Straylight assessment for the Venus Emissivity Mapper instrument: requirement definition, design mitigations, and analysis

Kurzfassung

The Venus Emissivity Mapper (VEM) instrument (on EnVision called VenSpec-M), is a multispectral imager for mapping of the Venus surface and its lower atmosphere, developed at DLR Berlin. This is realized by observation through narrow-band atmospheric windows present in the near-infrared spectral region. VEM will allow to detect thermal emissions like volcanic activity, surface rock composition, water abundance and cloud formation over Venus’ surface. The optical design of the instrument includes a single lens imaging Venus’s surface on a filter assembly composed by 14 individual filter stripes, and a 2-lenses relay optic re-imaging this spectrally filtered image onto an InGaAs detector. Considering the low intensity level of the scientific signal the instrument is aiming to detect, any external contributor to the effective Signal to Noise Ratio has to be studied and if possible mitigated. One of the major contributors is, as in most optical instruments, the straylight. The sources of this disturbance signal on the detector are various: e.g. the Sun, solar system bodies (moon, other planets), a mechanical part of the S/C, internal reflections inside the instrument, scattering effects. This paper will first describe the rationale behind the straylight requirement derivation based on simulated instrument performance and the design features implemented in the instrument to mitigate the straylight effects such as optical baffle, black coatings, diaphragms. In a second part, the straylight analysis performed using this design against the defined requirement will be presented. This includes both ghost images, in-field straylight and off-field straylight. The results of this analysis will finally be compared to the requirements and the design validity assessed.