the atmosphere. The radiation background seen by Gemini is orders of magnitude brighter than the limiting background due to thermal emission and scattered light from zodiacal dust (the lowermost curve in the bottom panel of Figure 3.1).

Infrared observations from space are limited only by a telescope’s collecting area, thermal emission, and the natural background set by zodiacal dust. Figure 3.2 shows the zodiacal background at an ecliptic latitude of 45 degrees, calculated using a standard set of assumptions.1 At wavelengths shorter than the minimum at approximately 3.8 microns, the background rises as scattering by the zodiacal dust becomes more efficient. At wavelengths longer than the minimum, the background rises due to thermal emission from optically thin zodiacal dust.

A primary goal of space infrared astronomy is to reduce the thermal emission from the telescope below the background. This reduction is readily achievable at wavelengths between 2 and 3 microns using passive cooling techniques. The natural background in this spectral region is more than five times darker than the darkest part of the optical spectrum near 0.5 micron.

Figure 3.2 shows the thermal background from the ATD/NTOT for two different operating temperatures: the baseline value of 200 K, and a scientifically more desirable temperature of 160 K (see Box 3.1, “Enhancing Infrared Performance”). The figure includes the background for the 8-meter, infrared-optimized Gemini telescope for comparison. If the ATD/NTOT operates at a temperature of 200 K, it will be background limited and therefore able to make the deepest infrared images, in the wavelength range from 2.0 to 2.4 microns, while still having substantially lower background at longer wavelengths than any other telescope.

The ATD/NTOT will be background limited in the 2.0- to 2.4-micron band only if its baseline InSb detector

FIGURE 3.1 Upper panel: A theoretical model of the absorption-line spectrum of Earth’s atmosphere, in the spectral region 2 to 10 microns, above Mauna Kea. Lower panel: Sources of the background radiation in the 2- to 10-micron region that will be seen by the infrared-optimized, 8-meter Gemini telescope on Mauna Kea.



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