Aditya-L1 Solar Coronagraph Calibration Verified Using Sun and Sirius Observations
Researchers have successfully validated the radiometric calibration of the Visible Emission Line Coronagraph (VELC) aboard India's Aditya-L1 spacecraft using observations of the Sun itself and the bright star Sirius-A at 5303Å. Unlike other operational space coronagraphs that calibrate using bright stars, VELC uses solar disk observations as its primary calibration source, with Sirius-A serving as an independent cross-check. The agreement between expected and measured detector counts for Sirius-A confirms that VELC's coronal brightness measurements can be reliably converted into absolute physical units, supporting the instrument's scientific validity.
A study accepted for publication in Solar Physics reports a consistency check of the radiometric calibration of the VELC instrument aboard India's Aditya-L1 solar observatory, which has been operating for approximately two years. The calibration at 5303Å — the wavelength of a prominent green coronal emission line — was performed using observations of the Sun's disk itself, a novel approach compared to the star-based calibration methods used by other space coronagraphs. The measured detector count at the center of the solar disk was approximately 1.70×10⁸ sec⁻¹Å⁻¹, corresponding to a solar flux of about 3.05×10⁶ erg sec⁻¹ cm⁻² Å⁻¹ at 1 AU. To verify this calibration, the team observed Sirius-A, finding a measured count of ~23±0.4 sec⁻¹Å⁻¹ against an expected value of ~19±1.4 sec⁻¹Å⁻¹ — a reasonable agreement given the initial nature of the check. Additionally, the Sirius-A observations were used to measure the instrument's point spread function, yielding a full width at half maximum of approximately 3.8 arcseconds for the 5303Å channel.
What's missing
The authors describe this as an 'initial consistency check' and do not fully account for the ~20% discrepancy between expected and measured Sirius-A counts, leaving open whether this offset reflects detector aging, stray light, or modeling uncertainties. Long-term stability of the calibration over the full mission lifetime is not yet assessed.
What different sources said
- arXiv astro-phCenter
A consistency check for the calibration of 5303{\AA} solar coronal emission line observations with Aditya-L1/VELC
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