INSTRUMENT_DESC gives a concise description of the instrument concept and working principle Written by Eddy NEEFS SOIR is a new compact space borne high resolution spectrometer developed for the ESA Venus Express spacecraft is described. It operates in the IR wavelength range of 2.325 to 4.2 um and measures absorption spectra of minor constituents in the Venusian atmosphere. It uses a novel echelle grating with groove density of 4 lines per mm in a Littrow configuration, in combination with an IR Acousto-Optic Tunable Filter for order sorting and an actively cooled HgCdTe Focal Plane Array of 256 by 320 pixels. It is designed to obtain an Instrument Line Profile, full width at half maximum (FWHM), of 0.2 cm-1 and a Sampling Interval of 0.1 cm-1 per pixel over the whole spectral range. The FOV in the spatial direction is limited to 30', in order to avoid solar limb darkening effects (apparent solar diameter at Venus is 44 arcmin). For the spectral direction, perpendicular to the spatial direction, the FOV is 2' (good height resolution during the inversion process of the Venus occultation data). Hence, a rectangular field of view of 2' by 30' is defined. The attitude of the VEX spacecraft during an occultation observation is controlled such that the boresight of the SOIR instrument is pointing the Sun and that the long side of the rectangular FOV remains parallel to the planet limb. The mass of SOIR is 6.5 kg and its volume to 414 mm by 254 mm by 210 mm. A periscope-like device is added to the side wall of the instrument, since the VEX solar viewing boresight is at an angle of 60 degrees with respect to the SOIR optical axis. SOIR is thermally qualified for an operational temperature range between -20 C and +40 C and a non-operational between -30 C and +50 C. SOIR uses an echelle grating as a diffracting device; this grating type is known to yield high reciprocal dispersion and high throughput in a compact design. Since an echelle, like all gratings, is generating overlapping orders, an order sorting filtering was required, in this case a tunable bandpass filter. Compared to orthogonal cross- dispersion solutions using prisms or low dispersion gratings, this technique allows the imaging of longer spectral lines on the detector and hence a larger binning factor along the lines, which results in higher signal-to-noise ratios. In SOIR the bandpass filter is an Acousto Optical Tunable Filter (AOTF). It has both the advantage of avoiding mechanical moving mechanisms and of a quick random access to any grating order by electronic control. Moreover, by deactivating the AOTF, one can stop all solar input to the spectrometer; this allows measuring a thermal background spectrum that can be subtracted from the observed spectrum. The spectrometer is composed of three main parts: (a) the front-end part which collects the solar light, defines the FOV and restricts the observed wavelength domain; (b) the spectrometer itself which realizes the free spectral range and the ILP and finally (c) the detector system that records the spectra according to a spectral sampling interval. The front-end starts at the AOTF entrance optics that reduce the diameter of the incoming light beam so that it becomes compatible with the AOTF acceptance aperture. In the intermediate image plane of the entrance optics a first diaphragm is placed that limits the FOV to slightly larger than the desired FOV. Then the beam enters the AOTF to filter out unwanted orders. When activated with an RF signal, the AOTF directs the small fraction of the beam that corresponds to the wavelength domain to be analyzed along its optical axis. The AOTF exit optics images the beam on the spectrometer entrance slit, which is the entrance aperture for the spectrometer part of the instrument. A collimating lens captures the light passing through the spectrometer entrance slit, collimates and transports it as a parallel beam to the dispersing element, the echelle grating located between collimator lens and imaging lens. Finally, the light diffracted by the grating is imaged via a camera lens on the detector. In order to obtain a compacted optical scheme for the high-resolution spectrometer part, a quasi-Littrow configuration was selected, in which the function of the collimation lens and of the imaging lens are merged into a single off-axis parabolic mirror. An additional benefit is that no co-alignment of two separate lenses is needed. In the detailed optical scheme and the associated ray tracing scheme, the main recognizable optical units are: the entrance optics, the diaphragm, the AOTF, the AOTF exit optics, the spectrometer slit, the off-axis parabolic mirror, the echelle grating, the folding mirror, the detector optics and finally the detector. Value or range Units Wavelength range 2.32 to 4.25 um Wavenumber range 2353 to 4310 cm-1 Spectral sampling interval 0.1 cm-1/pixel Instrument Line Profile 2 (FWHM) pixels Resolving power 23200 to 43100 Spectral field of view 2 arc minutes Spatial field of view 30 arc minutes Entrance aperture diameter 20 mm Mass 6.5 kg Dimensions (w/o periscope) 414 x 254 x 210 mm3 The SOIR instrument executes two operational phases in sequence: pre-cooling phase with duration T1 and observation phase with duration T2 (divided in a series of time equidistant 1 second observations, each composed of a number of spectral recordings packed into a single TM packet, together with housekeeping information related to the observation). After power-up 2 types of TC packets are sent to SOIR by the SPICAV DPU: the first type contains all parameters related to the pre-cooling phase of the SOIR detector system and the second type contains all the parameters to define the operation of SOIR observation phase. Upon reception of the first TC packet of type 1 the precooling phase starts, during which every second a TC packet of type 1 is sent to invite SOIR to send TM packets that do not contain scientific data. Only housekeeping is sent. Upon reception of the first TC packet of type 2 the observation phase starts during which every second a TC packet of type 2 is sent to invite SOIR to send TM packets that do contain scientific and housekeeping.