Optical design of a polarimeter for a 1.3m Telescope

Sambuddha Majumder

B.Tech Engineering Physics, Indian Institute of Space Science and Technology, Thrivandrum, Kerala 695547

Dr. Ravinder Kumar Banyal

Reader/Research Scientist, Indian Institute of Astrophysics, Sarjapur Main Road, Bangalore, Karnataka 590034

ABSTRACT:

The measurement of polarization provides a powerful diagnostic tool for the study of stars, interstellar matter and extragalactic objects. Polarimetry adds another facet to the traditional methods of spectroscopy and interferometry. It yields information on the geometric structure of astronomical sources that cannot be extracted in any other way. The technical implementation of polarimetry in the optical regime in concomittance with imaging or spectroscopy (“spectropolarimetry”) and also interferometry (“polarimetric interferometry”) almost always requires a quid pro quo on either side. Furthermore, polarization as a function of wavelength affords information about various properties in profuse media, such as cometary tails, interstellar as well as circumstellar environments. In this project, we intend to design an imaging polarimeter for the 1.3 m telescope belonging to the Indian Institute of Astrophysics, Bengaluru. Unlike other methods, our approach will be to use a pair of polarisation beam splitters, and fixed half-wave plates in the telescope pupil plane. This arrangment will allow simultaneous measurements of the polarized flux at angles 0, 45, 90, 135 degrees without traditional birefringent elements in the beam path. The first three elements of the Stokes vector (I, Q, U) can therefore be determined from data taken by a single shot and without any need for other moving elements which could be diffcult to incorporate in multimode astronomical instruments. Furthermore, the polarimetric module is targeted to cover a 5' field of view. In order to achieve this, we will make extensive use of a ray tracing software -"Zemax", which is one of the leading industrial standards for optical design and analysis of both imaging and illumination systems. In this project, we will explore three different ways in which the above can be achieved and also discuss the pros and cons in using this as a method to obtain the polarisation properties of astronomical objects compared to traditional polarimetry techniques.

Keywords: Birefringence, Beam Splitters, Zemax, Airy disc.