Design of the VenSpec-H instrument on ESA’s EnVision mission: development of critical elements, highlighting the FFCP, and grating

EnVision is ESA’s upcoming mission to Venus with a launch scheduled in 2031. One of the payloads on board is the VenSpec suite,1 containing three spectrometer channels, one of which is VenSpec-H. VenSpec-H (Venus Spectrometer with High resolution) performs absorption measurements in the atmosphere of Venus in four near-infrared spectral bands. VenSpec-H is developed under Belgian management and builds on heritage from instruments on Venus-Express and TGO. Techniques used in these precursor instruments are improved and complemented with new technologies to comply with the scientific goals of the EnVision mission. The operating wavelength range (1.15 - 2.5 <i>μm</i>) imposes stringent temperature requirements on the instrument to make nightside measurements below the Venus clouds possible. Most importantly, the spectrometer’s optical components are held in a separate cold section inside the instrument, cooled down to −45&deg;<i>C</i>, to remove thermal background from the signal. To avoid heat dissipation close to the spectrometer optics, the electronic boards are kept in a separate box. Besides that, some mechanisms, placed in the warmer part of the instrument at the entrance or exit of the cold section, had to be developed: a turn window unit to protect the interior of the instrument during the aerobraking phase of the mission, a filter wheel mechanism to select the spectral bands of interest, and an integrated detector-cooler-assembly to register the spectra. Some passive optical elements in the spectrometer had low technological readiness at the start of the project. One of them is a freeform corrector plate, used to compensate for aberrations introduced in the system by a parabolic mirror. This device is developed by the Brussels Photonics lab of VUB (Brussels) using a supply chain with shape adaptive corrective polishing and dedicated metrology. Another is the echelle grating, used to disperse the incoming light into its spectral components, which is built by AMOS. Both devices are highlighted in this article.