ACS offers two sets of polarizers, one optimized for near-UV/blue wavelengths (POLUV) and one optimized for visible/red wavelengths (POLV). These polarizers can be combined with most of the ACS filters (Table 6.1
), allowing polarimetry in both continuum and line emission. Rudimentary spectropolarimetry is possible by using the polarizers in conjunction with the dispersing elements. The large number of possible spectropolarimetric modes prevents automatic calibration of these modes by STScI, so observers must request additional orbits for their own spectropolarimetric calibrations. For normal polarimetric imaging, the location of the target remains fixed on the detector as successive images are obtained using a suitable filter and each of the constituent polarizers of the desired set. The varying intensities of the source in the successive images provide the basic polarimetric data for the source.
At least three polarized images are required to determine the degree and position angle of polarization as well as the total intensity of the source. Each set of ACS polarizers comprise three polarizing filters with relative position angles 0°
, and 120°
. The polarizers are aplanatic optical elements coated with Polacoat 105UV (POLUV set) and HN32 polaroid (POLV set). The POLUV set is effective throughout the visible region; its useful range is approximately 2000 Å to 8500 Å. The POLV set is optimized for the visible region of the spectrum and is fully effective from 4500 Å to about 7500 Å.
The relative performance of the POLUV and POLV polarizers are shown in Figure 6.1
. The POLV set provides superior perpendicular rejection in the 4500 Å to
7500 Å bandpass, while the POLUV set delivers lower overall rejection across a wider range from 2000 Å to
7500 Å. Performance degrades at wavelengths longer than about 7500 Å, but useful observations may still be obtained up to approximately 8500 Å. In such cases, imperfect rejection of orthogonally polarized light must be considered during data analysis.
A more detailed analysis, including allowances for imperfections in the polarizers, is given by Sparks & Axon (1999, PASP
, 111, 1298), who found that the important parameter in experiment design is the product of expected degree of polarization and signal-to-noise. For three perfect polarizers oriented at 60°
relative position angles (as in ACS), the uncertainty in the degree of polarization P (which ranges from 0 for unpolarized light to 1 for fully polarized light) is approximately the inverse of the signal-to-noise per image. Specifically, Sparks & Axon found
This analysis pertains to ideal polarizers with no instrumental polarization. However, the ACS polarizers (especially the POLUV polarizers) allow significant leakage of cross-polarized light and the instrumental polarization of the WFC is ~2% (see ACS ISR 2004-09
). The instrumental polarization of the HRC ranged from a minimum of 4% in the red to 14% in the far-UV. Other effects, such as phase retardance in the mirrors, may be significant as well. Please consult the STScI Web pages for more detailed information, especially the ACS Data Handbook
The polarizer sets were designed primarily for use with the HRC, where they offer a full unvignetted field of view (29 ×
26 arcseconds) with any allowed imaging, spectroscopic, and coronagraphic combinations. When used with the WFC, the polarizers provide an unvignetted circular field of view with a diameter 70 arcseconds. Although this field of view is significantly smaller than the normal WFC field of view, it is approximately five times larger than that obtained with the HRC. To avoid the gap between the WFC CCDs and to optimize readout noise and CTE effects, the scheduling system will automatically slew the target to pixel (3096,1024) on the WFC1 CCD whenever the WFC aperture and polarizer sets are selected. To reduce camera overhead times, a 2048 x 2048 subimage centered on the target will be readout from WFC1 (Table 6.2
lists the filters for which imaging polarimetry is available. STScI provides polarimetric calibration for the most popular of these filters. Filters not listed will not be calibrated, so users desiring imaging polarimetry in those bandpasses must include the necessary calibrations in their proposals.
UV polarimetry with ACS is challenging because the POLUV polarizers have relatively poor orthogonal rejection and the instrumental polarization of the HRC, which was 4% to 7% in the visible, rose to 8% to 9% in the UV and reached 14% at 2200 Å (see ACS ISR 2004-09
). Far-UV polarimetry is especially challenging because the POLUV properties are not well-characterized shortwards of 2800 Å, and they appear to change rapidly with wavelength. Moreover, the low UV transmission of the POLUV polarizers and the poor rejection in the far-red exacerbate the red leaks seen in the far-UV spectral filters.
Finally, the POL0V and POL60V filters contain bubbles which impact the PSF and flat fields. These bubbles are far out of focus and appear as large concentric bright and dark rings (400 pixels diameter in WFC, 370 pixels in HRC) and streaks in the flat fields. The worst case appeared in HRC POL60V images, where the amplitude of the artifacts reaches ±
4% in the flats and the affected region was roughly centered at pixel (835,430). Polarimetric combinations involving POL0V or the WFC are relatively minor, with typical amplitudes of ±
1%. Observers requiring precision polarimetry should avoid these regions of the field of view. Although the polarizer flats attempt to correct these features (see ACS ISR 2005-10
), the corrections are imperfect and dependent on the brightness and angular size of the target. The locations of these features and their effects can be discerned more accurately by examining the P-flats for the respective spectral filter crossed with the visual polarizers.