Removal of straylight from ExoMars NOMAD-UVIS observations

Mason, Jonathon; Patel, Manish; Leese, Mark; Hathi, Brijen; Willame, Yannick; Thomas, Ian R.; Wolff, Michael J.; Depiesse, Cédric; Holmes, James; Sellers, Graham; Marriner, Charlotte; Ristic, Bojan; Daerden, Frank; Lopez-Moreno, Jose Juan; Bellucci, Giancarlo and Vandaele, Ann Carine (2022). Removal of straylight from ExoMars NOMAD-UVIS observations. Planetary and Space Science, 218, article no. 105432.



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%.

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  • Item ORO ID
  • 82212
  • Item Type
  • Journal Item
  • ISSN
  • 0032-0633
  • Project Funding Details
  • Funded Project NameProject IDFunding Body
    Not SetST/V002295/1UKSA
    Not SetST/V005332/1UKSA
    Surface/atmosphere interactions from above and below.ST/S00145X/1UKSA UK Space Agency
    ExoMars TGO: CaSSIS operationsST/R003025/1UKSA UK Space Agency
    Not SetNot SetBelgian Science Policy Office (BELSPO)
    Not SetPEA 4000103401ESA Prodex Office
    Not SetPEA 4000121493ESA Prodex Office
    Not SetPGC2018-101836-B-I00Spanish Ministry of Science and Innovation (MCIU
    MINECO/FEDERESP2017-87143-RNot Set
    Not Set2018-2-HH.0Italian Space Agency
    ET_HOME30442502Belgian Fonds de la Recherche Scientifique
    Center of Excellence Severo OchoaSEV-2017-0709State Agency for Research of the Spanish MCIU
    Not SetNot SetNational Aeronautics and Space Administration
  • Keywords
  • Straylight removal; Ultraviolet–visible spectroscopy
  • Academic Unit or School
  • Faculty of Science, Technology, Engineering and Mathematics (STEM) > Physical Sciences
    Faculty of Science, Technology, Engineering and Mathematics (STEM)
  • Copyright Holders
  • © 2022 The Authors
  • Depositing User
  • ORO Import