INSTRUMENTAL POLARIZATION CAUSED BY TELESCOPE OPTICS DURING WIDE-FIELD IMAGING

When astronomical observations are made for a celestial object, with n on -zero field angle (wrt telescope axis), the beam of parallel rays f rom the celestial object strikes the telescope mirror obliquely. Each unpolarized ray of light when it strikes the metal coated mirror surfa ce gets polarized due to reflection. On the contrary, when the field a ngle is zero, these reflected rays for a held star, combine together t o produce an instrumental polarization effect. A 100% linearly polariz ed star when observed even at zero field angle, exhibits depolarizatio n due to the above effect. A detailed procedure has been worked out he re to estimate such polarization effects at the prime and Cassegrain f ocii, considering the case for linear polarizations only. Also to find the typical values of such polarization, a 2.3 m telescope having bea m sizes f/3.23 and f/13 at the prime and Cassegrain focii, has been co nsidered. The instrumental polarization values as calculated at these two focii are 0.000092 and 0.016104% at the field angles 300 and 90 ar csec, respectively. Furthermore, a 100% polarized star when observed a t the above two focii will appear to be 99.9999 and 99.9983% polarized respectively due to depolarization.