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Exposure times

Table 1 below shows the time taken for the background (sky + telescope) to reach 50% full-well capacity of the NIRI Aladdin array in spectroscopy mode. Table 2 shows the brightness of a stellar source (in magnitudes) that will fill 80% of the well of a NIRI array pixel in a one-second exposure (0.2 sec exposure at M).

The estimates are based on calculations made using the NIRI Integration Time Calculator. Background measurements made with NIRI on the telescope are very similar to the adopted values used by the ITC, but obviously span a range of values. The ITC estimates presented below should therefore be used as guidelines only. For these calculations it was assumed that L and M-band observations use the high-background bias voltage (deeper well); the low-background bias voltage (shallow well) is used for wavelengths shorter than 2.5 microns. Median observing conditions (CC50, IQ70) and airmass <1.2 were assumed. The thermal background depends sensitively on these assumptions, and the exposure time to saturation can differ somewhat from the tabulated values. Likewise the peak signal from a point source on a pixel of the array depends on sky conditions.

Two other factors that influence the choice of exposure time are:

  • Minimum possible exposure time due to array electronics (see Table 1 of the Imaging mode exposure times section)
  • Efficiency - ensuring that a much larger percentage of the time is spent exposing the array than reading it out (see Table 2 of Imaging mode exposure times section).

An important third additional factor is the time required for the noise from fluctuations in the sky+telescope background at the wavelength(s) of interest to equal read noise. This time may be larger than the value in Table 1, which applies only to the brightest pixel. In the infrared the background varies enormously with wavelength. For a program in which the wavelengths of interest all coincide with very low background compared to other wavelengths in the spectrum, one might choose to increase the exposure times above the values in Table 1 in order to be background-noise limited at the wavelengths of interest or simply to observe more efficiently. The ITC provides a spectrum of the background. It should be consulted in all situations, but especially if one is considering such an option.

Table 1 - Spectroscopy at f/6: Time in seconds for BACKGROUND to reach 50% full-well depth at brightest pixel1
Slit Width J H K L-L'3 M3
6 pix 10000 600 1000 1.0 0.3
4 pix 20000 900 1500 1.5 0.6
2 pix 30000 1800 3000 3 0.9

Table 2 - Spectroscopy at f/6: Magnitude of a point source filling 80% of the well1,2
[for an exposure time of 1 sec (0.2 sec in M band)4]
slit width J
6 pix 5.5 5.5 5 4 1
4 pix 5 5 4.5 3.5 0.5
2 pix 4.5 4.5 4 3 0.0
1 Assumes photometric conditions.
2 Assumes average seeing.
3 At these exposure times the array saturates in the L-L' band only at the very longest wavelengths(>4.1μm) and in the 4.5-5.1μm part of the M band only at wavelengths corresponding to opaque telluric emission lines. It is possible to obtain usable spectra at most wavelengths in these bands with exposures that are ~2X longer. Use ITC to obtain more detailed information.
4 Observations at M will generally require individual exposures of less than 1 second and in many cases the use of a subarray so as not to saturate on the thermal IR background.