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Author Raveendran, A. V. ♦ Srinivasulu, G. ♦ Muneer, S. ♦ Mekkaden, M. V. ♦ Jayavel, N. ♦ Somashekar, M. R. ♦ Sagayanathan, K. ♦ Ramamoorthy, S. ♦ Rosario, M. J. ♦ Jayakumar, K.
Source Indian Institute of Astrophysics
Content type Text
File Format PDF
Language English
Subject Domain (in DDC) Natural sciences & mathematics ♦ Physics
Subject Keyword Instrumentation ♦ Polarimeters-Techniques ♦ Polarimetric-Methods ♦ Observational, Data Analysis
Description A new astronomical photo-polarimeter that can measure line ar polarization of point sources simultaneously in three spectral bands was designed and bui lt in the Institute. The polarime- ter has a Calcite beam-displacement prism as the analyzer. T he ordinary and extra-ordinary emerging beams in each spectral band are quasi-simultaneou sly detected by the same photo- multiplier by using a high speed rotating chopper. The effect ive chopping frequency can be set to as high as 200 Hz. A rotating superachromatic Panchara tnam halfwave plate is used to modulate the light incident on the analyzer. The spectral ba nds are isolated using appropriate dichroic and glass filters. A detailed analysis shows that the reduction of 50% in the effic iency of the polarimeter because of the fact that the intensities of the two beams are m easured alternately is partly compensated by the reduced time to be spent on the observatio n of the sky background. The use of a beam-displacement prism as the analyzer complet ely 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 po larized and unpolarized stars show a very high mechanical stability for the polarimeter. T he position angle of polarization produced by the Glan-Taylor prism in the light path is found t o be slightly wavelength- dependent, indicating that the fixed super-achromatic half wave plate in the beam does not fully compensate for the variation in the position angle of the effe ctive 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-de pendent with a total amplitude of 0.271% in the U − I region; its mean value is 99.206%. The instrumental polariz ation 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 observa tions of polarized stars show that the agreement between the measured polarization values and tho se available in the literature to be excellent.
Educational Use Project ♦ Reading
Education Level UG and PG
Learning Resource Type Technical Report
Publisher Institution Indian Institute of Astrophysics
Issue Number 15
Starting Page 1