FOC Instrument Science Report #089

FOC Instrument Science Report #089       July 21, 1995

"Polarizer Throughputs and Image Shifts"
by P.E. Hodge

Abstract excerpted from the report:

Observations of an unpolarized globular cluster (NGC 5272)
were made with the three FOC polarizers in order to measure 
the relative throughput and position offset of each polarizer.
Aperture photometry was used to compare the brightnesses and
positions of stars. The relative positions were determined
to an accuracy of better than a quarter of a pixel. No useful
limits on the instrumental polarization were determined, however.
The measured star fluxes were very sensitive to the aperture size
and the sky subtraction. The change with aperture size is a
reflection of the differences in the PSFs with the various
polarizers. Other factors were the limited number of total counts
in the star images and the typical variation of about one percent
in the FOC sensitivity between exposures.