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Session 9 - Instruments, Techniques.
Display session, Monday, June 09
South Main Hall,
We discuss the ring focus measurements for the AXAF (Advanced X-ray Astrophysics Facility) X-ray optics -- the HRMA. The HRMA (High Resolution Mirror Assembly) is an assembly of four pairs of nested Wolter Type-I grazing incidence mirrors. The ring focus measurements are an essential part of the AXAF ground calibration carried out at the X-Ray Calibration Facility (XRCF) at the Marshall Space Flight Center in Huntsville, AL. The ring focus is a sharply focused ring formed by X-rays before they reach the focal plane. It is caused by spherical aberration due to the finite source distance (527 meters) at the XRCF. There are four rings due to the four mirror shells of the HRMA.
The ring focus measurements reveal aspects of the test system distortions and the mirror surface figures which are difficult or impossible to detect at the focal plane. The measurement results show periodic modulations of the ring width which could be caused by gravity, thermal, and/or strain in the epoxy bonds that are part of the mechanical support system. The strongest component of the modulation has 12-fold symmetry due to the 12 flexures that support each mirror shell. We discuss the ring focus model and compare it with the test results to understand the test system distortions and the mirror glass imperfection, and to predict the impact for the AXAF mirror on orbit performance.
