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GNIRS Detector Characteristics and Read Modes

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The properties of the Aladdin III InSb detector array for the GNIRS science channel are given in Table 1 below. The array has nicely uniform response, only a few regions of bad and dead pixels (see table below) very low dark current and low read noise in the lowest background mode. The bias voltage may be adjusted to increase the well depth for thermal IR (L and M band) observations or for very bright targets. InSb detectors have nearly 100 percent internal quantum efficiency (QE) from 0.4 to 5 microns, with a rolloff near the cutoff wavelength of 5.5 microns. The surface of the material is highly reflective (36 percent), but this is reduced by an anti-reflection coating that gives the array an effective QE of ~90 percent over the 0.9-5 micron interval.

Table 1: Detector Characteristics
Array Aladdin III InSb (Hughes SBRC)
Pixel format 1024x1022 27-micron pixels
Spectral Response <0.9 to 5.5 microns
Dark Current 0.15 e-/s/pix
Gain 13.5 e-/ADU
Well depth, 0.3V bias
90,000 e- (7,000 ADU)
non-linear response above 5,000 ADU
Well depth, 0.6V bias
180,000 e- (14,000 ADU)
non-linear response above 10,000 ADU
Quantum efficiency about 90%
Difference in odd/even row response 7%
Flat field repeatability TBD
Residual image retention Minimal impact on science; details
Regions of bad pixels (532< x <552, 708< y <716)
(177< x <210, 737< y <746)
(35< x <95, 876< y < 919)

There is a tradeoff between the detector read noise and the time required to read out the array. The best choice depends on the brightness of the object and the background at a given wavelength (see the Observing Strategies page for more guidance). The four read modes which can be used for GNIRS observations, and the read noise and exposure times for each are given in Table 2.

Table 2: Read Modes
Read Mode Low Noise
Read Noise
Min. exp time*
min. exp time* (sec)
Very Bright Objects;
high background
1 1 155 0.2 <1.0 low res. M
Bright Objects
medium background
1 16 30 0.6 ~2.0 Exp ~1 - 20s
sqrt(sig+bkgnd)>80 e/pix
Faint Objects
low background
16 16 10 9.0 20.0 Exp ~20 - 60 s
< 50 e/pix
Very Faint Objects
very low background
32 16 7 20.0 60.0 Exp > 60 s
< 20 e/pix

* The minimum exposure times are the times required to read the array. The recommended minimum exposure times allow efficient observing (spending at least 2/3 of the time actually exposing the array as opposed to reading it).

Some problems and features related to the detector and its controller are described on the "Known Issues" page.