A consistency check for the calibration of 5303Å solar coronal emission line observations with Aditya-L1/VELC

Abstract

We carried out radiometric calibration of the Visible Emission Line Coronagraph (VELC) onboard Aditya-L1 at 5303Å using observations of the Sun itself, a novel approach compared to the calibration using bright stars in the other currently operational space solar coronagraphs. The measured VELC detector count corresponding to the center of Sun's disk is ${\approx}(1.70{\pm}0.12){\times}10^{8}\, \rm sec^{-1}Å^{-1}$, which relates to Sun's flux at 1\,AU in 5303Å, ${\approx}3.0456{\times}10^{6}\, \rm {erg\,sec^{-1} cm^{-2} Å^{-1}}$. We verified the above calibration using observations of the bright star Sirius-A whose estimated flux at 1\,AU in 5303Å is ${\approx}1.44{\times}10^{-8}\,\mathrm{erg\,sec^{-1} cm^{-2} Å^{-1}}$. The expected detector count with VELC in this case is ${\approx}19{\pm}1.4\, \rm sec^{-1}Å^{-1}$, and the measured count is ${\approx}23{\pm}0.4\, \rm sec^{-1}Å^{-1}$. The reasonable agreement between the expected and observed values for Sirius-A in this initial consistency check after two years of observations with VELC, indicates that measurements of coronal brightness at 5303Å with the VELC and its conversion into absolute physical units using observations of the Sun disk light are in order. Using the Sirius-A observations we measured the point spread function (PSF) of the VELC in its 5303Å channel also. Its full width at half maximum (FWHM) is ${\approx}3.8^{\prime\prime}$.