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.
