The ACS has an imaging polarimetric capability. Polarization observations require a minimum of three images taken using polarizing optics with different polarization characteristics in order to solve for the source polarization unknowns (polarization degree, position angle, and total intensity). To do this, ACS offers two sets of polarizers, one optimized for the blue (POLUV) and the other for the red (POLV). These polarizers can be used in combination with most of the ACS filters (see Table 6.2) allowing polarization data to be obtained in both the continuum and in line emission; and to perform rudimentary spectropolarimetry by using the polarizers in conjunction with the dispersing elements. (Due to the large number of possibilities in combination with ramp and dispersing elements, and heavy calibration overheads, observers wishing to use those modes should request additional calibration observations). For normal imaging polarization observations, the target remains essentially at rest on the detector with a suitable filter in beam, and an image is obtained with each of the appropriate polarizing elements in turn. The intensity changes between the resulting images provide the polarization information.
Each set of polarizers comprises three individual polarizing filters with relative position angles 0º, 60º, and 120º. The polarizers are designed as aplanatic optical elements and are coated with "Polacoat 105UV" for the blue optimized set, and HN32 polaroid for the red set. The blue/near-UV optimized set is also effective all through the visible region, giving a useful operational range from approximately 2000 Ċ to 8500 Ċ. The second set is optimized for the visible region of the spectrum and is fully effective from 4500 Ċ to about 7500 Ċ.
The relative performance of the UV-optimized versus the visible optimized polarizers is shown in Figure 6.1.
Figure 6.1: Throughput and rejection of the ACS polarizers.The visible polarizers clearly provide superior rejection for science in the 4500 Ċ to 7500 Ċ bandpass, while the UV optimized polarizers deliver lower overall rejection across a wider range from 2000 Ċ to 7500 Ċ. While performance of the polarizers begins to degrade at wavelengths longer than about 7500 Ċ, useful observations should still be achievable to approximately 8500 Ċ in the red. In this case, allowance for imperfect rejection of orthogonally polarized light should be made at the analysis stage.
Imperfections in the flat fields of the POLVIS polarizer set have been found which may limit the optimal field of view somewhat. Potential users are encouraged to check ACS ISR 2005-10
the STScI ACS Web
site for the latest information.
To first approximation, the ACS polarizers can be treated as three essentially perfect polarizers. The Stokes parameters (I, Q, U) in the most straightforward case of three images obtained with three perfect polarizers at 60º relative orientation, 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 as follows:
These values can be converted to the degree of polarization P and the polarization angle , measured counterclockwise from the x axis as follows:
A more detailed analysis, including allowance for imperfections in the polarizers may be found in Sparks & Axon, 1999 PASP, 111, 1298. They find that the important parameter in experiment design is the product of expected polarization degree and signal-to-noise. A good approximation for the case of three perfect polarizers oriented at the optimal 60º relative position angles (as in ACS) is that the error on the polarization degree P (which lies in the range 0 for unpolarized to 1 for fully polarized) is just the inverse of the signal-to-noise per image. Specifically, they found
where
is the signal to noise of the ith image; and
The above discussion is for ideal polarizers with no instrumental polarization. Of course, the reality is that the polarizer filters, especially the UV polarizer, has significant leakage of cross-polarized light. The instrumental polarization of the HRC ranges from a minimum of 4% in the red to 14% in the far-UV, while that of the WFC is ~2% (see ACS ISR 2004-09
). Other effects, such as phase retardance in the mirrors, may be significant as well. Please consult the ACS Data Handbook (http://www.stsci.edu/hst/acs/documents/handbooks/DataHandbookv4/ACS_longdhbcover.html
), STScI Web pages, and ISRs (http://www.stsci.edu/hst/acs/documents/isrs
) for more detailed information.
The implementation of the ACS polarizers is designed for ease of use. The observer merely selects the camera (either HRC or WFC) and the spectral filter, and then takes images stepping through the three filters of either the VIS set (POL0V, POL60V, POL120V) or the UV set (POL0UV, POL60UV, POL120UV). Once the camera and polarizer are specified, the scheduling system automatically generates slews to place the target in the optimal region of the field of view.
Since the ACS near-UV and visible filter complement is split between two filter wheels, there are restrictions on which filters the polarizer sets can be combined with. The choices available were determined by the relative performance of the polarizers and the near-UV limitations of the WFC resulting from the silver mirror coatings.
The near-UV optimized polarizers are mounted on Filter Wheel 1 and may be crossed with the near-UV filter complement, which are mounted on Filter Wheel 2. The visible optimized polarizers are mounted on Filter Wheel 2 and can be crossed with filters on Filter Wheel 1, namely the primary broadband filters, and discrete narrowband filters H, [OII], and their continuum filters. Due to the calibration overhead required, we do not plan to support the use of ramp filters with either polarizer set. GOs are required to include calibration observations if they plan to use the ramp filters with the polarizers.
The polarizer sets are designed for use on the HRC where they offer a full unvignetted field of view, 29 ×26 arcseconds with any of the allowable filter and coronagraph combinations including those ramps and spectroscopic elements that may also be used on the HRC (although see above re. additional calibrations).
The same allowable combinations, either UV or visible optimized, may also be used on the WFC where an unvignetted field of view of diameter 70 arcseconds is obtained. This does not fill the field of view of the WFC due to the small size of the polarizing filters. However, it does offer an area approximately five times larger than that obtained on the HRC. In order to avoid the gap between the WFC CCDs, and to optimize the readout noise and CTE effects, the scheduling system will automatically slew the target to roughly pixel (3096,1024) on the WFC1 CCD whenever the WFC aperture is selected in conjunction with one of the polarizers. Also, to reduce camera overhead times, only a 2048 x 2048 subimage centered on the target location will be readout from WFC1 (see Table 6.1).
Occasionally observers will ask to obtain non-polarized images at the same physical location on the detector as their polarized images. This is straightforward for the HRC; one merely takes the exposure without the polarizer filter. However, for the WFC it is more complicated because specifying WFC together with a polarizer automatically invokes a large slew, whereas no slew is performed when the polarizer filter is omitted. To obtain a non-polarizer image at the same physical detector location as the polarizer image in the WFC, one needs to specify the aperture as WFC1-2K instead of WFC (see Table 6.1).
The filters specified in Table 6.2 are those that we expect users to choose for their polarization observations. We will calibrate the most popular of these filters. Filter combinations not on this list will most probably not be calibrated, so potential users who have a strong need for such a polarizer/filter combination should include any necessary calibrations in their proposals.
We anticipate that the most accurate polarization observations will be obtained in the visible band (i.e., F606W) with the HRC and the visible polarizers. This mode has the advantages of a very high rejection of perpendicular polarization, and known mirror coatings with readily modeled properties. The WFC may be capable of similar accuracy to the HRC; however, its proprietary mirror coatings will make modeling of the polarization properties, and hence calibration, much more difficult (e.g., unknown phase retardance effects in the WFC IM3 mirror are a concern).
Polarimetry in the UV will be more challenging for a number of reasons. The UV polarizer has relatively poor rejection in the UV, and the instrumental polarization of the HRC, which is 4% to 7% in the visible, rises to 8% to 9% in the UV, and reaches 14% at 2200 Ċ (see ACS ISR 2004-09
). Far-UV polarimetry will be especially challenging since the polarizer properties were not well-characterized shortwards of 2800 Ċ, and appear to change rapidly with wavelength. Moreover, the low UV transmission of the UV polarizer, and the poor polarization rejection in the far-red, work together to exacerbate red leaks which are normally seen in the far-UV spectral filters.
The polarizer filters contribute a weak geometric distortion which rises to about 0.3 pixels near the edges of the HRC field-of-view. This is caused by a weak positive lens in the polarizers, which is needed to maintain proper focus when multiple filters are in the beam. In addition, the visible polarizer has a weak ripple structure which is related to manufacture of its polaroid material; this contributes an additional ħ0.3 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 likely be less accurate in the polarizers due to residual errors and imperfect corrections.
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