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SIRTF Telescope Engineering And Trade Studies

This paper summarizes some of the basic system trade studies of the Space Infrared Telescope Facility (SIRTF) telescope. The basic requirements of the telescope are that it be background limited and have diffraction-limited imaging capability. This background limit is calculated as a function of wavelength in the 100- to 800- μm region and has been done versus primary mirror temperature. It is shown in the first section of this paper that the optics temperature must be less than 8 K. The contribution to the focal plane background by the other elements of the optical system have also been analyzed; this was done to enable determination of what effect the other parameters, e.g., temperatures and scattering properties of the sunshade, baffles, and other critical structures, have on the background noise. The noise generated by scanning the image motion compensation (IMC) systems is also discussed. This noise is generated by the effects of temperature and emissivity variations on the surfaces of the various optical elements. This paper also discusses the selection of materials usable for the primary mirror of cold optics systems. Fused silica is a particularly viable material for this type of system. Finally, there is a discussion on the fine guidance sensor (FGS) and its influence on the design of the telescope, including the optical system, the focal plane, and the potential heat load the focal plane may present to the system. The various locations of the FGS, both external and internal, are examined.