The POL0V and POL60V filters contain bubbles which impact the PSF and flat fields. These bubbles are far out of focus, and hence appear as large patterns (400 pixels diameter in WFC, 370 pixels in HRC) of concentric bright and dark rings, and streaks in the flats. The worst case is the HRC in the POL60V filter, where the amplitude reaches ±4% in the flats; the affected region for this case is roughly centered at pixel (835,430). Other combinations involving the POL0V filter or the WFC show more minor effects, with amplitudes typically ±1%. Observers requiring precision polarimetry would do well to avoid these regions of the field of view. While the polarizer flats attempt to correct these features (see ACS ISR 2005-10
), the corrections are likely to be imperfect and depend somewhat on the illumination provided by the target (i.e., stellar vs. extended target, etc.). The locations of these features and their effects can be discerned more accurately by examining the "pfl" flats for the respective spectral filter crossed with the visual polarizers.
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