Developing scanning-slit spectrograph for imaging the Sun

Abstract

For moderate resolution spectroscopy of the Sun, an imaging spectrograph is being developed at Indian Institute of Astrophysics. With this instrument images of the region of interest of the Sun can be obtained with low spatial and moderate spectral resolution. Dopplergrams can also be obtained with the acquired data to get line of sight velocity maps. The instrument is a back-end for a telescope with tracking system i.e. stable image of the Sun is projected onto the focal plane at all times. Modular approach is followed in the design, keeping sections of the instruments fairly independent. Scanning-slit assembly is a module that can linearly move in one direction to sweep the region of interest in the image. Spectrograph assembly consists of another slit, optics and dispersing element along with the detector so that spectral information about spatial locations on the slit can be obtained. This module is designed to obtain Intensity vs. (x,λ) (x is along the slit) and as the scanning-slit is swept along the y-direction, Intensity vs. (x,y,λ) information is built. The spatial resolution will be seeing limited as there's no correction system. Field of view is 6 arc minute along the slit direction, as the features of interest include sunspots and surrounding region. For testing, a front end system of 100mm clear aperture with f/22.5 is being used. The dispersing element is a reflecting grating with 1200 grooves/mm. For 6563 Å(H-alpha line) spectral resolution is 35 mÅ in second order. Linear dispersion is about 38 mÅ /pixel for the pixel size of 7.5μm, indicating that slit-width limited spectral resolution can be obtained.