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Optical Design of AXAF-I

Most readers will be familiar with the optical design, but a brief description may be helpful to some. The original design included six concentric mirror pairs, each pair consisting of a paraboloid and a hyperboloid. Two pairs were removed from the design as part of the 1992 restructuring of AXAF; we continue to use the original nomenclature to avoid confusion in the documents, so the four pairs of the present design are numbered 1, 3, 4, and 6, with pair 1 being the largest and pair 6 the smallest. The paraboloid and hyperboloid of pair 1 are called P1 and H1 respectively, and similarly for the other elements. The individual shells resemble shallow cones; all elements are 838.2 mm long, and the diameters at the pair intersections are approximately 1199 mm, 966 mm, 853 mm, and 634 mm for pairs 1, 3, 4, and 6 respectively. The deviation of any individual mirror element from a cone actually is quite small, about 36 m for H1, and less for the other elements. The paraboloids have a common focus which is located approximately 20 meters behind the center of the HRMA; the two foci of each hyperboloid are the (common) focus of its associated paraboloid and the system focus, the latter being approximately 10 meters behind the HRMA center. The design results in the on-axis rays making approximately equal grazing angles with both surfaces because this choice maximizes the throughput for a given area of polished glass; the typical grazing angles are 52, 42, 37, and 27 arcminutes for pairs 1, 3, 4, and 6 respectively. This geometry and the reflection properties of the Iridium coating result in HRMA effective areas of about 780, 445, 465 and 265 cm at 1, 3, 4, and 6.4 keV respectively (after allowance for obscuration by mechanical supports). The total polished area is about that of a five meter diameter normal incidence mirror.