Indian spectroscopic and imaging space telescope (INSIST): An optics design trade-off study

Large ground and space-based surveys in optical and near-infrared (NIR) wavelengths will revolutionize astronomy in the coming decade. Unfortunately, no ultraviolet (UV) facilities are planned to complement these surveys, which is crucial for studying the most active phase of the Universe that includes star formation in galaxies, active galactic nuclei (AGN), galaxy clusters, etc. A dedicated UV mission, the Indian Spectroscopic and Imaging Space Telescope (INSIST), is proposed to observe the UV sky. The compelling science objectives defined a set of high-level mission requirements. According to which, the INSIST is to have a wide field-of-view ( FoV≈ 0.25 square degree) comparable to India’s Ultraviolet Imaging Telescope (UVIT) and about two orders of magnitude larger than that of the Hubble Space Telescope, simultaneous imaging of the FoV in UV (150–300 nm), u (300–400 nm) and g (400–550 nm) bands, a Multi-Object-Slit medium resolution spectroscopy in a narrow FoV in UV and a simultaneous slitless spectroscopic capability in UV and u bands. To achieve these requirements, several optical design configurations were explored. Here, we present an optical design trade study conducted on various optical design configurations to achieve a sensitivity limit of m_AB> 26 mag, in the UV band and a spatial resolution better than 0.2 ^'' , using a 1-m size telescope aperture. We also present results from our fabrication and alignment tolerance analysis of the selected optical designs, and the design performance that meets the design requirements. Critical parameters like the encircled energy concentration, point spread function (its stability over the field), effects of the spiders supporting the secondary, etc., are explored during the design phase. Based on the trade study conducted in reference to various performance matrices, we down-selected the most optimal optical design for the INSIST.