A&A Supplement Series, Vol. 126, November II 1997, 113-119
Received November 19; accepted December 19, 1996
A.K. Sen
,
and M. Kakati
Send offprint request: A.K. Sen
Centre of Plasma Physics, Dispur, Guwahati 781 006, India
e-mail: 1. aks@ipprghy.ren.nic.in 2. mayur@ipprghy.ren.nic.in
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