Chandrayaan-2 Large Area Soft X-ray Spectrometer (CLASS): Calibration, In-flight performance and first results

Abstract The Chandrayaan-2 Large Area Soft X-ray Spectrometer (CLASS) onboard the Chandraayaan-2 spacecraft around the Moon, has been remotely measuring the lunar X-ray fluorescence spectra since September, 2019. The primary objective of the experiment is to provide global maps of O, Mg, Al, Si at a resolution of 12.5 km/pix and of Ca, Ti and Fe at localized regions during enhanced solar activity, using the lunar X-ray fluorescence measurements in the 0.5 to 10 KeV range. CLASS is an array of swept charge devices (SCDs), a variant of X ray Charge Coupled Devices (CCDs) that provide good spectral resolution and large area. The quality of X-ray measurements strongly depends on accuracy of its calibration techniques. In this work, the results from the pre-launch calibration of the instrument that combines experimental measurements and simulations are described. The spectral redistribution function of the swept charge device is simulated using an augmented version of a previously developed charge transport model (Athiray et al., 2015). Response matrices built from these models are verified with in-flight data. We study the background in SCDs arising from particles in the lunar orbit over many months and identify the sources. We demonstrate the in-flight performance of the instrument that enables generation of direct elemental maps. Elemental abundances for a region in the farside highland and in the nearside western mare are derived demonstrating the method and the instrument capability of deriving the elemental abundances at different spatial scales and at different solar activity levels.