Instrumental polarization caused by telescope optics during wide field imaging

Abstract

Abstract. 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 100% 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 100% polarized star when observed at the above two focii will appear to be 99.9999 and 99.9983% polarized respectively due to depolarization.

Analysis of polarization introduced due to the telescope optics of the Thirty Meter Telescope

“…They have considered x − y − z coordinate system as the global coordinate system and p − s as the local coordinate system, which is also followed in this paper. In a later work, 23 the IP and DP were calculated at the prime and Cassegrain foci. The polarization ray tracing algorithm (given in Appendix A) for TMT developed in the present work is actually based on the earlier work by Sen and Kakati.…”

“…The polarization ray tracing algorithm (given in Appendix A) for TMT developed in the present work is actually based on the earlier work by Sen and Kakati. 23 It involves tracing of the ray from the primary mirror of the TMT till the Nasmyth focus considering changes in the polarization at each surface. The direction cosine (DC) of the rays is determined at each interface for the incident, reflected rays, and surface normal (which together define a plane).…”

“…The DCs of the normal to the mirror surface at every point (x; y; z) can be found, as described in Sen and Kakati. 23 The DCs of the reflected rays from the primary mirror surface can be represented by the unit vector:…”

“…Using these conditions, we find the DC of the electric field vector, E dc for the polarized light as ðsin α; sin α cos η; − sin α sin ηÞ. 23 The DCs of p and s vectors are represented asŝ incpri ¼î ×n, p incpri ¼î ×ŝ incpri ,p refpri ¼ŝ incpri ×r. E p .…”

“…The DCs, incident angles, and reflection coefficients are determined for all the rays reflected from secondary (the details of the geometries can be followed from Ref. 23). The electric field of the light ray incident on the secondary mirror is the reflected electric field from the primary mirror having two components sE p and sE s given as follows: 23 E Q -T A R G E T ; t e m p : i n t r a l i n k -; e 0 0 9 ; 3 2 6 ; 5 7 5…”