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Focusing mirrors for X-ray astronomy are almost always located near the open aperture of the X-ray telescope. Such a mirror is typically a concentric nest of near-cylindrical paraboloids. Controlling the mirror temperature and reducing thermal radiation to space is essential to reducing optical distortion of the mirror assembly. This has been successfully done in the past by a partially open structure, termed a precollimator, between the mirror and space; or in the case of metal mirrors, by conduction from the support structure. As designs for future missions strive for more collecting area to “see” fainter objects, the individual mirrors become more numerous and thinner, presenting new challenges to thermal control. We report here studies by the Smithsonian Astrophysical Observatory on thermal control of a 1.6m-diameter X-ray mirror assembly for the Constellation-X mission. The mirrors are 0.3 mm thick, and the nest contains of order 100-200 mirrors.

The CHANDRA X-ray Observatory (formerly AXAF), one of NASA's “Great Observatories” was launched aboard the Shuttle in July 1999. CHANDRA comprises a grazing-incidence X-ray telescope of unprecedented focal length, collecting area and angular resolution - better than two orders of magnitude improvement in imaging performance over any previous soft X-ray (0.1-10 keV) mission. Two focal-plane instruments, one with a 150°K passively-cooled detector, provide celestial X-ray images and spectra. Thermal control of CHANDRA includes active systems for the telescope mirror and environment and the optical bench, and largely passive systems for the focal plane instruments. Performance testing of these thermal control systems required 1-1/2 years at increasing levels of integration, culminating in thermal-balance testing of the fully-configured observatory during the summer of 1998.

In the paper the we discuss how the thermal behavior of the Advanced X-ray Astrophysics Facility (AXAF) optical system has been modeled and tested, and how these efforts have influenced the design of the telescope, especially as it relates the imaging performance. This includes the passive/active system covering the space-facing aperture known as the thermal “precollimator”, the mechanical support system that allows the large optical elements to survive the rigors of test in one-G and launch yet minimally affecting on-orbit optical performance, and the active thermal control design of the telescope. Methodologies for the frequently difficult task of transferring results from thermal analysis software to mechanical finite-element analyzers to model thermal deformations are discussed. The complexity of these distortions of the surfaces of the mirror elements required the use of optical raytrace models to assess imaging performance of the telescope.

The large optical apertures required by many space-based telescopes make thermal control of these optics a significant challenge. One technique which has been used for x-ray telescopes involves placing insulating tubes forward of the entrance aperture. The reduction in both conduction and direct view produces a thermal gradient along the tubes, increasing the effective sink temperature for the optics and reducing the effective radiant source temperature and heat flow to space. In another configuration the “tubes” are formed by aperture slots in a stacked assembly of flat, low-conduction baffle plates. Because these apertures collimate both incident x-rays and thermal radiation, such an assembly has been termed a “thermal precollimator.” This paper describes precollimator design principles and design, analysis and testing of a precollimator for the Advanced X-ray Astrophysics Facility (AXAF).