The following paper deals with WF/PC-1 data and polarization artifacts which may be caused by features in the flat fields. While the WFPC2 flats are generally more accurate than the WF/PC-1 flats, it may still be useful to consider these issues when planning WFPC2 polarization observations.
-J. Biretta, WFPC2 Group Lead
WFPC Polarimetry of NGC1068 on 01/05/92 Revisted
Anthony J. Weitenbeck and Arthur D. Code
(U of Wisconsin, Madison)
Abstract:
This observation was analyzed by Capetti et al (ApJ 446,155). They blank out an area in their figures as having unreliable flat field. This shows on c191643ew.r6h flat field (which was made on 01/09/92, and from which the flat fields for the polaroids were made) as a roughly circular area about 0.6" across greater than the surrounding flat field values by as much as 60%. In their figures 7 and 8 of polarized flux and polarization an area coincident with Cloud D of Evans et al (ApJ 369,L27) has significant polarization. We would like to point out that there is a similar high area of the flat field at that position also (about 0.5" across, maximum about 20% higher than the surrounding flat field), which suggests that polarization observed at that position is also at least partly an artifact of the flat field. There is also a similar elevated area of the flat field almost exactly on the opposite side of Cloud B (Evans et al designation) as the area blocked off by Capetti et al. (but which is not on their plots because of their chosen flux and polarization value plotting limits), which also shows high polarization in our polarization and polarized flux plots.