FAQ on FOC Polarization Observations

Q:  Can polarization observations be made with the FOC and COSTAR,
    or do the oblique reflections off the COSTAR mirrors change
    the polarization too much?

A:  Light reflects off the primary and secondary mirrors of the
FOC f/96 relay at angles of incidence of about 5.3 and 11.1
degrees respectively, and the reflection off the fold and focus
mirror (which follows the polarizers) is at 11.5 degrees. In
contrast, the angles of incidence for the FOC COSTAR mirrors
are only about three degrees. The introduction of the COSTAR
reflections should therefore have a negligible effect on the
FOC polarization. Pre-COSTAR observations of an unpolarized
target (NGC 5272) showed that the instrumental polarization was
of order one percent or less. Post-COSTAR observations of the
same target show about three percent polarization, which is far
more than expected. The reason for this is not yet clear, but
as long as it remains constant it should not cause any
problem.

It should also be pointed out that a polarimeter based on three
separate polarizers cannot be expected to yield extremely
accurate results. The polarizers transmit different amounts at
different wavelengths, three separate images must be taken at
different times, and low level saturation may differ between
the images for a strongly polarized source. There may also be
differences in geometric distortion and focus.