Design aspects of multi-slit spectropolarimeter for national large solar telescope

Abstract

Magnetic field aids in coupling between different layers of the solar atmosphere. The magnetic field information simultaneously at multiple heights is important to understand many physical processes like flares, coronal heating, coronal mass ejections (CMEs) etc. on the Sun. Spectropolarimetry provides the capability of quantitative measurement of magnetic field to the telescope. The trend of bigger telescopes demands for faster temporal cadence to observe faster evolving structures of the Sun. This thesis aims at addressing the potentiality of multi-slit spectroscopy for National Large Solar Telescope (NLST) in order to achieve faster cadence without compromising on spatial or spectral resolution. Faster cadence is possible only with the raster scan of the entire field of view (FOV) by multiple slits. So the existing single-slit spectrograph design has been extended to five-slits spectrograph (FSS) design using ZEMAX. Acquisition of simultaneous multi-line spectropolarimetric observations in the wavelength coverage of 400-1600 nm is one of the major objectives of NLST. In the FSS design, we have been able to append the H-alpha line at 656.28 nm, to the four other spectral lines which had been already achieved with the single-slit design. The tolerance analysis of the optical design of FSS has been performed which is important to build up a real system from its optical design. It enables us to check the amount of error that is acceptable in the alignment or fabrication of optics without degrading the image quality beyond a certain extent. The thesis has also explained the various techniques to accomplish the spatial scanning in a slit-based spectrograph. A study of various polarization modulators has been presented with their advantages and disadvantages in order to be used in a solar polarimeter. Dual-Frequency Liquid crystal (DFLC) based retarders serve the purpose of higher modulation fre quency to overcome seeing-induced cross-talks. They can exhibit larger aperture than ferro-electric liquid crystal (FLC) based retarders, which will help in ensuring the required FOV for NLST. So, the characterization of a prototype DFLC based retarder is being planned to check its response time and calculate its retardance. For this purpose, laboratory setup has been worked out along with assembling and testing of the associated electronics.