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Session 55 - Space Instrumentation.
Display session, Wednesday, June 12
Tripp Commons,

[55.01] Transmission Maps of the ACIS UV/Optical Blocking Filter Flight Units

L. K. Townsley, P. S. Broos (PSU), J. F. MacKay (UW SRC)

The AXAF CCD Imaging Spectrometer (ACIS) employs filters made of Lexan coated on both sides with aluminum to block optical and UV light, so that the CCDs see only X-radiation from astronomical targets. These filters must be characterized by spatially mapping their transmission at various astrophysically and instrumentally important energies. The Penn State University ACIS team determined that a synchrotron, where a variety of well-determined X-ray energies is available, would provide the best calibration. We measured engineering grade UV/optical blocking filters at the University of Wisconsin Synchrotron Radiation Center (SRC) in June and October 1995, modified the hardware and software on a dry run in January 1996, and just completed the calibration of the flight filters in March 1996.

The Multilayer Beamline at the SRC was used for these measurements because it can access several energies important to the calibration and its built-in, computer-controlled x-z stage allows us to map the filters automatically with user-specified spatial resolution. These transmission maps formed the basis for choosing the actual flight filter units from the set of filters manufactured with flight specifications.

We obtained transmission measurements at five energies in the range 200-2000 eV. We present here best-fit models of the filter transmission based on these data points. Better than one percent accuracy in transmission as a function of energy was achieved over the entire filter area on scales corresponding to thirty arcseconds in the focal plane of AXAF (the amplitude of the planned aspect dither of the spacecraft). The pair of filters (one for the Imaging array and one for the Spectroscopy array) selected for flight will be installed on the ACIS focal plane in early summer.

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