Michele D.De La Peña, Alex Storrs, Anuradha Koratkar, and Charles Keyes (STScI)
A post-COSTAR polarization correction has been implemented for spectropolarimetry data acquired with the HST Faint Object Spectrograph (FOS). This correction essentially removes the wavelength-dependent COSTAR-induced instrumental polarization in the FOS Blue observations. In uncorrected data, the post-COSTAR polarization in Stokes Q was found to vary from 0 - 3% in the 1600 - 3300 Å spectral range; the Stokes U was found to vary from 0.0 - 0.5% over the same range.
The FOS calibration pipeline program, calfos, which is implemented as an IRAF/STSDAS task has been modified to process both pre- and post-COSTAR data properly. Further, the post-COSTAR correction is also available in the calpolar task which performs the same routine spectropolarimetric processing of FOS data as calfos, but calpolar provides the user with the additional freedom to manipulate the flux calibrated data prior to polarization processing.
Application of the post-COSTAR corrections depends upon the data being properly calibrated using the standard ``pre-COSTAR'' corrections, with the exception the data are left in the instrument frame rather than being rotated onto the sky frame. The Stokes parameters are then corrected to remove the additional polarization imposed by the COSTAR mirrors; the linear and circular polarization, as well as the phase angle are rederived; and the final results are rotated onto the sky frame.