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Session 57 - Astrophysical Diagnostics from Polarimetry.
Display session, Wednesday, June 12
In order to achieve a qualitative improvement in descriptions of the magnetic fields associated with small, dense molecular clouds, traced via background starlight polarization, a CCD-based imaging polarimeter was built and operated. This instrument simultaneously measured the linear polarizations of 25 - 50 stars per 5 arcminute CCD frame (Clemens and Leach 1987, Optical Engineering, 29, 923). The instrument consisted of a rotating polaroid in front of a quarter-wave plate, broadband V filter, and a TI 800 \times 800 pixel CCD. The observing mode involved polarization chopping and dual direction CCD charge shifting, resulting in images which contained two stellar objects for each star.
However, decoupling of the flat-field response of the CCD from the shifted-and-added images is not trivial. One solution was to ignore the flat-field step and proceed to photometric and then polarimetric analysis of the star light, as was performed by Kane in his dissertation (1995, Boston University).
Revisiting the flat-fielding problem, we have developed a new approach which borrows from techniques employed in ground-based infrared astronomy. In this method, we analyze the shape of the stellar background (sky) imaged onto the CCD, via comparison with dome flats, to develop a detailed sky model. Subtraction of this sky model from the raw images leaves sky corrected stellar images. Stellar pair matching and classification is used to extract the individual stellar PSFs for direct flat field correction before performing photometry.
This detailed analysis results in lower random and systematic errors than our previous method. In this poster, we present the polarimetric map deduced from the light of stars behind the periphery of the star forming globule B335, and compare the polarimetric limits obtained using our two methods.
Program listing for Wednesday