A&A Supplement Series, Vol. 126, November II 1997, 113-119
Received November 19; accepted December 19, 1996
A.K. Sen, and M. Kakati
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Centre of Plasma Physics, Dispur, Guwahati 781 006, India
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When astronomical observations are made for a celestial object, with non -zero field angle (wrt telescope axis), the beam of parallel rays from the celestial object strikes the telescope mirror obliquely. Each unpolarized ray of light when it strikes the metal coated mirror surface gets polarized due to reflection. On the contrary, when the field angle is zero, these reflected rays for a field star, combine together to produce an instrumental polarization effect. A linearly polarized star when observed even at zero field angle, exhibits depolarization due to the above effect. A detailed procedure has been worked out here to estimate such polarization effects at the prime and Cassegrain focii, considering the case for linear polarizations only.
Also to find the typical values of such polarization, a 2.3 m telescope having beam sizes f/3.23 and f/13 at the prime and Cassegrain focii, has been considered. The instrumental polarization values as calculated at these two focii are 0.000092 and 0.016104% at the field angles 300 and 90 arcsec, respectively. Furthermore, a polarized star when observed at the above two focii will appear to be 99.9999 and 99.9983% polarized respectively due to depolarization.
keywords: techniques: polarimetric techniques -- telescopes -- polarization