Purpose of VenSpec-H
VenSpec-H is a high-resolution infrared spectrometer working in nadir viewing mode. It is an echelle grating spectrometer coupled to a high-performance, actively-cooled SOFRADIR HgCdTe detector, with an Acousto-Optic Tunable Filter (AOTF) at its entrance for order selection (. These components are optimised for Venus atmospheric observations in the spectral range 1.0–2.5 μm.
VenSpec-H instrument will detect and quantify SO2, H2O and HDO in the lower atmosphere, to enable characterisation of volcanic plumes and other sources of gas exchange with the surface of Venus, complementing VenSAR and VenSpec-M surface and SRS subsurface observations. A nadir pointed high-resolution infrared spectrometer is the ideal instrument for these observations at the1·0 μm, 1·7 μm, and 2·0 – 2·3 μm atmospheric windows that permit measurements of the lower atmosphere during the night, and above the clouds during the day. Baseline observations will be performed on the night side but observations at all times of day are possible.
The heart of the instrument is the spectrometer section where the incoming uniform light is diffracted into its spectral components, using an echelle grating. The entrance of the spectrometer section is the spectrometer slit, the image of which is projected on the detector. The output of the spectrometer section is an opening interfacing to the detector assembly. The selection of the spectral band is carried out using filters on a filter wheel situated in front of the slit in the warm section, and a butcher’s block attached to the slit in the cold section. The detector is an Integrated Dewar Detector-Cooler Assembly whose window sits in the exit aperture of the spectrometer section. The focal plane array located at the focal plane of the spectrometer exit optics, is cooled by means of a cryocooler. VenSpec-H has a rectangular field of view (defined by the spectrometer slit) of 7.32° (length of slit) by 0.084° (width of slit). VenSpec-H will observe close to nadir direction on both day and night side.
The instrument heavily builds on the LNO channel of NOMAD on board ExoMars Trace Gas Orbiter.