Flux calibration of Aditya-L1/VELC 5303Åobservations

We carried out radiometric calibration of the Visible Emission LineCoronagraph (VELC) onboard Aditya-L1 at 5303Å using observations of thesolar disk through a neutral density filter, and the bright star Sirius-A in the deepspace. The measured detector counts due to the Sun, after correcting for filter−1transmission and aperture size, is ≈(1.70±0.06)×108 sec−1 Å . Using this, andthe reported flux values for Sun and Sirius-A, we calculated the expected detector−1counts for Sirius-A at 1 AU (i.e. at the location of Sun) to be ≈19 sec−1 Å . The−1measured Sirius-A count with VELC is ≈11.6±0.4 sec−1 Å . The close agree-ment between the expected and observed values indicates that measurementsof coronal brightness at 5303Å with the VELC can be reliably converted into absolute physical units.