In-Lab Photometric Calibration of the SUIT Payload on-board Aditya L1

The Solar Ultraviolet Imaging Telescope (SUIT) on board Aditya-L1 shall perform spatially resolved full disk imaging of the Sun within the 200-400 nm wavelength band for the first time. Eleven bandpasses within this spectrum will help probe different heights of the solar atmosphere, giving us a picture of the mass and energy transfer mechanisms between them. O2 and O3 in the upper layers of the Earth's atmosphere considerably absorb UV wavelengths shorter than 310 nm. The solar emission in this wavelength range is highly significant for understanding the Ozone-Oxygen chemistry and the Sun-Earth climatic connection, making SUIT an unique addition to the currently available instruments. In addition, SUIT aims to closely observe dynamic features of various scales in the solar atmosphere, such as flares and jets, along with magnetic features like plages, active regions, and network regions. For the plethora of observations SUIT aims to perform, it is necessary to have reliable photometric calibration of the payload for deriving the absolute radiative energies of various dynamical processes on the Sun. Here, we present the methodology developed for the in-lab photometric calibration of SUIT while maintaining stringent contamination control as necessary for a NUV payload. We also model the instrument throughput using available sun-as-a-star spectra from SOLSTICE and SOLSPEC and find the measured results to have > 85% coherence with the model. We also measure the spectral response of the payload as a whole for its various scientific bandpasses and find them complying satisfactorily with the requisite science needs. Our work on absolute flux calibration of the SUIT payload has the scope to open new doors in solar astronomy, and the technology and methods employed to achieve this will be significant for the calibration of ground and space-based instruments in the future.