Optical Design of the Dual Beam Imaging Polarimeter (DBIP) for the 1.3m JCB telescope in Kavalur

Abstract

Imaging Polarimetry of Extended sources is not very common. This project entitled "Dual Beam Imaging Polarimeter" is an attempt to build one for the 1.3m telescope in Kavalur. The Instrument will make use of the Wide field Imaging capabilities of the telescope, The Corrected field of view of the telescope is 30 arcmin. The Instrument is slated to measure the Polarization from Circumstellar and Interstellar environments, which are almost invisible in normal imaging. One reason being the glare from their host star and second reason being they are not self-luminous. The light that gets scattered off from the Dust that is filling up the voids of circumstellar and interstellar medium is what induces polarization (particularly Linear Polarization) and this polarization is what we are ought to measure. The Measurement of Polarization from Dust scattering will reveal a lot of information about its properties. The percentage (degree) of polarization and position angle of polarization will depend on the size of the scatterers and their alignment respectively. The scattering is More predominant in the Infrared, because the size of dust particles is in the order of microns. Polarimetric measurements of this Astrophysical environments can reveal properties of dust such as their distribution, alignment, composition and soon. Polarimetry is an Important diagnostic tool in astronomy, where the Astrophysical process breaks the symmetry of light. This include Anisotropic (Birefringence or Dichroism) properties of the Interstellar medium, An Exoplanet transiting its parent star breaking the symmetry of light emitted by its parent star and resulting in a Non-zero net polarization, The Geometry of bodies emitting light, for example highly oblate stars will give rise to intrinsic polarization and the measurement of polarization of light from these stars is a measure of their oblateness (asphericity) (or) deviation from exactly being a sphere. Fast rotating stars can be one of the contenders for this class of objects because they are oblate. The envelope of a supernova explosion (or) the geometry in to which a supernova got exploded. Direct emission from Astrophysical bodies can also give rise to polarized signals, for example Synchrotron emission from pulsars. Only difference in all these phenomena is the level of polarized signals that we receive and one instrument cannot be made sensitive for Measuring Polarization for all the above said Astrophysical processes. The Design challenges imposed by this Instrument are its Wide FOV of 6 arcmin diameter or more with a Broad wavelength region of 500nm-900nmm, The space available for the instrument to be mounted at the cass focus of the telescope is around 500mm, The two Images should be separated unoverlapped at the detector and Image quality of two beams. The Instrument Design has been done for a FFOV of 6 arcmin using ZEMAX optical design software and the two beams are very well separated at the Image surface, so they don't even overlap while tolerancing. Wollaston prism has been Achromatized by employing a pair of Wollaston prisms in place of one to correct for the Chromatic aberrations introduced by the first prism. The reduced chromatic aberration allowed me to work on a single design to be used with all the filter bands as opposed to previous design which used two designs one for B,V filters and other for R,I filters. The correct set of materials that can make up this Wollaston prism pair has been found out and the combination of their wedge angles is computed which results in a beam deviation of 10° at a central wavelength of 700nm and resulting in least variation in CVSA(Chromatic variation in splitting angle). Some of ZEMAX programming features has been used to make the design process more efficient, for example Glass Substitution. Accelerating the tolerance analysis by using tolerancing features like Polynomial sensitivity tolerancing, Tolerancing Cache etc. The future work to be done is to complete the Toleracing of the design with acceptable degradation in the system performance and simultaneously attaining tolerances which are manufactured without increasing the cost of manufacture.