Studying the Imaging Characteristics of Ultra Violet Imaging Telescope (UVIT) through Numerical Simulations

Ultra Violet Imaging Telescope (UVIT) is one of the five payloads aboard the Indian Space Research Organization (ISRO)'s ASTROSAT space mission, with broad science objectives extending from individual hot stars and star-forming regions to active galactic nuclei. The imaging performance of UVIT would depend on several factors in addition to the optics: e.g., resolution of the detectors, satellite drift and jitter, image frame acquisition rate, sky background, source intensity, etc. The use of intensified CMOS-imager-based photon counting detectors in UVIT put their own complexity over reconstruction of the images. All these factors could lead to several systematic effects in the reconstructed images. A study has been done through numerical simulations with artificial point sources and archival image of a galaxy from the GALEX data archive, to explore the effects of the above-mentioned parameters on the reconstructed images. In particular, the issues of angular resolution, photometric accuracy, and photometric nonlinearity associated with the intensified CMOS-imager-based photon counting detectors have been investigated. The photon events in image frames are detected by three different centroid algorithms with some energy thresholds. Our results show that in the presence of bright sources, reconstructed images from UVIT would suffer from photometric distortion in a complex way, and the presence of overlapping photon events could lead to complex patterns near the bright sources. Further, the angular resolution, photometric accuracy, and distortion would depend on the values of various thresholds chosen to detect photon events.