GNIRS Exposure Times

Maximum Exposure Times: Acquisition Imaging

To avoid saturation, acquisitions of bright stars (H < 11) should be done using the narrow band H2 filter. The broadband H filter is recommended for fainter acquisions. The sky background is ~3000 ADU in 10 sec through the H filter in nominal conditions, however we recommend single exposures should not exceed 6 seconds to minimize the impact of persistence left by the acquisition images and allow for sky variability. The table below summarizes the recommended exposure times for bright and faint acquisitions.  The table below assumes CC50/IQ70 conditions; for programs accepting worse conditions remember to adjust the brightness of your object by the extinction expected under these conditions.

Acquisitions are nominally done in H as this offers the best compromise between detecting fairly red targets while avoiding very high sky background.  However, the choice of acquisition filter rests with the PI.  For targets that are brighter at J or X (around 1um), these broadband filters can be used and background levels will be approximately 1/5 of the H value.   In these cases, follow the same guidelines below for target magnitudes but maximum exposure times can be up to 30 seconds before multiple co-adds are needed; sky subtraction is not typically necessary.  K band acquisition is not recommended; if needed, keep maximum single exposure to 1 second.  Acquisition at L-band is possible with the narrowband PAH filter; maximum exposure time is tbd. Broadband L and M filters can not be  used for acquisition.  See OT instructions and Observing Strategies for details on creating acquisition observations. 

*Fainter than H~14mag, sky subtraction is usually needed.  In this case, select the "faint acquision" case from the GNIRS OT library corresponding to your particular instrument configuration.

Maximum Exposure Times: Spectroscopy

For faint targets in low background situations (J, H, high-resolution K), the maximum exposure time is determined by sky variability and radiation events with the short cameras. In this case, we recommend a maximum exposure time of 15 minutes and dithering (either along the slit or off the slit) to facilitate sky subtraction and cosmic ray (and radiation event) corrections.

In the thermal infrared, the maximum exposure time is always set by the background (even with the deeper well) and depends on the wavelength and the instrument configuration.  In this case, one can "co-add" several very short exposures to build up signal in their target without saturating on the background. For M band observations, the 110 l/mm grating is recommended with the short (0.15"/pixel) camera, giving a maximum exposure time of 0.5 second with the 0.3" slit (and 0.3 sec with the 0.45" slit). If the 32 l/mm grating is desired, the maximum exposure time with the short camera is 0.2sec with an 0.3" slit. Wider slits are not possible in the M band with the short camera. In the L band, the background varies strongly with wavelength, but one can use the short camera with the 32 l/mm grating and a maximum exposure time of ~3 seconds with a 0.3 arcsec slit.

The values below are specified for R=1700 and a 0.3 arcsec slit.

* Because the M exposures are so short at R=1700, and because the band is not very wide, it is strongly recommended that you work at R=6000 for all M band observations (maximum exposure = 0.6 sec). You can reduce the resolution after data reduction by binning pixels, and your overhead per integration goes from 100% to 33%.

** To avoid numerous very short M band exposures, use coadds to combine multiple exposures until you dither the telescope; e.g. 0.6x50 coadds, dithering every 30 seconds.

*** The times at R=6000 are 3x longer at L and M but the same at the shorter wavelengths. This is because at the longer wavelengths the highest levels of background are thermal continuum or resolved emission lines, while at the shorter wavelengths the highest background is due to unresolved airglow lines. The times with a wider slit scale inversely as the slit width, though with the widest slits the airglow lines smear out and the maximum times decrease less.

For the simplest case (one low-noise read, short exposure times, shallow well), the saturation level is ~7500 ADU, however one should keep the counts below 6000ADU to remain safely in the linear regime.  This corresponds to ~75,000e- per pixel (note that the ITC reports electrons per spectral pixel which is a function of slit width).  For bright objects, such as standard stars, one can use the ITC to determine the appropriate exposure time; some examples are given in the table below.

Recommended Exposure Times

The following table gives an example of times appropriate for a 5.0 magnitude star (in all wavelength bands) with the short blue (SB) camera, 32 l/mm grating, and 0.3 arcsec slit under good observing conditions (CC = 50, IQ = 70). This table is meant to give an idea of typical exposure times for some particular GNIRS cases and can be scaled to account for other modes or slit widths.

Typical Exposure Times for a 5.0 mag A0 standard (with a 0.3 arcsec slit using the 32 l/mm grating)

* Background limited

• Scale the peak electron flux in ITC output spectrum (i.e. e- per exposure per spectral pixel) to e- per spectral & spatial pixel. (see below)
• Run ITC with "Software aperture of diameter (or slit length)" selected.
• In the ITC output, divide peak e- by "derived image size" value in pixels (scale from arcsec using the pixel scale you selected in the ITC setting).
• Select appropriate exposure time (per single co-add) so that max e- per SINGLE PIXEL (i.e. spectral/spatial, calculated above) are no more than 75,000 e- (shallow well) or 150,000 e- (deep well).

Last update July. 28, 2005; Greg Doppmann