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Exposure time estimation

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Integration Time Calculator (ITC)

The GNIRS Integration Time Calculator (for spectroscopy only) can be used to determine limiting magnitudes, exposure times, S/N ratios, background levels, etc. for a wide range of source properties, observing conditions and GNIRS configurations. When using the ITC please note the caveats and guidelines at the top of the ITC web page.

A table of spectroscopic sensitivity estimates is also available. Note that it applies to specific wavelengths at which the atmosphere is nearly transparent, and should not be taken as indicative of the sensitivity at other wavelengths in its band. The ITC must be used to determine sensitivities at other wavelengths within a band.

It is important to note that very high values of S/N predicted by the ITC are unrealistic; i.e., sensitivity and S/N are only interchangeable for faint sources / low S/N. To achieve very high S/N requires stability of the instrument against small wavelength shifts and shifts in the optical path, stability over time of the pixel-to-pixel response, the accuracy of removal of telluric lines (and thus to some extent the stability of atmospheric conditions), and other factors, none of which are included in the ITC. In practice ground-based infrared observations rarely achieve signal-to-noise ratios greater than several hundred. Scientists who wish to obtain values of signal-to-noise ratio considerably higher than 100 should consult with GNIRS scientists prior to proposal submission.

The GNIRS ITC may be used to determine if an observation will saturate the array. 

The estimated image quality delivered to the instrument is provided as part of the observing condition constraints .

In the four sections of this form, select the appropriate astronomical source, telescope and instrument configuration, observing conditions, and observation parameters. Click on the calculate button (calculate button) at the bottom of any section to submit the parameters from all the sections to the server or the reset button (reset button) to reset all parameters to their defaults. The results are reported in a separate web page that can be resized and printed.

See the ITC Help for general guidance on use of the Integration Time Calculators, and the (more info) links for specific help on each section of the form.

Important Notes:

  • For a discussion of the effect of slit width on sensitivity in the JHK bands see here.
  • The atmospheric transmission and extinction files used in the ITC only extend down to 0.9 um, therefore the ITC plots artificially cut off at 0.9 um, even though the instrument itself is somewhat sensitive to shorter wavelengths (but dropping off rapidly). The Sensitivity Table gives approximate sensitivities for orders 7 (0.93um), 8 (0.83um), and 9 (0.75um) for the 32 and 111 l/mm gratings and short camera only, as those orders are automatically observed in cross-dispersed mode.
  • Very high values of signal-to-noise ratio (>several hundred) are usually not realistic, as the values actually achieved depend on many factors besides those included in the ITC. 

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GNIRS Optical Properties

The optical elements of GNIRS (e.g. cameras, gratings, cross-dispersing prisms and slits) are described in detail on the GNIRS spectroscopy pages.

The choice of camera, grating and slit width set the spectral resolution. Not all combinations are viable. Use of the cross-dispersing prism may be toggled on/off (again, only some combinations of prism, grating and camera are viable). Given a specific central wavelength, the choice of "blue" or "red" optimised camera, grating order and order sorting filter is made automatically. A broad-bandpass filter is used with the cross-dispersing prisms.

Note that even though specific capabilities are listed in the ITC, they may not be available in every semester.

Caution: the atmospheric transmission and emission files currently used by the ITC have a resolution of 1nm and therefore produce telluric features substantially broader than the actual instrumental resolution, especially at R=5900 and 18000. The inter-line signal and noise calculations are correct.

GNIRS Detector Properties

The four combinations of well depth and read mode (more specifically, the number of non-destructive reads) define the minimum exposure time, read noise and saturation limit. At this time, there is no checking that the specified exposure time is more than the minimum allowable.

Values of the read noise and minimum exposure times are given on the GNIRS science detector page.

Dark current: laboratory characterisation indicates a value of 0.1e-/s at the adopted operating temperature. This may include some internal cryostat background (e.g. a low-level light leak).

GNIRS On-Instrument Wavefront Sensor

At this time, the GNIRS OIWFS is not available. The facility Peripheral Wavefront Sensor is required for tip-tilt image stabilization.

Choosing optimal exposure times

This page gives information on the limitations to spectroscopic exposure times due to sky and telescope background and due to source signal. (Acquisition exposure times are given here.) The information can be used to determine optimal exposure times for science observations. Examples are given below the two tables.

Limitations due to Sky and Telescope Background

When observing very faint targets at 1.0 - 2.3 µm, OH emission lines dominate the flux on the detector and for sufficiently long exposures will saturate the array at many wavelengths. In the X and J bands with the short blue camera and in X, J, H, and K with the long blue camera the exposure times for this to happen are far longer than should be used. In the thermal infrared (3 - 5 µm), except for the very brightest objects the maximum exposure time is determined by the background.

The table below gives, for 2-pixel wide slits, the exposure times that would fill at least some of the array detectors to half of their full well capacities. Saturation / non-linearity effects are noticeable beyond that level. The values for the L and M bands assume the deep well is used. Note that exposure times are generally inversely proportional to slit width. For example, for the 31.7 l/mm grating, short blue camera, and 0.45 arcsec wide slit, the the value for 3.2 - 3.5 µm in the table implies that the maximum recommended exposure time for that slit is 8 seconds in that wavelength region.

As shown in the table below the background depends strongly and not necessarily monotonically on wavelength. The table gives approximate maximum exposure times for different portions of the L and M bands. For details of the dependency on wavelength the Integration Time Calculator should be used. Note that when short exposures are required one normally should "co-add" many such exposures to build up signal in their target without saturating on the background. Very short (< 0.5 sec) exposures are not recommended, as the shortest read time possible is 0.2 seconds and thus overheads are large.

X J H K L (deep well) M (deep well)
1.0-1.2µm 1.2-1.4µm 1.4-1.9µm 1.9-2.5µm 2-8-3.2µm 3.2-3.5µm 3.5-3.8µm 3.8-4.15µm 4.5-4.8µm 4.8-5.4µm
31.7 1500(b) 1800(b) 300 900(b) 30 12 12 3 0.8 0.25(c)
110.5 1500(b) 1800(b) 300 900(b) 60 30 25 10 2.5 0.8
10(d) 10000(b) 7000(b) 1000(b) 4000(b) 100(b) 50 50 10 1 0.6
32(d) 13000(b) 10000(b) 1000(b) 4000(b) 300(b) 150 150 30 3 2
110.5(d) 13000(b) 10000(b) 1000(b) 4000(b) 1000(b) 500 500 100 10 6
10.44 13000(b) 16000(b) 5000(b) 5000(b) 5.0 2.0 1.0 0.5 - -
31.7 13000(b) 16000(b) 5000(b) 8000(b) 15 6 3 1.5 - -
110.5 13000(b) 16000(b) 5000(b) 8000(b) 45 18 9 5 - -

(a) Exposure times generally scale inversely with slit width.
(b) Actual individual exposure times should not exceed 600 sec.
(c) This configuration and exposure time not recommended because of large overheads.
(d) This configuration (without ALTAIR) often used for high spectral resolution (R up to 18,000), especially at L and M.

Limitations due to Source Brightness

The following table gives approximate brightnesses of stars for which the signal in a 1 second exposure fills the detector's well to the 50% level in the peak spectral row. The table assumes "good" (IQ70) image quality and narrow slits (0.3" for the short cameras and 0.1" for the long cameras). Note that:

  • (1) because the minimum exposure time is 0.2 sec, the absolute magnitude limits are 1.7 mag brighter than in the table for these narrow slits;
  • (2) limiting brightnesses are similar for low and medium resolution in the X, J, H, and K bands with and without AO (when using AO the smaller pixel size is approximately balanced by the more concentrated image).

For estimates of limiting brightness (or maximum exposure time) with other slit widths and sky conditions use the GNIRS ITC with "Analysis Method: software aperture of diameter" set to either 0.15" or 0.05" depending on the camera you are using. The ITC will then generate a spectrum of the peak row.

Camera AO? X
shallow well
shallow well
shallow well
shallow well
deep well
deep well
31.7 Short No 4.6 5.0 4.7 4.6 2.9 2.3
110.5 Short No 3.4 3.8 3.5 3.4 1.7 1.1
110.5 Long No(b) 0.9 1.0 0.6 0.3 -1.4 -1.9
10.44 Long Yes 5.5 5.8 5.8 5.7 -0.6(c) n/a
31.7 Long Yes 4.3 4.6 4.6 4.5 -2.3(c) n/a
110.5 Long Yes 3.1 3.4 3.4 3.3 -3.4(c) n/a

(a) Assumes IQ70, CC50, WV80, airmass 1.5, 0.30 arcsec slit with short camera, 0.10 arc sec slit with long camera. Wider slits increase signal in peak row.
(b) Large light loss without AO. However, better sensitivity than with AO at L and M.
(c) Transmission of ALTAIR is ~0.05 in the L band.

Choosing an appropriate exposure time

In general, optimal exposure times can be determined using the integration time calculator, with the constraints that individual exposures cannot be shorter than 0.2 sec and (at present) should not be longer than 600 sec. The tables on this page also can be used for such a determination. Two examples are given below.

Example 1: The goal is to obtain the highest S/N on a K=10.0 mag point source in 30 minutes of actual exposure time, using the 32 l/mm grating, the short blue camera, and the 0.45 arcsec wide slit. From the upper table one can determine that the background through the 0.45" slit limits the exposure to 900 x (0.30/0.45) = 600 seconds. From the lower table, a 600 second exposure will fill the well to the 50% level for a K=4.6 +2.512 log(600) =11.6 mag source in a 0.3" slit and with the 1.5 times wider 0.45" slit a somewhat fainter star (K~12) would do this. Thus, a shorter exposure is required to avoid saturation on the continuum of the K=10 point source. One probably should reduce the individual exposure at least by a factor of 6 (~2 mag), to ~ 100 sec, and then choose to obtain 30 such exposures (or somewhat fewer as there will be overheads associated with multiple exposures). If the source is suspected of having bright line emission, one would choose larger numbers of shorter exposures (say 60 seconds, or less) to avoid saturation on the emission lines.

Example 2: The goal is to get a spectrum of an L=10 star in the 3.8-4.2 micron interval using the 110.5 l/mm grating and the short camera with the 0.45 arcsec wide slit. From the upper table, one can see that individual exposures must be limited to about (0.30/0.45) x 20 ~ 13 seconds to avoid the onset of saturation on the background. The lower table indicates that in a 13 second exposure only stars brighter than L=4.7 mag (i.e., 1.9 + 2.5 log(13)) will be a problem. Because the star is much fainter than this, exposures of 13 seconds are OK. Because these are rather short exposures, individual frames should probably consist of several exposures in order to improve efficiency.

Guideline Exposure Times for Standard Stars

Because spectra in the 1.0-2.5 micron region are often obtained in better conditions than those specified in Phase2, it is recommended that exposure times on standard stars be set conservatively. The exposure and coadd settings in the following table will provide high S/N in poor conditions without saturating the detector in good conditions. It is assumed that the observations consist of 1 ABBA sequence and are made at airmass 1.2. Minimum S/Ns are for 2.35 microns, near the long wavelength end of the K window; near the centers of the XJHK windows the S/N will be considerably higher (without saturating).

GNIRS Mode A0V star magnitude no. coadds x single exposure time ABBA min S/N
IQ70 CC50
ABBA min S/N
IQany CC80
32 l/mm, SXD, 0.30 arcsec slit 6.0 5 x 2.0 sec 900 300
32 l/mm, SXD, 0.45 arcsec slit 6.0 5 x 1.2 sec 900 300
32 l/mm, SXD, 0.675 arcsec slit 6.0 5 x 1.0 sec 900 300
111 l/mm, longslit 0.30 arcsec slit, J-band 6.0 2 x 8 sec 700 225
111 l/mm, longslit 0.30 arcsec slit, K-band 6.0 2 x 15 sec 1100 400
10 l/mm, LXD, 0.05" pix, 0.10 arcsec slit, no AO 5.0 4 x 5.0 sec 1200 200
111 l/mm 0.05" pix, 0.10" slit, no AO, J,H, or K 5.0 1 x 40.0 sec 700 150

The above exposure times may be scaled by a factor of 2.5 per magnitude difference. E.g., for a magnitude 7.0 star the exposure times in the first two rows should be 5.0 and 3.0 seconds, with the same number of coadds to achieve the same S/N as in the table. For the first three rows in the table increase exposure times by a factor of three if the 111 l/mm grating is substituted for the 32 l/mm grating and SXD is used.