Optical Filters

3.5 Polarizer Quad Filter

The polarizer quads were also designed to map onto a four-faceted WFC configuration and, consequently, also require a partial filter rotation of -33 degrees to move the filter quadrant 1 (nominal polarization angle 135 degrees ) into WFCs 2 and 3, with some vignetting of both camera fields. Several additional partial rotations have been added to allow observations with different polarization angles on the same CCD.

The polarizer quad may be used in several ways: by observing the target with each camera, by observing the target with the same camera using different partial rotations of the polarizer quad, or by observing the target with the same camera using different roll angles of the spacecraft. The first method has the drawback that calibration is complicated by uncertainties in the relative photometric calibration between cameras, while the second method uses the same camera but has non-optimal polarization angles and limited fields of view. The third method may present scheduling difficulties due to constraints on the spacecraft roll angle, and the need to rotate undersampled images. (See Biretta and Sparks 1995, "WFPC2 Polarization Observations: Strategies, Apertures, and Calibration Plans," WFPC2 Instrument Science Report 95-01.)

Table 3.10: Polarizer Quad Filter. Polarization angle 0 degrees lies along +X direction in Figure 3.11

The polarizer is designed for problems where large polarization angles are observed, and is not suitable for problems requiring precision of order 3% or better. The required polarization angle is selected by filter name and aperture location. The transmission of the quad polarizer is shown in Figure 3.7. The polarizer is afocal and must therefore usually be used with another filter which will largely define the shape of the passband.

Figure 3.6: Polarizer Quads. The schematics show the filter projected onto the field-of-view for all rotated positions. Apertures are marked. Dashed lines indicate the central region of each quad which is free of vignetting and cross-talk. Greyscale images are VISFLATs of the polarizer with F555W.

Figure 3.7: Polarizer Transmission for light polarized perpendicular (dotted curve) and parallel (solid curve) to the filter polarization direction.