The 0.7-1.7 nm spectral range of the solar spectrum obtained from Mars thanks to TGO-ACS observations
Résumé
The Trace Gas Orbiter (TGO) was launched in 2016 to probe the atmosphere of Mars. It has on board the Atmospheric Chemistry Suite (ACS), which is a set of observation instruments in the infrared domain, in particular, the NIR (Near Infrared) and the MIR (Mid-Infrared) photometers. These photometers detect the atmospheric components of Mars when they are pointed at the Sun and the Line Of Sight (LOS) crosses the atmosphere. The solar spectrum is directly measured when the LOS is above the atmosphere of Mars and serves as a spectral reference for calibration. We consider here particular observations made with the NIR photometer to construct the solar spectrum allowed by its spectral range, i.e. 0.7 to 1.7 nm. This is motivated by the need to have a solar spectrum with high spectral resolution for NIR calibrations but also useful for models (Sun, climate...) or for other remote sensing experiments (Earth or other planets). We have acquired all the diffraction orders of the NIR by continuously varying the frequency of its AOTF (Acousto-Optic Tunable Filters, a component used to separate the orders). They were then successively imaged on the CCD and series of its useful part were recorded for each order. We will present how we process this data to extract the solar spectrum, in particular how we calculate the flat field useful for image correction. We will then present how to overcome the contamination of successive spectral orders using a geometric method. We will then show how to correct the order intensity variations to obtain the solar spectrum. We will end by discussing our results, in particular by comparing them with other existing spectra on the same band.