A new three-band, two beam astronomical photo-polarimeter

G. Srinivasulu1*, A. V. Raveendran2, S. Muneer1†, M. V. Mekkaden3, N. Jayavel4, M. R. Somashekar1, K. Sagayanathan1, S. Ramamoorthy1, M. J. Rosario5 and K. Jayakumar6‡
1Indian Institute of Astrophysics, Bangalore 560034, India
2399, "Shravanam", 2nd Block, 9th Phase, J P Nagar, Bangalore 560108, India
3No 82, 17E Main, 6th Block, Koramangala, Bangalore 560095, India
4No 22, Bandappa lane, New Byappanahalli, Bangalore 560038, India
5210, 4th Main, Lakshmi Nagar Extn, Porur, Chennai 600116, India
624, Postal Nagar, Ampuram, Vellore-632009, India

Received: July 23 2014 | Accepted: May 05 2015

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We designed and built a new astronomical photo-polarimeter that can measure linear polarization simultaneously in three spectral bands. It has a Calcite beamdisplacement prism as the analyzer. The ordinary and extra-ordinary emerging beams in each spectral bands are quasi-simultaneously detected by the same photomultiplier by using a high speed rotating chopper. A rotating superachromatic Pancharatnam halfwave plate is used to modulate the light incident on the analyzer. The spectral bands are isolated using appropriate dichroic and glass filters.
We show that the reduction of 50% in the effciency of the polarimeter because of the fact that the intensities of the two beams are measured alternately is partly compensated by the reduced time to be spent on the observation of the sky background. The use of a beam-displacement prism as the analyzer completely removes the polarization of background skylight, which is a major source of error during moonlit nights, especially, in the case of faint stars.
The field trials that were carried out by observing several polarized and unpolarized stars show the performance of the polarimeter to be satisfactory.

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Keywords : instrumentation: polarimeters – techniques: polarimetric – methods: observational, data analysis