A stellar photo-polarimeter

Abstract

A new astronomical photo-polarimeter that can measure linear polarization of point sources simultaneously in three spectral bands was designed and built in the Institute. The polarimeter has a Calcite beam-displacement prism as the analyzer. The ordinary and extra-ordinary emerging beams in each spectral band are quasi-simultaneously detected by the same photomultiplier by using a high speed rotating chopper. The effective chopping frequency can be set to as high as 200 Hz. 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. A detailed analysis shows that the reduction of 50% in the efficiency 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 a very high mechanical stability for the polarimeter. The position angle of polarization produced by the Glan-Taylor prism in the light path is found to be slightly wavelengthdependent, indicating that the fixed super-achromatic halfwave plate in the beam does not fully compensate for the variation in the position angle of the effective optical axis of the rotating plate. However, the total amplitude of variation in the U − I spectral region is only 0.◦92. The polarization efficiency is also found to be wavelength-dependent with a total amplitude of 0.271% in the U −I region; its mean value is 99.211%. The instrumental polarization is found to be very low. It is nearly constant in the V − I spectral region ( 0.04%), and apparently, it increases slightly towards the ultraviolet. The observations of polarized stars show that the agreement between the measured polarization values and those available in the literature to be excellent.