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PlanetPol: A Very High Sensitivity Polarimeter
We have built and used on several occasions an optical broadband stellarpolarimeter, PlanetPol, which employs photoelastic modulators andavalanche photodiodes and achieves a photon-noise-limited sensitivity ofat least 1 in 106 in fractional polarization. Observations ofa number of polarized standards taken from the literature show that theaccuracy of polarization measurements is ~1%. We have developed a methodfor accurately measuring the polarization of altitude-azimuth mountedtelescopes by observing bright nearby stars at different parallacticangles, and we find that the on-axis polarization of the WilliamHerschel Telescope is typically ~15 × 10-6, measuredwith an accuracy of a few parts in 107. The nearby stars(distance less than 32 pc) are found to have very low polarizations,typically a few ×10-6, indicating that very littleinterstellar polarization is produced close to the Sun and that theirintrinsic polarization is also low. Although the polarimeter can be usedfor a wide range of astronomy, the very high sensitivity was set by thegoal of detecting the polarization signature of unresolved extrasolarplanets.
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