Removal of straylight from ExoMars NOMAD-UVIS observations

We present an in-flight straylight removal method for the Ultraviolet and Visible Spectrometer (UVIS) channel of Nadir Occultation Mars Discovery (NOMAD) instrument aboard ExoMars Trace Gas Orbiter (TGO). The presence a ‘red-leak’ signal in UVIS was discovered post-launch ground calibration measurements spectral lamps; observations lamps with negligible UV light emission (RS12) showed significant at wavelengths. Subsequent analyses nadir martian atmosphere revealed that wavelengths red-leak excess 300% true signal, jeopardising primary science (retrievals atmospheric ozone). By modifying readout to obtain region interest around illuminated on Charge-Coupled Device (CCD) detector, instead binned one-dimensional spectrum, utilising profiles derived from RS12 lamp we show majority can be successfully removed self-consistent manner. corrected radiances are compared coincident Color Imager (MARCI) residuals between two instruments generally remaining within 15%.