7.9 Polarization Observations

Polarization observations require three or more images with the polarizing filter spanning a large range of position angles on the sky. For WFPC2, this may be achieved by using different quads of the polarizing filter (each quad being oriented 45° to the others), by rotating the spacecraft though different angles, or by a combination of these methods. Rotating the spacecraft through use of ORIENTs provides the simplest calibration, as a single polaroid can be used for all images. However, in practice, it will be the most difficult method to schedule. For further information see Biretta and Sparks (1995, ISR WFPC2 95-01).

We note that WFPC2 has significant instrumental polarizations which will make measurements on targets with less than 3% polarization difficult. The pick-off mirror introduces about 6% instrumental polarization. Furthermore, the pair of mirrors in the calibration channel, which is used to generate the polarizer flat fields, introduces ~12% polarization. In principle these effects can be calibrated out, but this has yet to be demonstrated.

The polarizers are most effective in the range from 3000Å to 6500Å; this corresponds roughly to filters in the range F255W to F675W. At shorter wavelengths the transmission decreases sharply, and at longer wavelengths they cease to polarize the incoming light.