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: http://www.stsci.edu/instrument-news/handbooks/wfpc2/W2_75.html
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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.