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Many ACIS OBFs manufactured by Luxel Corporation were calibrated at the SRC Multilayer Beamline. The technique was established and tested using Lexan-based engineering filters, then all flight-candidate Lexan filters were mapped. In the summer of 1996, we discovered that the Lexan-based filters suffered light leaks after acoustic testing, which led to the new polyimide-based filter design. We mapped the final suite of polyimide-based flight-candidate filters in December 1996. Several witness samples were mapped then and in January 1997, for use in other transmission tests.
The Multilayer Beamline is ideally suited for these measurements due to its ability to access several energies in the 200-2000 eV range and its built-in, computer-controlled x-z stage, which allows us to map the filters automatically with excellent spatial resolution. Complete transmission maps were obtained on all filters at 273 eV, 522 eV, and 775 eV, with a spatial resolution of 0.76mm x 0.76mm, roughly 32 ACIS pixels (15 arcseconds) square. Additionally, small swaths of the filters were mapped at 1330 eV and 1860 eV, in an attempt to measure the filter thicknesses by fitting transmission curves to points at many energies.
These energies were chosen because they spanned the wavelength range of interest, they bracketed absorption edges of important elements (carbon, oxygen, and aluminum), and because they were readily accessible with existing beamline hardware. The spatial resolution was chosen to match the planned aspect dither amplitude of the spacecraft and to yield a complete map of each filter in a reasonable time (6-8 hours). Coincidentally, this pixel size was well-matched to the size of the striations in the filters, oversampling them adequately. We searched the Lexan-based filters for variations on finer spatial scales (0.2mm x 0.4mm) but saw none. The pixel size we used is too coarse to reveal pinholes in the filters, which are usually only a few microns in size.