ACS contains a set of six filters that are sensitive to linear polarization; there are three visible polarizer filters with their polarization directions set at nominal 60°
angles to each other, and three UV polarizer filters arranged in a similar manner. These filters are typically used in combination with a spectral filter which largely defines the spectral bandpass. In most cases observers will obtain images of the target in each of the three filters. The initial calibration steps for polarization data are identical to that for data taken in any other filter—the data are bias-corrected, dark-subtracted, and flat-fielded in the normal manner. The polarization calibration itself is accomplished by combining the set of images (or the resulting counts measured on the images) in the three filter rotations to produce a set of I, Q, and U images, or equivalently, a set of images giving the total intensity, fractional polarization, and polarization position angle.
The instrumental polarization, defined as the instrument's response to an unpolarized target, provides a simple measure of some of these effects. Figure 5.5
shows the instrumental polarization derived for the HRC through on-orbit observations of unpolarized stars (HST
). The instrumental polarization is approximately 5% at the red end of the spectrum, but rises in the UV to about 14% at the shortest wavelengths. Also shown is a rough model for the effects of the M3 mirror together with a very crude model of the CCD. The mirror is aluminum with a 606
Å thick overcoat of Magnesium Fluoride and has an incidence angle of 47°
. Since details of the CCD are proprietary, it has been simply modeled as Silicon at an incidence angle of 31°
; no doubt this is a serious over-simplification. Figure 5.6
shows the same plot for the WFC, which has an instrumental polarization around 2%. Here the IM3 mirror is a proprietary Denton enhanced Silver Coating with an incidence angle of 49°
, and the CCD has an incidence angle of 20°
. While the lower instrumental polarization of the WFC seems attractive, users are cautioned that the phase retardance effects are not known for the Denton coating, and have some potential to cause serious problems—if sufficiently large, the retardance could produce a large component of elliptical polarization which will be difficult to analyze with the linear polarizers downstream.
An extensive series of on-orbit polarization calibration observations were carried out in Cycles 11 and 12 (programs 9586
, and 10055
). These included observations of unpolarized and polarized standard stars, the star cluster 47
Tuc, and an extended reflection nebula. Additional observations of polarized standards were taken over a wide and well-sampled range of HST
roll angles to help quantify the angular dependences which are expected as the wavefront interacts with the diattenuation and phase retardation in the mirrors and CCD.
Preliminary calibrations, based largely on data in programs 9586
, are available for use by polarization observers. The number of polarimetric observations obtained with ACS is very small compared to other modes. As a result of this, the polarimetric mode has not been calibrated as precisely as other modes because of limited resources. For details, please see ACS ISR 2007-10.
, spectral filter
) were applied to the observed count rate robs
in each of the three polarizers (POLnUV or POLnV, where n = 0, 60, 120). These corrections are tabulated in Table 5.6
, and have been scaled such that Stokes I will approximate the count rate seen with no polarizing filter.
Finally, the position angle on the sky of the polarization E-vector is computed. The parameter PAV3 is the roll angle of the HST
spacecraft, and is called PA_V3
in the data headers. The parameter
contains information about the camera geometry which is derived from the design specifications; for HRC,
, and for the WFC,
. Note that the arc tangent function must be properly defined; here, the result is defined as positive in quadrants I and II, and negative in III and IV.
The full instrumental effects and the above calibration have been modeled together in an effort to determine the impacts of the remaining uncalibrated systematic errors. These will cause the fractional polarizations to be uncertain at the one-part-in-ten level (e.g., a 20% polarization has an uncertainty of 2%) for highly polarized sources; and at about the 1% level for weakly polarized targets. The position angles will have an uncertainty of about 3°
. (This is in addition to uncertainties which arise from photon statistics in the observer's data.) This calibration has been checked against polarized standard stars (~5% polarized) and found it to be reliable within the stated errors. Better accuracy will require improved models for the mirror and detector properties as well as additional on-orbit data. No calibration has been provided for F220W, F250W, or F814W, as they are believed to be too unreliable at this time. There is also some evidence of a polarization pathology in the F625W filter, and observers should be cautious of it until the situation is better understood. In addition, one incidence of a 5°
PA error for F775W has been observed, suggesting this waveband is not calibrated as well as the others.