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 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.
Polarimetric observations require at least three images taken with
polarizing optics that allow the determination of the degree and position angle of polarization as well as the total intensity of the source. Each set of ACS polarizers comprises three individual 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 is 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 begins to degrade 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.
To first approximation, the ACS polarizer sets comprise three
essentially perfect polarizers. In such cases, the Stokes parameters (I, Q, U) can be computed using simple arithmetic. Using im1, im2, and im3 to represent the images taken through the polarizers POL0, POL60, and POL120 respectively, the Stokes parameters are:
These parameters can be converted to the degree of polarization, P, and
the polarization angle, [theta], measured counterclockwise from the x axis as follows:
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 polarization degree and signal-to-noise. For three perfect polarizers oriented at 60°
relative position angles (as in ACS), the uncertainty in the polarization degree 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, they found
This above 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 ACS polarizers are designed for ease of use. The observer selects
the camera (either HRC or WFC) and the spectral filter, then takes successive images with the three polarizers of the VIS set (POL0V, POL60V, POL120V) or the UV set (POL0UV, POL60UV, POL120UV). Once the camera and polarizer set are specified, the scheduling system automatically generates the slews that place the target in the optimal region of the field of view.
Because the POLUV and POLV sets are housed on separate filter
wheels, the number of spectral filters available to each set is restricted. The available filters were determined according to the relative performance of the polarizers and the near-UV limitations of the WFC caused by its silver mirror coatings.
The POLUV polarizers are mounted on Filter Wheel 1 and may be
crossed with the near-UV filters mounted on Filter Wheel 2. The POLV polarizers are mounted on Filter Wheel 2 and may be crossed with filters on Filter Wheel 1, namely the primary broadband filters, and discrete narrowband filters Hα
, [OII], and their continuum filters. Because supported calibration time is limited, STScI does not support the use of ramp filters with either polarizer set. GOs must plan their own calibration observations if they use the ramp filters with the polarizers.
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 a unvignetted circular field of view of 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
Occasionally observers desire non-polarized images of targets at the
same location on the detector as their polarized images. Doing so was straightforward with the HRC; one merely had to take an exposure without the polarizer in place. However, WFC polarimetry automatically invokes a large slew from the non-polarimetric imaging position. To obtain a non-polarized image at the same physical detector location as the polarized images, one must specify the WFC1-2K aperture instead of the WFC aperture (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.
The most accurate ACS polarimetry was obtained in the visible bands
(i.e., F606W) with the HRC and POLV polarizers. This mode had the advantage of very high rejection of perpendicular polarization, and known mirror coatings with readily modeled properties. Because the WFC mirror coatings are proprietary, models of their polarization properties (e.g., the phase retardance of the IM3 mirror) are unavailable. Consequently, calibrating WFC polarized images is much more difficult than calibrating the HRC images.
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 be 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.
The polarizers contribute a weak geometric distortion that rises to about
0.3 pixels near the edges of the HRC field-of-view. This distortion is caused by a weak positive lens in the polarizers that is needed to maintain proper focus when multiple filters are in the beam. The POLV polarizers also have a weak ripple structure intrinsic to their polaroid coatings. These ripples contribute an additional ±
0.3 pixel distortion with a very complex structure (see ACS ISR 2004-10
and ACS ISR 2004-11
). All these geometric effects are correctable with the drizzle software. However, astrometry will be less accurate with the polarizers because of residual errors and imperfect corrections.
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 likely to be 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.