The spectrograph uses a 1024x1024 Aladdin 3 Insb array. The saturation well-depth is about 8000 Data Numbers (DN) for the default bias of 0.4V, the gain 13.0 electrons/DN, the dark current 0.2 electrons/sec over long integrations and the readnoise 50 e RMS per read. With 32 non-destructive reads (NDRs) (minimum itime about 17 sec with default SlowCnts of 20 - see below) the readnoise is reduced to about 12 e RMS. The number of NDRs is automatically selected by the software. Shorter itimes reduce the number of NDRs. If efficiency is a concern then the number of NDRs can be set manually (see C). The SlowCnts parameter allows the array to be readout at different speeds. With 20 SlowCnts the minimum ITIME is 0.51sec, with 3 SlowCnts the minimum itime is 0.18sec. The penalty for faster read out is higher readnoise although in practice the reason for increasing the readout rate is because of higher background where readnoise is not an issue. At 0.4V bias the array is linear to about 5 percent up to 4000DN. FOR GOOD LINEARITY CORRECTION (in Spextool) AND TO SAFELY AVOID SATURATION, IT IS STRONGLY RECOMMENDED THAT COUNTS BE KEPT BELOW ABOUT 4000 DN.
The SpeX guider uses a 512x512 Aladdin 2 Insb array. The saturation well-depth is about 5000 Data Numbers (DN) for the default bias of 0.4V, the gain 13 electrons/DN, the dark current 1.5 electrons/sec over long integrations and the readnoise 65 e RMS per read. Up to 2500DN the device is linear to about a 10 percent. The full guider array (512x512 pixels) minimum ITIME is 0.24sec using the default value of 12 SlowCnts. With 3 SlowCnts the minimum ITIME is 0.12sec. However, at this speed fixed pattern noise becomes noticeable. Subarrays can be used to reduce the ITIME further For example, a 256x256 subarray has a minimum ITIME of 0.04sec using 3 SlowCnts. For this particular device, increasing the bias does not usefully increase the well-depth. TO SAFELY AVOID SAFELY AVOID SATURATION, IT IS STRONGLY RECOMMENDED THAT COUNTS COUNTS BE KEPT BELOW ABOUT 2000 DN (although IR guiding works well on saturated images). Observers should note that the guider array is currently not corrected for linearity. To get photometry to 1% level the object and standard need to be exposed to approximately the same well depth, otherwise photometry can be off by up to 10%. Work is underway to include linearity correction (ask). Flat fielding can be done on the sky or dome. For wavelengths longer than 2.5 microns flat fielding does not work but dithering images can achieve a few percent photometry.
Spectrograph array 1024x1024 For the default number of SlowCnts (20) the spectrograph array takes 0.51sec to read out per NDR. If the ITIME is long enough, up to 32 NDRs will be taken. The maximum number of NDRs is set in the SETUP menu. For example, the minimum ITIME for 32 NDRs is 0.51x32=16.3sec, so if ITIME=17sec the program will automatically set NDR=32 and the real time for the 17sec on-chip integration is 17+16.3=32.3sec - an efficiency of about 50%. The trade-off is between readnoise and efficiency. In theory, the readnoise is reduced by the square root of the number of reads. In practice, we see little reduction beyond 32 NDRs (50 e RMS for 1 NDR compared to 12 e RMS for 32 NDRs for the spectrograph array - see plot). The observer has the option of manually setting the number of NDRs in the Setup menu to improve efficiency which might be an issue for time resolved observations. Example, SlowCnt 20 (default) ITIME=4.0sec, default NDRs=4.0/0.51=7, real time=4.0+(7x0.51)=7.6sec per coadd efficiency = 53% SlowCnt 20 (default) ITIME=4.0sec, manually set NDRs to 2 real time=4.0+(2x0.51)=5.0sec per coadd efficiency = 80% Example, SlowCnt 20 (default) ITIME=120sec, default NDRs=32, real time=120+(32x0.51)=136.3sec per coadd efficiency = 88% With SlowCnts manually set to 3, the array read out takes 0.18sec so the efficiency is higher (but so is the readnoise due to the faster read out of the array). Example, SlowCnt 3 ITIME=4.0sec, NDRs=1, real time=4.0+(1x0.18)=4.18sec per coadd efficiency = 96% The setup for the fastest read out possible is: SlowCnt2 ITIME=0.1sec, NDRs=1, real time=0.1+(1x0.1)=0.2sec per coadd efficiency = 50% This works well on very bright stars. IF YOU CHANGE THE DEFAULT SETTINGS OF NDRs AND SlowCnts REMEMBER TO CHANGE THEM BACK FOR LONGER ITIMES OTHERWISE NOISE WILL BE ADDED. Guider/Imager array 512x512 For the default number of SlowCnts (12) the guide array takes 0.24sec to read out per NDR. If the ITIME is long enough, up to 16 NDRs will be taken. The maximum number of NDRs is set in the SETUP menu. For example, the minimum ITIME for 16 NDRs is 0.24x16=3.84sec, so if ITIME=3.9sec the program will automatically set NDR=16 (if the default has not been changed) and the real time for the 3.9sec on-chip integration 3.9+3.84=7.74sec - an efficiency of about 50%. The best setup for thermal imaging in the L' filter is ITIME=0.2 SlowCnt=9 (NDRs automatically set to 1). real time=0.2+(1x0.2)=0.4 per coadd efficiency = 50% The efficiency in this mode can be increased by decreasing SlowCnts to the acceptable minimum of 3 for the guider which has a minimum ITIME of 0.12sec, but at the cost of increased column structure (noise): ITIME=0.2 sec, NDRs=1 real time=0.2+(1x0.12)=0.32 per coadd efficiency= 63%
Depending upon the number of counts 1024x1024 data files are 2MB or 4MB in size. They are saved in FITS format as filename0001.a, filename0002.b etc., where .a and .b refer to the the telescope beam position and 'filename' is string of unix characters up to 20 in length. All the instrument parameters are saved to the fits header (integration time parameters, telescope position, mechanism positions etc.). Depending on the ITIME, up to several hundred 1024x1024 images can be acquired per night, so observers should be prepared to backup ~2GB of data per full night of observing. 1024x1024 images take about 0.25sec to save to /scrs1 across the network and about 0.10sec to save to the /data local disk. The time taken to read the TCS information varies from 0.1-0.5sec so for the best efficieny TCS communication can be turned off in the Setup menu. DV display can also be turned off but the effect on efficiency is small.
Data is usually saved to the 70 GB scratch disk on stefan eg. /scrs1/bigdog/your_name/date, where bigdog indicates spectrograph data. The path is setup by clicking AUTOSAVE ON in the bigdog xuiwindow and then typing in the path. Similarly for the IR guider/imager e.g. /scrs1/guidedog/your_name/date, where guidedog indicates guider data. These data are owned by bigdog and guidedog, irrespective of the path name, and so you must open a bigdog or guidedog window to erase data. For better efficiency movie mode data are usually saved to the local disks on bigdog or guidedog eg. /data/your_name/bigdog/date and /data/your_name/guidedog/date. The local /data disk have much smaller capacities and so the space must be managed carefully.
Note that the header parameter DIVISOR is the product of NDRs and COADDS. Fits files must be divided by ITIME x DIVISOR to get DN/sec. The SpeX Data Viewer (DV) is set by default to divide by the DIVISOR (under SETUP) but check to make sure. Data reduction packages such as IRAF do not automatically divide by ITIME X DIVISOR to get DN/sec, this must be done explicitly. Observers frequently make this mistake and find that their counts (DN/sec) are too large by a factor of the DIVISOR. Spextool automatically accounts for the DIVISOR.
It is important to make sure that signal levels on the both arrays remain in the linear operating range. FOR THE SPECTROGRAPH COUNTS MUST BE LESS THAN ABOUT 4000DN. FOR THE GUIDER ARRAY COUNTS MUST BE LESS THAN ABOUT 2500DN. (An exception to this is when guiding. Guiding works even on saturated objects.) The spectrograph calibration macros keep signal levels below about 4000DN. Check the signal by running the cursor across the spectrum or image and by drawing an x_linecut through the image or spectrum. Make sure that the DV window is set to 5000 when checking for saturation and then zoom on window to measure individual pixlels. Always check to make sure that dividing by the DIVISOR is set on in DV 'SETUP' window(see below). Always do this on an unsubtracted object frame since subtracting off a sky frame can lead to an erroneously low value (particularly when observing at thermal wavelengths). If the signal level is too high reduce the ITIME. In the guider the signal can also be reduced by using a narrower filter in the guider filter wheel. A complication is that in heavy saturation the measured signal can actually fall to zero. (Signal = first read - last read = saturation - saturation = 0) However, heavy saturation is usually obvious, appearing as a central dip or valley in the PSF. Signal levels do not change with NDRs or COADDS since DV is set by default to divide by the DIVISOR (= NDRs x COADDS). However, dividing by the DIVISOR can be turned off in the DV 'SETUP' window, in which case saturation level will vary with NDRs and COADDS. Always check to make sure that dividing by the DIVISOR is on.
These effects are more apparent in the spectrograph array due to the lower backgrounds and longer integration times. PERSISTENCE The arrays suffer from image persistence at the level of about one percent. The residual image decays with time over a period of minutes. We have reduced the effect to 1-2% by continuously resetting the array at 1Hz when the array is idle. However, image persistence can still dominate dark current when following calibrations by long integrations on faint objects. When nodding objects up and down the slit a 1-2% residual of the spectrum is always seen in an unsubtracted image if the object is sufficiently bright. DOME EFFECT As a result of background resets to minimise persistence, the array cools slightly at the start of long integration sequences since there is less power dissipation. The cooling causes the array bias level to change slightly during the first pair of an observing sequence. For faint objects the first A-B pair subtraction shows a slight dome-like structure instead of appearing flat. By the second A-B pair the bias is stable and the subtraction appears flat. TACHYONS Tachyons are bright or dark circular features of up to about 100 pixels diameter which occasionally appear in long integration time subtracted image pairs. We originally called these features "tachyons" because they might have been coming from another dimension for all we knew. We now understand them to be due to outgassing of adsorbed molecules cryo-pumped onto the array during cooling. Following cooldown the frequency of tachyons decays with time and are usually not detectable in long dark exposures after days to a couple of weeks. However, we now completely eliminate them by turning on array temeprature control to prevent cooling below the operating temperature of 30K, or by warming the array slightly (above 80K) should the array cold soak for anytime below 30K during cool-down.
ITIME Integration time. The time between array readouts, ie. the on-chip integration time. The minimum ITIME depends on array readout rate (Slowcnts) and number of pixels (sub-arrays). The maximum ITIME is arbitrarily set to 1800sec. For the spectrograph the minimum full array itime is 0.51 sec with the default SlowCnts=20 and 0.10 sec with SlowCnts=2 (see discussion of SLOWCNTS below).
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COADDS (co-additions) The number of integrations summed together per image/data file. Individual coadds lose their identity, only the sum is stored. The reason for doing COADDS rather than increasing the ITIME is usually to avoid on-chip saturation and at the same time avoid the additional data storage and time overheads involved in doing CYCLES.
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BEAM.PATTERN Beam.Pattern A integrates at the A (object) beam position. Beam.Pattern B integrates at the B (sky) beam position. Beam.Pattern A&B integrates at the A beam position and then moves the telescope to the B beam and integrates there. At each position the integration is given by the ITIME and number of COADDS
CYCLES The number of times a given OBSMODE is repeated. Example: ITIME=5.0sec COADDS=6 OBSMODE=2 CYCLES=3. 6 images will be stored, 3 object and 3 sky. Each image will consist of 6 summed 5.0sec on-chip integrations for a total time per image of 30 sec (not including the small overhead involved in doing coadds).
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NDRs (Non-Destructive Reads) The number of times the array is read out per coadd per image. (For a description of the read out modes see SpeX User's Guide Volume Two in Users Documentation section of SpeX webpage). Increasing NDRs lowers the readnoise but increases the minimum ITIME.
Bigdog Screen Guidedog ScreenSLOWCNTS (Slow-counts) This variable sets the number of NOPs (No Operations) or delays in the DSP clocking algorithm. Increasing SlowCnts therefore slows down the readout which reduces readnoise. Reducing SlowCnts from increases readnoise and column structure. However it decreases the mimimum full array integration time from 0.51 sec (20 SlowCnts) to 0.1 sec (2 SlowCnts). This is useful for obtaining spectra spectra on very bright objects and for thermal imaging with the guider. If SlowCnts are reduced ALWAYS remember to set SlowCnts back to the default values (20 for the spectrograph and 12 for the guider) this is not done automatically. SlowCnts is set in the OBS menu to the right of AutoSave. Bigdog Screen Guidedog Screen