GNIRS Observing Strategies and Guidelines

This page brings together information that might affect decisions on observing strategy in various GNIRS observing modes, and provide guidelines/tips to maximize observing efficiency and avoid common errors.  The justification for most of the guidelines here can be found on the various GNIRS web pages and OT/Phase II instructions-- if you discover inconsistencies, please let us know! 

As always, once your Phase II definitions are complete, please be sure to consult the GNIRS checklist.

Planning you observing sequences:

• Choose an appropriate read mode that minimizes your observing overheads and read noise
• Be Aware of the maximum exposure time for your observing case
• Create acquisition sequences for all your targets
• Choose stars for guiding
• Offsetting the telescope is needed to effectively remove sky background.
• To maximize the source signal spectroscopy, follow the recommended max. observation times for checking the alignment of the source in the slit.

Acquisition

Target acquisition is done in the following sequence:

1. The acquisition mirror is inserted with the requested slit and decker in position to take a slit image to measure slit center.
2. The slit and decker are moved to "acquisition" position to image the entire science FOV, nominally in the H (1.65um) filter, but the choice of filter is up to the PI.  Any filter but the broadband L and M filters can be used for acquisition; see the acquisition exposure times page for more information.
   
3. The object is centered at the measured slit center position. Nominally, this requires 2 images (to measure initial position and to confirm offset) for objects visible above the background; 3 images if a sky exposure is needed for background subtraction.
4. Optionally, the slit and decker can be reinserted to image the object through the slit.  However, this is not usually done as the slit position is repeatable to <0.5pixel. (The acquisition mirror is repeatable to 1-2pixels, which is why step 1 is usually repeated with each acquisition.)
5. The acquisition mirror is removed and the instrument is configured for spectroscopy.

Guiding

The peripheral wavefront sensors (PWFS) currently must be used for guiding. Please adhere to the following guidelines when selecting a guide star:

• Guide stars must be within a 4.7'-6.9' annulus; where the lower limit is somewhat dependent on slit orientation and location of PWFS1 or PWFS2. If necessary, a smaller inner radius can be used with PWFS2 - check for vignetting in the OT Position Editor AND please add a note alerting the observer.
• Only select one guide probe. PWFS2 is usually the better choice - it has better performance and can be used to V<16. The limiting magnitude for PWFS1 is V~11. PWFS2 also has smaller vignetting field.
• Ensure that the guide star selected works for ALL offset positions. If guiding is not required in some positions (offsetting to sky), you must change the PWFS entry in the OT offset iterator to "freeze" for those positions.
• Pick the brightest star within the aperture that meets all other constraints for best performance.
• Always check the guide stars "auto-selected" by the Phase I tool. The GS catalogs have many spurious entries! Reject extended objects and binaries.
• Guide star problems or mistakes are probably the most common time-waster during acquisition - this time is charged to the program!

Offseting (Nodding, Dithering)

All infrared observations require background subtraction. This is accomplished by moving the telescope slightly from one integration to the next (a.k.a offsetting, dithering, nodding). Optimal dither patterns can improve efficiency at the telescope and maximize the signal-to-noise-ratio of the combined data. The offset sequences that you set up depend on the size of the object and the length of slit available to you. Ideally, you should take observations at several (not just two) positions on the slit. This provides better sky subtraction and rejection of bad pixels. This is not possible, though, where the slit is short or the object is extended. Here are some guidelines:

General:

• If you only need to spend modest amounts of time on your object to reach the desired signal to noise, doing a longer sequence or 2 repetitions is generally better than a couple of positions at the maximum allowable time. Use the ITC to see whether you lose predicted S/N by doing more, shorter exposures. If the loss is small (5% or less), you should get better sky removal with the shorter exposure times. This is mainly an issue at the shorter wavelengths, since you are always doing very short exposures at L and M.
• For very faint objects, it is better to maximize the exposure time (see below) at each dither position to minimize readnoise effects (and improve efficiency), even at the expense of slightly better sky subtraction. Often this is an important trade-off; again, use the ITC to experiment with your particular situation.

Long Slit:

• When nodding along the slit remember that smaller nods more accurately retain the target in the slit than do large nods. Although the slit is 50-99 arcsec long it is not necessary to use all of it on small targets. For a point source, nods of 3-5 arcsec are ample.
• For point sources or compact objects, aim to do 3-5 different positions on the slit, separated by 3-5 arcsec. It is probably better to go through a dither sequence twice with a smaller number of positions than once with a larger number. If the object is too large to do 3 different positions conveniently (>10 arcsec or so across), but you can still fit 2 positions on the slit, use two slightly different positions at both the “A” and “B” positions in the sequence. That is, to dither +/- 15 arcsec, make your sequence +16, -16, -14, +14 arcsec. Likewise, for “on” and “off” (sky) sequences, shift both the on and off positions by an arcsec or two.
• For objects of moderate extent (<50 arcsecs), large offsets can be used to keep the object in the slit. In this case, guiding should be enabled at all positions. This reduces efficiency somewhat for each nod, but is compensated by collecting source photons at all positions.
• For larger extended objects, offsets to sky are necessary which nominally do not require guiding - specify to "freeze" the PWFS at these positions for maximum efficiency. It is a good idea to dither the sky positions by a few arcseconds to facilitate removal of accidental point sources.

Cross Dispersed Mode:

• The cross-dispersed slit is so short (6 arcsec) that the best you can do with a point source is 2 different positions, separated by 3 arcsec (offsets +1.5, -1.5 arcsec). For an object with any measurable extent, you are forced to go to an “on”/”off” sequence. If the object is compact enough, it will be advantageous to do more than one “on” position, even if the two positions are shifted by only a few pixels (it’s probably desirable to make any shifts an integral number of pixels). Always do different “off” positions and spend the same amount of time on object and on sky for best sky subtraction.

Integral Field Unit (IFU):

• The same approach should be used for the IFU as for the cross-dispersed mode for extended objects (there is no gain in signal to noise using the IFU for a point source, and a lot more effort in data reduction). Here, however, you can dither in 2 dimensions, and may want to set up an 8-point on/off sequence which uses a total of 4 different “on” positions offset by a few pixels in both directions.

Read Modes

The read mode itself is automatically set by the exposure time per coadd, so the users should evaluate the best strategy when setting the integration time: at or below a wavelength of 2.5um, spectroscopy of faint sources should make use of multiple read pairs ("Faint Object " or “Very Faint Object” read modes). The very faint source mode provides slightly better read noise, but adds 18 seconds of overhead per exposure (or coadd). If  exposures of 60secs or longer are feasible, it will give slightly better performance between airglow lines. (Look at the noise plot in the ITC; if the regions of interest to you have noise around 10-20 electrons/spectral pixel, use this mode). For shorter exposures (20-60secs), the faint object mode will be satisfactory. (See detector page for overheads associated with each readmode.)

Bright standard star observations are much shorter and will use the single read pair ("Bright Objects"). 

All of these modes use the same bias (well depth), so you can calibrate objects observed in one mode with standards observed in another. This is not true when objects and standards are observed with different bias.

Spectroscopy in the L and M bands should be done in the “high background mode”, which provides greater well depth but higher dark current and read noise (which is irrelevant since all images are background-limited).

The "Acquisition" mode is available only within the GNIRS sequence iterator and is intended for very short (0.2sec) acquisition images with shallow bias.

Maximum Exposure Times

The maximum exposure time you should use is set by three constraints:

(1) Saturation on the sky,
(2) Saturation on the object, and
(3) Sky variations during a dither sequence.

The GNIRS maximum exposure guide gives safe limits for currently available configurations. You can check background in the ITC for any configuration by squaring the background value shown in the result, and then dividing by the number of pixels in the software aperture used. The background per pixel should be kept at 50,000 electrons maximum for shallow well (K and shorter wavelengths), and 100,000 for deep well (L,M). In general, avoid very long exposure times, because the sky will vary significantly between frames, and you will accumulate a larger number of radiation events per image. Unless the ITC shows a significant loss in formal signal to noise, a good maximum value is 900 seconds.

Bright objects may saturate the array before the background does. Check the recommended exposure times and scale the values to your particular case. You can check for object saturation with the ITC by dividing the signal counts by the number of pixels in the software aperture. It is helpful to set the aperture directly to a value around the image FWHM to ensure that you are looking at peak counts. If you have asked for cloud or seeing conditions worse than the median, you should do the saturation check under median conditions (70% IQ and 50% cloud). In general, check anything brighter than 12^th magnitude in K band and below, 7^th magnitude in L, and 1^st magnitude at M. So long as the integration times are only a few seconds, you might as well do 2-3 co-adds per position as well. This is especially advisable if you have set allowable cloud percentile to 70% or worse.

Continuous Elapsed Time on Source

Science observations should not exceed ~60 minutes elapsed time for the 0.1" slit, ~90 minutes for the 0.3" slit, and 120 minutes for the 0.675" or larger slits. This is to allow for re-centering the object in the slit (correcting for flexure between the PWFS and GNIRS). Targets requiring more time should be broken into multiple observations, with accompanying acquisition observations. These observations may be executed on separate nights, but even when executed sequentially the acquisition obs. (and accompanying overhead) will be used to check slit centering.

Overheads

Overheads associated with each new science target (for target acquisition, telescope, WFS and instrument re-configuration etc) total 15-20 min for spectroscopy. The shorter time applies for bright objects (H<14), while complex acquisitions (such as blind acquisition) can take longer. The program is always charged the actual time used (including errors in the OT definition, but not including telescope-related problems).

Overheads during an observing sequence are usually dominated by telescope motions (nominally about 10 seconds/motion, for small offsets) and by array readout. The readout overhead is equal to the minimum exposure time given in the OT, and is incurred per coadd. The lower noise readout modes have longer overheads because multiple reads are involved, but since the exposure times are usually longer, the fractional overhead is still low. The time to write the file is of order 2 seconds and is usually consumed in the ~10 seconds for telescope motion. For most observations you can assume an overhead (after acquisition) of 10%, provided you set things up rationally. M band observations are an exception to this: assume an overhead of 40% because of the short exposures. (If you insist on working at R=1700 in M band, assume 110% overhead – which is why it's not recommended.)

The OT adds a fixed 20 minute overhead per observation for acquisition, regardless of the type of observation. Since GNIRS programs separate acquisition and science data collection into separate observations, this overhead appears twice, but the second instance is ignored until the observation is actually executed. The OT also assumes a fixed 10 sec overhead for every telescope move and instrument config change in the observation sequence, which is approximately correct, but it does NOT include readout. For long sequences and/or very short exposures, the PIs should calculate the readout overhead and take it into account in their total time allocation.

Note about detector readout overheads: These overheads are NOT currently included in the OT total time calculation.  For most observations these are likely to be negligible, with the exception of very short exposure times, such as one uses in the M-band, or very long sequences.  For an 0.5sec observation, the total time per exposure is ~0.7seconds, nearly a 50% overhead.  The readout overhead is the same for each coadd, however increasing coadds reduces the overhead writing to disk (~2secs per file), as well as the total number of files.  For very short exposures one should generally use coadds to accumulate the data in one file until it is time to dither on the sky.  For lowest readnoise, the overhead is 18secs per exposure; e.g. 8 hours on-source with 600sec exposures adds about 16 minutes total readout overhead.

Calibrations

It is now required that Phase II OT files contain all flat, arc, and standard star calibrations needed for your program. Examples are provided in the OT library available on the OT help page. Baseline calibrations will not be charged to the program (although the OT doesn't know this).

For science observations longer than ~20 minutes, two telluric standard per object must be defined: one appropriate telluric standard suitable for observation before your science observation, and a second suitable for use afterward. Only one will be observed as part of the baseline (if both are desired, this must be requested in a note, and your program will be charged the time used on the second standard). If a single telluric will suffice before or after (for short science observations), only one need be provided. Notes may be used to provide guidance on the sequencing or selection of telluric standards, and, especially, if multiple targets can use a single telluric standard.

Flux calibration is inaccurate because of slit losses, which vary somewhat with wavelength (due to differential refraction and the wavelength dependence of seeing). For this reason, spectrophotometric standards are not done as part of the standard baseline calibration. The accuracy for which the relative flux density of a telluric standard is known is probably better than 10% for a given atmospheric window, 10-20% for the full range of a cross-dispersed spectrum. If the project requires more accurate flux calibration than this, the observer has two choices:

• Ensure that photometry is already available for the science targets, and normalize the spectra.
• Request time with photometric conditions for additional observations of the target and a flux calibrator with a wide slit to provide the normalization.

PIs should check that the baseline calibrations are sufficient. Additional calibrations can be added as needed, but the program will be charged the time required to obtain anything beyond the baseline.

Last update December 2005; Bernadette Rodgers and Greg Doppmann
Original content, Jay Elias