FIGURE 3.2 The zodiacal background at an ecliptic latitude of 45 degrees is compared with the thermal emission from the ATD/NTOT (assuming the effective emissivity from the 10 mirrors in the optical path to the InSb detector is 0.20) when operating at 200 K (baseline) and also at a more desirable value of 160 K (enhanced). Note that the ATD/NTOT is background limited at wavelengths up to 2.4 and 3.0 microns for the baseline and enhanced temperatures, respectively. Thermal emission from the Hubble Space Telescope and the ground-based, 8-meter, infrared-optimized Gemini telescope is included for comparison.

has sufficiently low dark current and read noise. The task group calculated the noise requirements by assuming that the bandpass of a filter at 2.2 microns is 0.4 microns and that half the 2.2-micron photons incident on the aperture are detected by the InSb array. The zodiacal background rate is then ~0.2 electrons per second per 50-milliarc-sec pixel. To be background limited, the dark current in the array must be lower than ~0.2 electrons per second per pixel. Assuming a 1000-second integration, the background shot noise will be ~14 electrons per pixel. Consequently, the rms read noise per pixel must be lower than this value to realize background-limited exposures.

Box 3.1 Enhancing Infrared Performance

There is considerable advantage in lowering the temperature of the telescope from the baseline value of 200 K to 160 K. As Figure 3.2 indicates, the telescope is then background limited to a wavelength of 3 microns where the zodiacal background is twice as dark as at 2.4 microns. Cooling the telescope would, as indicated in Figure 3.3, result in a limiting magnitude approximately 2 magnitudes fainter over the interval from 3 to 8 microns than can be achieved in the baseline. If the telescope flies with low-noise detectors that operate at these wavelengths, every effort should be made to reach the goal of 160 K.

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