive than the old intensified video cam era The sensitivity is best demonstrated with an example Figure 2 compares a portion of the DSS scan of the red POSS2 plate with images with the slit camera taken in dark time It shows that stars with R 16 can be clearly seen in 1 second images stars with R 18 can be clearly seen in 8 second images and faint galaxies can be seen in 64 second images If you can see something on a POSS plate you can probably s
5. right of the window The sense of the offset is from your object to the reference object Thus if the coordinates of your object are 12 52 32 66 15 52 25 1 and the co ordinates of your reference object are 12 52 33 21 15 52 11 6 the offsets 23 4 7 should be 13 5 north and 7 9 east Make sure you remember the factor of cos6 Select the Mark Reference option This will place the reference mark a second smaller cyan cross in the image display The reference mark will be offset from the slit mark by an offset equal to the offset of the reference object from your object If you move the slit mark the reference mark moves to to maintain the relative offset You now have to move the telescope so that the reference mark coincides with your reference object Take an exposure and identify your reference object in the image Click on your reference object with the left mouse button which will move the yellow region of interest marker to that position and then click on the Move Reference to ROI button which will command the telescope to move so that the reference mark is coincident with the reference object Note that the Move Reference to ROT button is different from the Move Slit to ROI button Take another image Your reference object should be roughly coincident with the reference mark However because the telescope does not offset precisely your reference object will likely not be pre
6. should be able to see it in exposures of at most a few tens of seconds with the slit camera This makes it easy to move the slit to your object even if the spectrograph is rotated 1 Make sure you have set the correct rotator angle See page 16 for more information 2 Make sure you have an adequate dark See page 16 for more information 3 If necessary adjust the slit mark to coincide with the actual slit position The slit mark is the large elongated cyan cross Initially the slit mark will be the center of the field and will almost certainly not correspond with the actual slit position You can adjust the slit position by clicking in the image with the right mouse button The position of the slit can be seen clearly in images taken with the slit illuminated by the HeAr lamp and in images of the sky of more than a few seconds You can make the slit more obvious by decreasing the display range by clicking on the z button 4 Take an exposure and identify your object in the image Click on your object with the left mouse button which will move the yellow region of interest mark to that position and then click on the Move Slit to ROI which will command the telescope to move the slit to the object 5 Take another image Your object should be roughly coincident with the slit position However because the telescope does not offset precisely your object will likely not be precisely coincident with the slit position Adjust
7. the position of the telescope until you see the slit over your object You can adjust the position of the telescope using the x x y and y buttons to move the telescope which move it in the frame of the slit camera regardless of the rotation of the spectrograph That is the x and x buttons always move the telescope parallel to the slit camera rows and the y and y buttons always more the telescope parallel to the slit columns Move the slit until the slit mark and hence the slit is coincident with your object 6 You are now ready to start guiding and taking spectra 22 4 6 Moving the Slit to a Relatively Faint Object If your object is too faint to see easily in the slit camera normally this will only apply to continuum sources fainter than magnitude 19 or equivalently faint emis sion line sources you have to adopt a strategy of offsetting the telescope from a brighter source that can more easily be seen in the slit camera However the tele scope does not offset especially accurately so blind offset from a brighter source is not especially reliable If your object is sufficiently close to a brighter source you can use the brighter object as a reference This avoids most of the uncertainty associated with blind offsets The basic idea is to place a reference mark in the slit camera offset from the slit by an amount that precisely corresponds to the offset from your object t
8. Characteristics and Use of the CCD Slit Camera on the Italian B amp C Spectrograph Alan Watson Leonel Guti rrez Enrique Colorado Jorge Valdez Gerardo Sierra Esteban Luna amp Alfonso Ginori Instituto de Astronomia UNAM Edition 1 5 of 4 June 2002 Abstract This document describes the characteristics and use of the CCD slit camera for the Italian Boller and Chivens spectrograph on 2 1 m telescope of the Ob servatorio Astron mico Nacional on Sierra San Pedro M rtir This camera replaced the old intensified video slit camera in March 2002 The slit camera has a field of about 1 5 x 1 0 and pixels of 0 346 and can detecting magni tude 18 stars in exposures of a few seconds The slit camera makes it easier to focus the telescope position the slit on the object and check the position of the slit while exposing a spectrum If a suitable star is present in the field the slit camera can be used in guide in place of the offset guide camera Contents 1 2 Introduction Components 2 1 Mount and Optics 2 2 Camera 23 Filters sisca 2 4 Control Software 2 4 1 CCD Controls and Displays 2 4 2 Image Controls and Display 2 4 3 Telescope Controls and Displays Performance 3 1 Sensitivity 3 2 Saturation Limit Procedures 4 1 Starting Up 4 2 Shutting Down 4 3 Taking an Image 4 4 Focusing the Telescope 2 2 0000 4 5 Moving the Sli
9. Figure 7 Rotate the instrument and take more images until the trailed star is exactly parallel to the slit Have the mechanical engineer remove the rotator angle display reset it to zero and replace it 27 5 Problems and Solutions This section describes some problems we ve encountered and suggests some solu tions 5 1 Some Necessary Details First Pll describe the control software and physical and network connections in more detail so you can better reason about the cause of a problem and about pos sible solutions The computers involved in the control software are sonaja and alpha the two Linux PCs in the control room router2m a Linux PC and buscador an em bedded Linux PC on the back of the telescope primary mirror There are two separate networks at the 2 1 meter sonaja alpha and router2m are on the observer network router2m and buscador are on the instrument network router2m serves as a communication conduit between the otherwise separate ob server and instrument networks The camera hardware is connected to buscador by a parallel cable The low level control software runs on buscador When buscador reboots it auto matically runs usr local sbig sbig tcl The high level software actually runs on router2m rather than sonaja or alpha as it needs to be able to talk to buscador When you click on the B amp C Slit Cam era on sonaja or alpha you execute xhost to allow r
10. See page 16 for more information Take bias and dark images See page 16 for more information Find and mark the slit Turn on the HeAr lamp and take an exposure of a 1 second Images with the lamps have strong gradients perpendicular to the slit but the HeAr lamp seems to give a slightly more even illumination than 17 Figure 4 A slit camera dark image of 64 seconds Figure 5 Slit camera images that show the importance of subtracting a suitable dark The left image has no dark subtracted the right image has a dark subtracted Both images have exposures of 64 seconds and are displayed over the same range about their median value The hot pixels in the left image make it difficult to see the faint galaxies in the lower right of the image Figure 6 A slit camera image of the slit illuminated by the HeAr lamp The HeAr lamp gives a slightly more even illumination than the quartz lamp 18 4 2 the quartz lamp Figure 6 shows an example slit camera image with the HeAr lamp Click on the position of the slit with the right mouse button to move the cyan slit mark to coincide with the slit Shutting Down Shutting down the camera basically consists of shutting down the components in order from the high level control program to the camera hardware with appropriate delays There is no general need to shut down the camera during the day unless a storm threatens 1 Stop the high level control program by clicking the Exi
11. an of a red POSS2 plate showing the field of the Landolt standard PG 0918 029B The image shows three stars with magnitudes R 13 5 R 15 9 and R 17 9 and three faint galaxies b A 1 second image with the slit camera The R 13 5 star has S N 100 and the R 15 9 has S N 20 c An 8 second image with the slit camera The R 15 9 star has S N 100 and the R 17 9 star has S N 20 d A 64 second image with the slit camera The galaxies can be seen faintly but clearly Each image is roughly 1 5 x 1 0 and each pixel is 0 346 The filter was the clear filter the seeing was about 1 0 and the sky was dark The dark horizontal stripe is the slit 13 Table 2 Count Rates Filter 15 mag Star Sky electron second electron second binned pixel Dark Grey Bright Clear 4700 4 5 18 0 32 0 Red 620 0 5 3 7 7 0 Green 1300 0 5 3 7 7 0 Blue 1050 0 2 2 2 4 3 Table 3 Limiting Magnitudes for S N 20 Filter Exposure Time second 1 2 4 8 16 32 64 Dark Clear 15 7 165 17 2 17 9 18 6 19 2 19 7 Red 13 5 143 15 0 15 7 164 17 1 17 6 Green 14 4 15 1 15 8 165 17 2 17 9 184 Blue 141 149 156 163 17 0 17 6 182 Grey Clear 15 7 164 17 2 178 184 189 19 4 Red 13 5 143 15 00 15 7 164 17 0 17 5 Green 14 3 15 1 15 8 165 17 2 17 8 18 3 Blue 141 149 156 163 17 0 17 6 18 1 Bright Clear 15 7 164 17 1 17 7 18 3 188 19 2 Red 13 5 143 15 0 15 7 16 3 169 17 4 Green 14 3 15 1 15 8 165 17 1 17 7
12. ange the CCD temperature set point using the menu The panel shows the CCD set point the current CCD temperature and the current air temperature The temperatures are quantized in units of 0 4 C You almost certainly don t want to change the CCD set point 2 4 2 Image Controls and Display The image is displayed in the center of the window The field is 1 5 x 1 0 Each pixel is 0 346 The z and z buttons increase and decrease the display range The Normal z button restores the default display range of 50 The display range is always symmetric about the mean value in the image If you need finer control over the image display you should display the image in IRAF When you click on the image the coordinates and value of the pixel under the cursor are displayed in the above the image The magenta arrow shows the direction of north This is determined from the rotator angle specified in the telescope panel The orientation of the image is the same as the sky if north is up east is left The yellow square marks the region of interest ROI This is used to mark the object prior to moving the slit to it with the Move Slit to ROI button in the telescope panel and to mark the guide star prior to guiding You can move the region of interest by clicking in the image with the left mouse button The large elongated cyan cross is the slit mark This is used obviously to mark the position of the slit for
13. ark your image will have lots of noise from hot pixels The dark should be be at least as long as the exposures you plan to take Figure 5 shows an image which shows inadequate dark subtraction The dark rate depends on the CCD temperature so if the CCD temperature changes you should take a fresh bias image and dark image Dark and bias frames are currently lost when the embedded Linux PC is rebooted so to be safe you should get into the habit of taking a bias and dark whenever you restart the control software Starting Up Starting the camera basically consists of starting up the components in order from the camera hardware to the high level control program with appropriate delays Turn on the camera by plugging the camera power cable into the slit camera body This cable is the rightmost cable on the slit camera body when you face the spectrograph Turn on the embedded Linux PC on the back of the telescope primary mirror cell If you are not sure which one this is follow the parallel cable from the slit camera body Wait one minute for the embedded PC to complete booting Check that the high level control program is not already running on either of the Linux PCs in the control room If it is stop it Start the high level control program on either of the Linux PCs in the control room by clicking on the B amp C Slit Camera icon in the Instruments folder Make sure you have set the correct rotator angle
14. ark current The read noise the dark rate and the sky make similar contributions to the noise in exposures of a few tens of seconds at 0 C with the clear filter in dark time If you wish you can change the set point or disable cooling completely As the camera has no heater if the ambient temperature drops below the set point the CCD will follow To mitigate the dark rate and especially the tail of hot pixels the camera software automatically subtracts a bias image and a dark rate image scaled to the exposure time from each exposure Table 1 Camera and Filter Characteristics CCD Kodak KAF 0401LE Unbinned format 765 x 510 pixels of 9 um x 9 um Binning 3x3 Binned format 255 x 170 pixels of 27 um x 27 um Binned pixel size 0 346 Field 1 5 x 1 0 Orientation North up and east left ADC 16 bits Gain 2 5 electrons Read noise 14 1 electrons Median dark rate Unbinned pixel full well 8 0 electrons second binned pixel at 5 C 4 4 electrons second binned pixel at 0 C 2 4 electrons second binned pixel at 5 C 50 000 electrons Read Time Minimum exposure Maximum exposure 7 seconds 0 11 seconds 3600 seconds Cooling Heating Temperature range Temperature regulation Default temperature Single stage thermo electric cooler None Up to 35 C below ambient 0 1 C 0 C or ambient whichever is cooler Quantum efficiency 0 18 at 4000 0 25 at 5000 0 35 at 6000 0
15. ation 5 Over exposure 5 Pixel coordinates 9 Pixel scale 5 Pixel values 9 Position angle 9 Problems 28 Control software 28 Displaying images with IRAF 29 Guiding 31 imexam 30 Moving the telescope 30 Seeing your object 29 Procedures 16 Aligning the camera 26 Focusing the camera 26 Focusing the telescope 20 Guiding 25 Monitoring the slit position 24 Moving the slit to a relatively bright object 22 Moving the slit to a relatively faint object 23 Setting the origin of the rotator angle display 26 Shutting down 19 Starting up 17 Taking an image 19 Quantum efficiency 6 Quartz lamp 17 Read noise 6 Read time 5 6 Reference mark 9 10 24 Reference offset 10 23 Region of interest 9 ROI see Region of interest Rotator angle 9 16 26 Saturation 5 Saturation limit 12 Save button 8 Scale 5 Sensitivity 12 Set point 5 8 Set point menu 8 34 Setting the origin of the rotator angle display 26 Shutting down 19 Slit mark 9 Slit position angle 9 Start Guiding button 10 Start Movie button 8 Starting up 17 Stop Guiding button 10 Stop Movie button 8 Taking an image 19 Telescope controls 9 Telescope offset 10 Temperature 5 8
16. button changes into a Stop Guiding button To stop guiding click on the Stop Guiding button Guiding will stop after the current exposure 25 4 9 Focusing and Aligning the Slit Camera Each time the camera is mounted on the spectrograph it will require aligning with the optical axis of the transfer lens rotating so that the CCD rows are parallel to the spectrograph slit and focusing This can be done during the day and should be done by the resident astronomer and the technicians when they mount the spectro graph The reasons for focusing the camera and aligning it optically are obvious The reason for rotating the camera so that the slit is accurately parallel to the slit is so that telescope motions in the camera frame will be calculated correctly and so that the reference mark will appear in the correct place 1 Turn on the HeAr lamp and take an exposure of a 1 second Images with the lamps have strong gradients perpendicular to the slit but the HeAr lamp seems to give a slightly more even illumination than the quartz lamp Figure 6 shows an example slit camera image with the HeAr lamp 2 Close to slit so that it is as narrow as possible This gives a clear target for focusing 3 Start taking images Examine the focus and slit alignment while moving the mount until you encounter an adequate focus and alignment For reasons that should be obvious this is known as Twist and Shout With the current stop gap mount t
17. cal engineer to reset it Below the rotator angle field is a display of the slit position angle The rotator angle and slit position angle are related by slit position angle 90 rotator angle The rotator angle also determines the direction of the north arrow in the image panel The x x y and y buttons move the telescope parallel to the axes of the slit camera by the offset specified in the central field The offset is in arcsec onds Thus the x and x buttons move the telescope parallel to the slit and the y and y buttons move the telescope perpendicular to the slit even when the slit is rotated The Move Slit to ROT button moves the telescope so that the object in the region of interest is under or at least near the slit mark The telescope does not move precisely even over small distances and so all of the commands that move the telescope necessarily are equally imprecise For example you should not rely on the Move Slit to ROP command to move the slit precisely to the region of interest it should move it roughly to the region of interest but then you ll have to make fine adjustments The Start Guiding button starts guiding on the star in the region of interest While guiding the button changes to a Stop Guiding button and clicking on this button will stop guiding at the end of the current exposure The Mark Reference option
18. cisely coincident with the reference mark Adjust the position of the telescope until the reference object and reference mark are coincident You can adjust the position of the telescope using the x x y and y which move it in the frame of the slit camera regardless of the rotation of the spectrograph When the reference object is coincident with your reference mark the slit should be over your object You are now ready to start guiding and taking spectra Monitoring the Slit Position Happiness is a black stripe through your object To achieve this you will want to monitor the position of the slit while you take spectra Make sure you have an adequate dark See page 16 for more information Take an image Adjust the exposure time and the display range so that you can see your object centered on the slit or better so that you could see your object if all of its light wasn t going down the slit 24 3 Click on the Start Movie button This will repetitively take images You 4 8 should be able to monitor the position of the slit If the slit drifts off your object you can correct it by stopping guiding with the offset guide camera if you are using it moving the slit with the the 6 99 66 99 66 x x y and y buttons which move the telescope in the frame of the slit camera regardless of the rotation of the spectrograph and restarting g
19. d on the spectrograph opposite the science CCD The current mount is a stop gap The slit is imaged with a 1 1 lens 2 2 Camera The camara is a Santa Barbara Instruments Group SBIG ST 7E camera with a non intensified Kodak KAF 0401LE CCD The characteristics of the CCD are given in Table 1 More information on the camera and CCD is provide in the SBIG catalog and the Kodak data sheet The field of the camera is 1 5 x 1 0 The scale is 0 346 per pixel The orientation is the same as the sky if north is up east is left The slit camera CCD has similar characteristics to a science CCD except that it is physically small has a lower quantum efficiency because the anti blooming charge drain covers about 30 of the pixel area and is operated at a higher tem perature The CCD is not intensified so over exposing will not damage it Furthermore each pixel has an anti blooming drain which grounds charge in excess of the pixel full well and so saturation should not result in blooming up the columns The camera maximum frame rate is one frame every seven seconds This may seem slow but in practice it does not present a problem especially given the capabilities of the software to automatically place the slit on or at least near an object The camera has a thermo electric cooler and the CCD temperature set point is set to 0 C by default This balances avoiding generating excessive heat in the dome while maintaining a reasonable d
20. dded Linux PC attached to the back of the telescope primary mirror cell and the high level part running on either of the Linux PCs in the control room You interact with the high level program the high level program interacts with the low level program and the low level program interacts with the camera hardware To start the high level program click on the B amp C Slit Camera icon in the Instru mentos folder on the desktop of either of the Linux PCs in the control room The low level program starts automatically when the embedded Linux PC is booted Figure 1 shows the window of the control program It looks complicated but in practice it is fairly natural to use The CCD controls and displays are on the left and the telescope controls and displays are on the right In the middle is the image display a status line and an Exit button The following sections briefly describes each item in the window 2 4 1 CCD Controls and Displays The CCD exposure controls are on the upper left of the window Clicking the Expose button takes an exposure You can change the exposure time using either the Exposure Time menu or by editing the adjacent entry field The exposure time is given in seconds There is no way to cancel an exposure once it is in progress You can select the filter using the Filter menu After an exposure is read a bias image and a dark rate image scaled by the exposure time are subtracted The resulti
21. e See page 16 for more information Make sure you have really set the really correct rotator angle See page 16 for more information Make sure you understand that the control software always moves the tele scope and not conceptually your object Remember that the telescope offsets imprecisely 30 5 7 Problems Guiding If you have problems guiding with the slit camera control software consider the following 1 Make sure you have set the correct rotator angle See page 16 for more information 2 Make sure you have really set the really correct rotator angle See page 16 for more information 3 Make sure your star is bright enough I don t really know how bright stars have to be to guide well but I d guess the limit would be magnitude 17 with 8 second exposures 4 Make sure your integrations are not too long I don t really know how long integrations can be and still result in good guiding but I d guess 8 seconds 31 6 Future Plans Planned modifications to the slit camera include e Acquiring an Ho filter to reduce the background when looking at emission line nebulae and to allow bright stars to be observed without saturation e Doubling the field by replacing the 1 1 lens with a 2 1 focal reducing lens e Making focusing and aligning the camera easier by replacing the current stop gap mount with one that allows fine control of the position of the cam era e Modifying the guidin
22. ee it with the slit camera More quantitatively Table 2 gives the count rates in each filter of a star with mag nitude 15 and of the sky The dark sky rates were measured directly from obser vations with the moon below the horizon The grey and bright sky rates are scaled from observations about 13 from the 35 illuminated moon and refer to the 50 and 100 illuminated moon Table 3 shows the limiting magnitudes at 0 C for each filter over a range of exposure times The limiting magnitudes quantitatively corresponds to S N 20 in a 2 diameter aperture and empirically corresponds to the faintest level at which stars can be clearly seen see Figure 2 for examples of stars at this S N 3 2 Saturation Limit The slit camera has a saturation limit corresponding to stars that saturate the ADC in the shortest exposures of 0 11 seconds see Figure 3 Obviously the saturation limit depends on the image quality Table 4 shows the saturation limits for image quality of roughly 1 FWHM Saturating the CCD will not damage it and should not result in blooming up the columns as the CCD is not intensified and has anti blooming gates Furthermore tests show that it is possible to accurately center saturated stars 12 a DSS scan of POSS red plate b Slit camera 1 second image c Slit camera 8 second image d Slit camera 64 second image Figure 2 Example slit camera images that illustrate its sensitivity a The DSS sc
23. g algorithm to better account for systematic errors in the telescope tracking Other suggestions which have been made and which are still under consideration include e Allowing exposures to be aborted e Encoding the instrument rotator e Improving the frame rate 32 Index x button 10 y button 10 z button 8 x button 10 y button 10 z button 8 Aborting an exposure 8 Aligning the camera 26 Anti blooming drains 5 Bias and Dark button 8 16 Bias image 5 8 16 CCD 5 6 CCD controls 8 Control software 7 28 Cooling 5 6 8 19 Coordinates 9 Count rate 12 Dark image 5 8 16 Dark rate 5 6 Display in IRAF option 8 29 Display range 8 Exit button 7 19 Expose button 8 Exposure time 8 Exposure Time menu 8 Field 5 32 Filter 8 Filter menu 8 Filter wheel 7 Filters 6 7 FITS 8 Focusing the camera 26 Focusing the telescope 20 33 Frame rate 5 6 32 Future plans 32 Gain 6 Guiding 25 31 32 Hardware 28 HeAr lamp 17 Image display 8 Image display range 8 imexam 30 imexam 8 IRAF 8 29 30 Limiting magnitude 12 Mark Reference option 10 24 Monitoring the slit position 24 Mount 5 Move Reference to ROI button 10 Move Slit to ROI button 10 Moving the slit to a relatively bright object 22 Moving the slit to a relatively faint object 23 Moving the telescope 10 30 Normal z button 8 North 9 Optics 5 Orient
24. gain knob but a manual is necessary for the CCD camera One of the aims of the new camera was to be more sensitive that the old camera and this manual explains how to achieve this sensitivity and another aim was to provide capabilities to aid you in focusing and placing your object on the slit and again this manual explains these However if reading manuals is beneath your dignity you can get something that works for bright objects simply by starting up 84 1 clicking the Start Movie button in the control program window and shutting it down at the end of the night 84 2 You ll get exposures every seven seconds and should be able to see stars down to magnitude 15 or 16 If you want to do better unfortunately you ll have to read this manual The CCD slit camera is the result of work by Leonel Guti rrez Enrique Colorado Jorge Valdez Gerardo Sierra Esteban Luna Alfonso Ginori and Alan Watson with the assistance of the mountain technical staff of the OAN SPM If you have prob lems with or comments on the slit camera please address them to Leonel Guti rrez leonel astroesen unam mx and Alan Watson a watson astrosmo unam mx This manual was written by Alan Watson with input from Philippe Eenens Ana Hi dalgo Michael Richer and Sergei Zharikov If you have comments on this manual please address them to Alan Watson a watson astrosmo unam mx 2 Components 2 1 Mount and Optics The CCD and filter wheel are mounte
25. his is somewhat haphazard which the main motivation for the plans to replace the current mount If you display the images in IRAF you can use imexam and especially the k command to examine the focus more precisely 4 10 Setting the Origin of the Rotator Angle Display Each time the spectrograph is mounted on the telescope the rotator angle display needs to be set so that it reads 0 when the slit is exactly east west This cannot be done during the day and should be done by the resident astronomer and the technicians on the engineering night that normally precedes each run The reason for setting the origin accurately so that telescope motions in the camera frame will be calculated correctly and so that the reference mark will appear in the correct place 26 Figure 7 Checking the alignment of the slit by trailing a star along the slit The basic idea is to rotate the spectrograph until a star trailed east west is parallel to the slit Find a relatively bright star between say magnitude 6 and magnitude 10 Place it at one end of the slit and slightly above or below the slit Turn on the HeAr lamps to illuminate the slit jaws and select the narrowest slit 60 um This will allow you to clearly distinguish the slit Start an exposure of say 4 seconds and immediately offset the telescope east or west as appropriate by 60 to trail the star along the slit during the exposure The result should look something like
26. is running on the computer on which you started the high level control program Click on the Expose button If you asked for an exposure of longer than 64 seconds you will be asked to confirm the exposure time This is because it is impossible to abort an exposure once it has started The camera will take an exposure read the image and display it in the image panel The field is 1 5 x 1 0 Each pixel is 0 346 The image should also be displayed in IRAF if you selected this option and you can examine with imexam For this to work you have to run imexam without a file name argument so that imexam knows to use the image cur rently being displayed If you see lots of hot pixels it s probably because you didn t take a dark because your dark is not long enough or because the CCD temperature has changed Figure 5 shows an example of an image with inadequate dark sub traction The solution to all of these problems is to take another dark possibly longer and retake you image If you wish to change the display range click on the z and z buttons To revert to the standard display range click on the Normal z button If you wish to save your image click on the Save button The image will be saved as a FITS file in the directory home observa imagenes on the Linux PCs in the control room The image name will be a four digit sequence number followed by s fit e g 0001s fit
27. n each should appear both in the image panel and in IRAF Focus the telescope while using the r a and e commands in imexam to de termine the image quality For this to work you have to run imexam without a file name argument so that imexam knows to use the image currently being displayed There is no need to exit from imexam between images measures are taken so that IRAF notices that each image is different The scale is 0 346 per pixel You should focus the telescope by first lowering the secondary and then rais ing it slowly When you find an optimum position you should lower the secondary and then raise it once more to the optimum position This pro cedure keeps the secondary actively supported against gravity and results in more accurate and repeatable positions The secondary is lowered with the Baja button and raised with the Sube button on the hand paddle The encoded secondary position is displayed in the patch panel in the control room raising the secondary causes the dis played numbers to become more negative 5 Stop the sequence of exposures by clicking on the Stop Movie button The sequence will stop after the current exposure 6 If you wish disable the Display in IRAF so that subsequent slit camera images will appear only in the image panel and not in IRAF 21 4 5 Moving the Slit to an Relatively Bright Object If your object is relatively bright roughly magnitude 19 or brighter you
28. nd dark images See page 2 for more information Make sure you re using the clear filter The other filters are much less sensi tive Make sure you re taking a long enough exposure Make sure you know the offsets between the telescope offset guide camera and slit camera by looking at a bright star Make sure the camera is working by taking an image with the HeAr lamp on you should see something that looks like Figure 6 Problems Displaying Images in IRAF If you have problems displaying slit camera images in IRAF consider the follow ing 29 5 5 Make sure you ve selected the Display in IRAF option Make sure you re running IRAF on the computer on which you re displaying the high level control program Restart ximtool or ds9 Make sure home observa imagenes is mounted Make sure home observa imagenes is not full Problems with imexam If you have problems with imexam consider the following 5 6 Stop imexam and start it again making sure you don t give it a file name argument i e just type imexam and press return so that imexam knows to use the image currently being displayed Stop imexam issue the lpr command a few times and start it again Restart IRAF Problems Moving the Telescope If you have problems moving the telescope with the slit camera control software consider the following Make sure you have set the correct rotator angl
29. ng image is is displayed in the central panel If you select the Display in IRAF option the image will also be displayed in IRAF If you display a slit camera image in IRAF you can examine with imexam For this to work you have to run imexam without a file name argument so that imexam knows to use the image currently being displayed Each time you click the Save button the current image is saved as a FITS file The image will be saved in the directory home observa imagenes on the Linux PCs in the control room The image name will be a four digit sequence number followed by s fit e g 0001s fit Each time you save an image the sequence number is incremented You can change the sequence number by editing the field adjacent to the Save button Clicking the Start Movie button starts a repetitive sequence of exposures During this sequence the button changes into a Stop Movie button and clicking this button will stop the movie after the end of the current exposure Clicking Bias and Dark button takes a bias image and dark image Only the dark image is displayed The images are used to calculate a dark rate image and the bias image and the dark rate image scaled by the exposure time are subtracted from subsequent exposures You should select a long exposure time when taking dark images to reduce the noise in the dark rate image The CCD temperature controls are in the lower left of the window You can ch
30. o the reference object Then if you place the reference mark over the reference object the slit should be over your object You will need accurate relative astrometry of your object and reference object and you will also need the CCD rows aligned with the slit see page 26 and the instrument rotator origin accurately set to zero when the slit is east west see page 26 Your reference object has to be close enough to be visible in the slit camera while your object is on the slit remember that the field of view of the slit camera is only 1 5 x 1 0 1 Make sure you have set the correct rotator angle See page 16 for more information 2 Make sure you have an adequate dark See page 16 for more information 3 If necessary adjust the slit mark to coincide with the actual slit position The slit mark is the large elongated cyan cross Initially the slit mark will be the center of the field and will almost certainly not correspond with the actual slit position You can adjust the slit position by clicking in the image with the right mouse button The position of the slit can be seen clearly in images taken with the slit illuminated by the HeAr lamp and in images of the sky of more than a few seconds You can make the slit more obvious by decreasing the display range by clicking on the z button 4 Enter the reference offset in arcseconds north and east from your object to the brighter reference object in the fields in the lower
31. outer2m access to the X dis play then rsh to router2m and there execute usr local boller_sbig boller or the experimental version home observe boller_sbig boller with DIS PLAY set back to sonaja or alpha To display images in IRAF the high level program does an rlogin to the computer to which is it displaying and executes some IRAF commands 5 2 Problems with the Control Software If you have problems with the control software you can t start it it is hung or confused or it complains that there are problems consider the following 1 Check that you re not already running the high level program possibly on the other Linux PC 28 5 3 Check that buscador and the camera have power Try shutting down and restarting the high level control program buscador and the camera according to the procedures on page 17 Check that you can ping router2m from the Linux PC If not there s a prob lem either with the network or with router2m Problems Seeing Your Object If you have problems seeing your object consider the following 10 5 4 Check your coordinates Check the pointing of the telescope Make sure you re not looking at the back of the offset guide camera mirror Checking your object in the field of the slit camera Remember that the field of the slit camera is only 1 5 x 1 07 Checking that the slit is not over your object Make sure you re got adequate bias a
32. s not read 0 in this orienta tion you should ask the mechanical engineer to reset it If the spectrograph is rotated the rotator angle should be set to the angle given by the rotator angle display on the back of the primary mirror cell Get into the habit of checking the direction of north shown by the magenta arrow in the image display This is derived from the rotator angle If the software s idea of north doesn t agree with your idea of north at least one of you must be confused 2 Make sure you have adequate bias and dark images To take bias and dark images set a suitably long exposure at least 64 seconds using the Exposure Time menu or the adjacent field and then click on the Bias and Dark button If you asked for a dark of longer than 64 seconds you will be asked to confirm the exposure time This is because it is impossible to abort a dark once it has started The camera will take first a bias image and then a dark image Once the dark image is read it should appear in the image panel Figure 4 shows an example of a dark image The purpose of taking bias and dark images is to reduce the effects of hot pixels on your exposures After you take a bias image and a dark image the control software uses them to calculate a dark rate image When you 16 4 1 subsequently take an exposure the bias image and the dark rate image scaled to the exposure time are subtracted If you don t have an adequate d
33. t button or by clicking 4 3 the x button in the title bar You ll be asked if you want to exit and you should reply that you do Turn off the embedded Linux PC on the back of the telescope primary mirror cell If you are not sure which one this is follow the parallel cable from the slit camera body Wait at least a minute to give the thermo electric coolers time to settle and then turn off the slit camera by unplugging the camera power cable from the camera body This cable is the rightmost cable on the camera body when you face the spectrograph Taking an Image The fundamental operation of the slit camera is not surprisingly to take images of the slit This is straightforward and most of the time only involves the Ex pose button and the Exposure Time menu and occasionally the Bias and Dark button 1 Make sure you have adequate bias and dark images See page 16 for more information Select the exposure time from the Exposure Time menu or by editing the adjacent field Exposure times are specified in seconds Exposure times of less than the minimum of 0 11 seconds are treated as 0 11 seconds Select the filter from the Filter menu The clear filter is the default and gives the best sensitivity so you probably don t need to change it 19 4 If you wish to display the images in IRAF then you should select the Dis 4 4 play in IRAP option and make sure that IRAF
34. t to an Relatively Bright Object 4 6 Moving the Slit to a Relatively Faint Object 4 7 Monitoring the SlitPosition 4 8 Guiding 4 9 Focusing and Aligning the SlitCamera 4 10 Setting the Origin of the Rotator Angle Display Problems and Solutions 5 1 Some Necessary Details oo oo cromo ee ee 5 2 Problems with the Control Software 5 3 Problems Seeing Your Object 6 0c we ee ee 5 4 Problems Displaying Images in IRAF 5 5 Problems with imexam naaa 5 6 Problems Moving the Telescope 4 5 7 Problems Guiding Future Plans Index Oe O ON NAUA 12 12 12 16 17 19 19 20 22 23 24 25 26 26 28 28 28 29 29 30 30 31 32 33 1 Introduction The Italian Boller and Chivens spectrograph is one of the most heavily used in struments on the 2 1 m telescope of the Observatorio Astron mico Nacional on Sierra San Pedro Martir In March 2002 the old intensified video slit camera was replaced by a CCD camera The most important characteristics of the CCD slit camera are e It has a 1 5 x 1 0 field and 0 365 pixeles See 2 2 e Is much more sensitive than the old camera In dark time it allows you to see stars of magnitude 15 7 in 1 second exposures 17 9 in 8 second exposures 19 7 in 64 second exposures If you can see something on a POSS plate you can probabl
35. uiding once more Once you ve finished taking spectra of this object Stop the sequence of exposures by clicking on the Stop Movie button The sequence will stop after the current exposure Guiding If a suitably bright star falls in the field of the slit camera you can guide using the slit camera rather than the offset guide camera The advantage is that you avoid problems of flexure and so guide more accurately I ve only very limited experience with guiding It works relatively well with bright stars My advice would be to guide on stars that have good S N and to not take exposures longer than 8 seconds to avoid a slow correction rate which limits you to stars brighter than about magnitude 17 Make sure you have set the correct rotator angle See page 16 for more information Make sure you have an adequate dark See page 16 for more information Take an image and select the star on which you wish to guide Adjust the exposure time so that the guide star is well exposed Click on the star in the image with the left mouse button which will move the yellow region of interest marker to the position at which you clicked then click the Start Guiding The software will repetitively take images determine the position of the star in each calculate the correction necessary to restore the star to its original position and send this correction to the telescope While guiding the Start Guiding
36. y see it with the slit camera in exposures of a few seconds or tens of seconds See 3 1 and especially Figure 2 e The limiting magnitudes in bright time are only slightly worse being 19 2 in 64 second exposures See 3 1 e It will saturate on stars brighter than about magnitude 5 but tests show that saturated stars can still be accurately centered on the slit See 3 2 e It can display images in the control window of the camera and optionally in IRAF where you can be examine them with imexam See 2 4 1 e It can save images as FITS files See 2 4 1 e It enables you to quickly and accurately place the slit over visible objects even when the spectrograph is rotated See 4 5 e It enables you to quickly and accurately place the slit a known offset from visible objects even when the spectrograph is rotated This allows you to ac curately place the slit over very faint objects that are close to visible objects provided the offset between the objects is known and provided the offset is sufficiently small that the visible object falls in the field See 4 6 e It enables you to quickly and accurately focus the telescope See 4 4 e It gives you the option of guiding using the slit camera rather than the offset guide camera See 4 8 The improved capabilities of the slit camera come at a small cost in complexity A manual for the old intensified video camera would have been absurd as all it had was a
37. your own reference and also to mark the slit prior to moving the slit with the Move Slit to ROT button in the telescope panel Initially the slit mark is located at the center of the field and will almost certainly not correspond to the actual slit position You can move the slit mark by clicking in the image with the right mouse button If you enable the Mark Reference option in the lower right of the window the reference mark appears in the image display This is a small cyan cross This reference mark will be offset from the slit mark by the offsets specified in the fields associated with the Mark Reference option The reference mark is used when placing the slit on faint objects see 4 6 for more information about this 2 4 3 Telescope Controls and Displays The telescope controls and displays are located on the right of the window The most important telescope control is the field that specifies the rotator angle Unless this is correct none of the other telescope controls will work correctly When you rotate the spectrograph you should read the rotator angle display on the back of the primary mirror cell and enter the rotation angle here The initial value of the rotator angle is 0 which is correct for the most common orientation of the spectrograph with the slit east west and the science CCD to the north If the rotator angle display on the telescope does not read 0 in this orientation you should ask the mechani
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