PSS (2022) https://doi.org/10.1016/j.pss.2022.105432
Jonathon P. Mason, Manish R. Patel, Mark R. Leese, Brijen G. Hathi, Yannick Willame, Ian R. Thomas, Michael J.Wolff, Cédric Depiesse, James A. Holmes, Graham Sellers, Charlotte Marriner, Bojan Ristic, Frank Daerden, Jose Juan Lopez-Moreno, Giancarlo Bellucci, Ann Carine Vandaele
We present an in-flight straylight removal method for the Ultraviolet and Visible Spectrometer (UVIS) channel of the Nadir and Occultation for Mars Discovery (NOMAD) instrument aboard the ExoMars Trace Gas Orbiter (TGO). The presence of a ‘red-leak’ straylight signal in the UVIS instrument was discovered post-launch in ground calibration measurements of spectral lamps; UVIS observations of lamps with negligible UV light emission (RS12) showed a significant signal at UV wavelengths. Subsequent analyses of nadir observations of the martian atmosphere revealed that at UV wavelengths the red-leak straylight was in excess of 300% of the true UV signal, jeopardising the primary science observations of the instrument (retrievals of atmospheric ozone). By modifying the UVIS readout method to obtain a region of interest around the illuminated region on the Charge-Coupled Device (CCD) detector, instead of a binned one-dimensional spectrum, and utilising straylight profiles derived from measurements of the RS12 calibration lamp we show that the majority of the straylight at UV wavelengths can be successfully removed for the nadir channel in a self-consistent manner. The corrected UVIS radiances are compared to coincident Mars Color Imager (MARCI) instrument observations with residuals between the two instruments generally remaining within 15%.
Full frame image of the UVIS detector during a typical nadir measurement. The in-axis light dispersion is defined as the illuminated region (LR) and the off-axis area outside this region is defined as the non-illuminated region (NLR). The off-axis straylight can be seen between the LR and the detector readout register and the white dotted lines show the typical ROI of the CCD that is read out. The visible stratified lines seen in the LR is the illumination from the individual fibres.
