HRMA Ring Focus
HRMA Ring Focus Measurements
and Mirror Surface Distortions
The Advanced X-ray Astronomy Facility (AXAF) mirror and science
instruments ground calibration was carried out at the NASA/Marshall
Space Flight Center's X-Ray Calibration Facility (XRCF) to ensure a
scientifically successful mission. The AXAF mirror -- the High
Resolution Mirror Assembly (HRMA) is an assembly of four pairs of
nested Wolter Type-I grazing incidence mirrors coated with iridium.
Each of the eight mirror shells is held by 12 invar pads outside the
shell with epoxy bonding. During the calibration, the HRMA was placed
horizontally, and was distorted by a small but measurable amount by
the mechanical support system due to gravity, thermal, and strain in
the epoxy bonds. To predict the AXAF on-orbit performance, these
distortions need to be measured carefully and their impact needs to be
assessed.
As part of the calibration, the HRMA ring focus measurements were
designed and carried out for the above purposes. 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. The ring focus measurements reveal
aspects of the test system distortions and the mirror surface figures
which are difficult or impossible to detect in the focal plane.
During the calibration, 6 long exposure images (5 HSI and 1 HRC) were
taken with a time span of 3.5 months, in order to monitor the change
in the strain of the epoxy bonds which could distort the mirror
surface. The measurement results show periodic modulations of the
ring width and its change during that time span. With careful
analysis of all the data and comparing them with the ring focus
models, it is found that the epoxy shrinkage change is very small
(less than 0.3%) and the epoxy bonds are stabilized after the first
month and half of testing.
The small change due to the epoxy strain and its subsequent
stabilization observed during the calibration interval lead us to
expect no more serious changes between the calibration and the flight
performance. We should be able to predict the HRMA on-orbit
performance according to its calibration data. The HRMA has been
tested as an excellent X-ray mirror. The ring focus test and its
results provided us with the confidence that HRMA will stay that way.
SPIE '97 paper:
AXAF HRMA Mirror Ring Focus Measurements, SPIE
Proceedings Vol. 3113, 106 (1997).
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