Thermal Control Study of the Constellation-X Telescope Aperture

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 annular slots between mirrors would normally behave as blackbody cavities of small depth, but we found that the high-reflectance mirror surface produces direct thermal effects at a depth of 100 times the slot width, greatly complicating the task of producing uniform mirror temperature. The elastic behavior of such thin mirrors can produce unacceptable distortion even in areas that are nearly isothermal. We show that a combination of passive and actively-controlled blades can produce temperature uniformity of about 2C.