A consistency check for the calibration of 5303{\AA} solar coronal emission line observations with Aditya-L1/VELC
We carried out radiometric calibration of the Visible Emission Line Coronagraph (VELC) onboard Aditya-L1 at 5303{\AA} 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 ${\approx}(1.70{\pm}0.12){\times}10^{8}\, \rm sec^{-1}{\AA}^{-1}$, which relates to Sun's flux at 1\,AU in 5303{\AA}, ${\approx}3.0456{\times}10^{6}\, \rm {erg\,sec^{-1} cm^{-2} {\AA}^{-1}}$. We verified the above calibration using observations of the bright star Sirius-A whose estimated flux at 1\,AU in 5303{\AA} is ${\approx}1.44{\times}10^{-8}\,\mathrm{erg\,sec^{-1} cm^{-2} \AA^{-1}}$. The expected detector count with VELC in this case is ${\approx}19{\pm}1.4\, \rm sec^{-1}{\AA}^{-1}$, and the measured count is ${\approx}23{\pm}0.4\, \rm sec^{-1}{\AA}^{-1}$. 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 5303{\AA} 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 5303{\AA} channel also. Its full width at half maximum (FWHM) is ${\approx}3.8^{\prime\prime}$.