Photometric Calibration & Spectral Validation of the Solar Ultraviolet Imaging Telescope onboard Aditya-L1

Abstract: The Solar Ultraviolet Imaging Telescope (SUIT) is one of the seven payloads on board Aditya-L1 mission of the Indian Space Research Organization (ISRO). SUIT provides full and partial disk images of the Sun in the 200-400 nm wavelength range. This would help us probe the solar atmosphere at different heights and understand the mass and energy transfer process between its layers. For the first time, SUIT will also help us measure spatially resolved solar spectral irradiance at this wavelength band, which is significant for studying the sun-climate relationships. To perform these studies, it is necessary to photometrically calibrate the payload and validate the spectral coverage of the various bandpasses. We perform the photometric calibration and spectral validation of 8 bandpasses using light of known intensity and spectral coverage. For photometric calibration, the telescope throughput is modeled using sun-as-a-star spectrum from SOLSTICE and SOLSPEC. The modeled throughput is compared with in-lab measurements taken with light of known intensity. The ratio of measured photoelectrons gathered with the modeled prediction agree within 20%. For spectral validation, readings are taken across the transmission spectrum of each filter, keeping adjacent readings independent of each other. The relative intensity measured at each wavelength is seen to trace the modeled telescope bandpass for that filter. These tests could not be performed for filters with bandpasses operating below 250 nm (NB01, BB01 and BB02), primarily due to heavy atmospheric attenuation in these wavelengths leading to decreased SNR of the data. The experimentally measured results agree closely with the modeled values, validating SUIT's optical performance and presenting the reliability of the developed throughput model.