Determination of polarimetric capabilities of astronomical telescopes

Abstract

Polarimetric observations of celestial objects reveal information regarding

magnetic field, scattered dust and planetary atmosphere. The degree of po- larization, which is of interest to astronomers, ranges from 10−5

to a few tens of percent. A potential problem in carrying out polarimetry in astronomy is

the polarization introduced by the telescope and instrument optics to the in- coming light. Hence, it is necessary to estimate the polarization effects for any

telescope prior to the design of a polarimeter. We have developed a polarization model to estimate the polarization effects from the telescope optics of one of the next generation large telescopes, the Thirty Meter Telescope (TMT). Analysis has been carried out to study the effects of the segments of the primary mirror of TMT and to understand their

impact on instrumental polarization and crosstalk. The dependence of polar- ization effects on the field of view and zenith angle of the telescope at different

instrument ports of the TMT has been determined. Polarization aberrations due to telescope optics have been estimated for the adaptive optics system to ascertain their effect on the final PSF of the telescope. This is useful in the design of second generation instruments for TMT with high contrast and high spatial resolution capabilities. We also propose a design technique for the mitigation of the polarization effects due to the telescope optics in future polarimetric instruments.

The polarization ray tracing algorithm which was used to develop the po- larization model for TMT was also used to develop a polarization model for

the Multi-Application Solar Telescope (MAST). The results of the analyti- cal model were verified experimentally to understand the deviations between

the model and the observations. The Stokes parameters were measured for different input polarizations at the wavelength of 6173 ̊A using an imaging spectro-polarimeter. A fairly good match is seen between the model and the observed Mueller matrix elements. Finally, for an understanding of the science requirements, we have studied

the polarimetric properties of a few nova systems as a case study. The stud- ies were conducted using linear polarization data obtained with imaging and

photopolarimeters available on the existing 1-2 m class telescopes.