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The ACIS-I and S arrays are covered by UV/Optical blocking filters (OBF).
These filters are necessary because CCD's are sensitive to UV and optical radiation
(see Lumb et al., 1991).
The ACIS filters consist of polyimide, a polycarbonate plastic with a chemical
composition of C22H10O4N2, with a layer of aluminum coated on
each side to provide optical light blocking.
The x-ray transmission properties of the AXAF CCD Imaging
Spectrometer(ACIS) UV/Optical blocking filters were
measured in the energy range of 260 to 3000 eV at the National
Synchrotron Light Source at Brookhaven Laboratories.
The main purpose of the calibration
was to determine model transmission functions for the ACIS OBF's
in the energy range of 0.05 to 10 keV with an accuracy of
better than 1%. We present a model transmission function
that fits the data to better than 1% in the measured energy band
of 260 to 3000 eV. The transmission above 3 keV is expected to vary smoothly
and is predicted by the model from the measured filter
parameters.
Detailed fine energy scans above the Al-K and C-K
absorption edges revealed the presence of fine oscillations
of the X-ray transmission. These features are most likely
extended X-ray absorption fine structure (EXAFS).
The amplitude of the EXAFS oscillations above the Al absorption edge is about
5% of the mean value of the X-ray transmission.
The ACIS OBF's were measured at room temperature while the
on orbit temperature of the filters is expected to be
about -60 C. We predict that the amplitude of the EXAFS
will increase by less than 0.5
when the filters are
in orbit at normal operating temperatures.
Optical transmission measurements on a multilayer filter composed of Al/Polyimide/Al = 330Å/2000Å/330Å were made at PSU by Gordon Garmire, Leisa Townsley and Pat Broos. We have used the results from the optical measurements to determine the optical constants for polyimide in the measured wavelength band and predict the optical transmission performance of the flight filters.
Mark Bautz