Hyperspectral and Spectropolarimetric Instrumentation for the Solar Atmosphere

Abstract

The Sun is the closest star whose surface can be resolved to great a detail. Understanding the working of Sun helps in understanding a large fraction of stellar system as it falls under the spectral class G2V which is the second most abundant type found in our galaxy. It has been speculated for some time that the layers in the solar atmosphere are coupled to each other and the coupling mechanism would reveal a lot about the dynamics of the Sun and the long standing problem of coronal heating. Thus physical parameters of the solar atmosphere should be obtained simultaneously at different heights. Estimates on the parameters at a particular region in the solar atmosphere is carried out using spectroscopic or polarimetric measurements of the atomic emission or absorption lines in that region. Traditionally there are two types of spectrograph used in solar astronomy to perform spectroscopy. They are the Fabry-Pérot interferometer and Slit-Grating based spectrograph. Both these instruments have their share of advantages and disadvantages. With telescopes getting bigger, the spatial resolution and light gathering power is increasing. Due to increased size in the telescope aperture, the instruments will be able to resolve small scale structures of the sun. But the small scale structures in the Photosphere and Chromosphere are dynamic in nature and have short time scales. Thus fast time cadence spectro-polarimetry is a very important capability for new instruments in solar observation. Coupling between the layers of the solar atmosphere can be studied with simultaneous observation at multiple wavelength regions corresponding to different height in the solar atmosphere. A traditional grating spectrograph is incapable of achieving high time cadence but can operate at multiple wavelength bands simultaneously. A Fabry-Pérot spectrographs is incapable of simultaneous wavelength measurement but has high time cadence of observation. This thesis aims at addressing the disadvantages of both the instruments and coming up with a solution for the same. An instrument for each type (i. e. Fabry-Pérot and Grating) is built and their capabilities are demonstrated through the data obtained from the engineering run. The slit and grating based spectrograph is extended to include polarimetry in order to measure the vector magnetic field of the solar atmosphere. Although the instruments are developed and their capabilities are demonstrated for photospheric and chromospheric observations, they can be modified to observe the corona. But the corona is a million times fainter than the photosphere with weak fields of the order of tenths of Gauss in contrast to the kilo Gauss fields on the photosphere. The polarimeters required to measure such field have to be sensitive. A case study of polarimetric accuracy needed to measure the corona polarization and the possible sources of noise affecting the signal is also under-taken