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The primary goal of SIRTF is to conduct astronomical observations to a sensitivity limited by the photon noise produced by zodical dust emission. This fundamental natural background limit to sensitivity cannot be exceeded, regardless of telescope design. To approach this limit a cryogenic, baffled telescope is a necessity. For the SIRTF Technology Integrated System (TIS) Study for NASA, Perkin-Elmer studied the implications of obtaining natural background limited sensitivity with the baseline Cassegrain design. The major contributions to an infrared signal originating from the telescope that could exceed this level are as follows: 1) Stray Light from the sun and moon that results from scatter and diffraction; 2) Photon noise due to thermal emission from the telescope; 3) Scan noise due to a dc offset that results from telescope chopping in the presence of thermal gradients; 4) Thermal drift noise, which results from the offset caused by telescope temperature changes. Our major results are as follows: 1) The noise specification for the 200p - 300p band was the major system driver 2) Chopping removes stray light as a major noise contribution; 3) The black surfaces associated with the secondary mirror constitute the major source of photon, scan, and drift noise in the far infrared and should be masked with a cold stop from the view of the focal plane; 4) Heat dissipation flowing into the secondary mirror from the chopper can be a major source of noise in the far infrared and low dissipation and thermal isolation should be chopper requirements; 5) The telescope should be cooled to less than 5K; 6) Thermal control to hold the mirror surface thermal changes to the level of a mK/s is necessary; 7) Barrel baffle temperatures as high as 30K are within reauirements; the sunshade is a negligible contributor to the background.
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