FOS IDT REPORT CAL/FOS-078

FOS POLARIMETRY CALIBRATIONS


Richard G. Allen and Paul S. Sinith

March 1, 1992


Abstract

Ground-based calibrations of the FOS polarizer were performed at the
Martin Marietta Company in Denver, Colorado in 1983 and 1984 and at
the Lockheed Missiles and Space Company in Sunnyvale, California in
1988. The required SV/OV calibrations have now been completed as weil,
and the poiarizer is being routinely scheduled for astronomical
observations. Since the orbital calibrations have shown that the
polarizer can be used effectively with the 4.3-arcsec A-1 target
acquisitiou aperture, the overall efficiency of the polarizer is only
slightly degraded by the poor imaging of the HST. The geomagnetic
image motion problem of the red Digicon, however, precludes the use of
the A-side of the instrument for polarimetry a.t the present
time. Observations should thus be taken on the B-side with the
4.3-arcsec A-1 target acquisition aperture whenever possible. When
better background rejection is needed or higher spectral resolution is
required, tbe polarizer can be used with the 1.O-arcsec B-3 circular
aperture. Calibrated observations can be taken in the wavelength range
1280-3310 Ä with the B-waveplate and gratings H13, H19, and H27.