PlanetPol: A very high sensitivity polarimeter

J. H. Hough*, P. W. Lucas, J. A. Bailey, M. Tamura, E. Hirst, D. Harrison, M. Bartholomew-Biggs

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

83 Citations (Scopus)


We have built and used on several occasions an optical broadband stellar polarimeter, PlanetPol, which employs photoelastic modulators and avalanche photodiodes and achieves a photon-noise-limited sensitivity of at least l in 106 in fractional polarization. Observations of a number of polarized standards taken from the literature show that the accuracy of polarization measurements is ∼1%. We have developed a method for accurately measuring the polarization of altitude-azimuth mounted telescopes by observing bright nearby stars at different parallactic angles, and we find that the on-axis polarization of the William Herschel Telescope is typically -15 × 10-6, measured with 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 little interstellar polarization is produced close to the Sun and that their intrinsic polarization is also low. Although the polarimeter can be used for a wide range of astronomy, the very high sensitivity was set by the goal of detecting the polarization signature of unresolved extrasolar planets.

Original languageEnglish
Pages (from-to)1302-1318
Number of pages17
JournalPublications of the Astronomical Society of the Pacific
Issue number847
Publication statusPublished - Sept 2006


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