SOIR - The instrument

SOIR is an innovative, compact, high resolution spectrometer for remote IR measurements of gaseous species, using solar occultations. The system consists of an Echelle grating spectrometer working in the infrared, combined with a TeO2 Acousto-Optic Tunable Filter (AOTF) for the selection of the diffraction grating orders.

Such an instrument combines the advantages of a high spectral resolution and a fast and flexible access to any part of its spectral domain, compactness and low mass. The selected diffracted beam enters the Echelle grating spectrometer through the spectrometer entrance slit.

Optical scheme of the SOIR spectrometer:
  • (1) AOTF entrance optics;
  • (2) Diaphragm;
  • (3) AOTF;
  • (4) AOTF exit optics;
  • (5) Spectrometer entrance slit;
  • (6) Collimating and camera lens merged into one off-axis parabolic mirror;
  • (7) Echelle grating;
  • (8) Folding mirror;
  • (9) Folded detector optics;
  • (10) Detector assembly placed upright

(credit: BISA)


In order to obtain a compact optical scheme for the high-resolution spectrometer a Littrow configuration was implemented, in which the usual collimating and imaging lenses are merged into a single off-axis parabolic mirror, with the additional benefit that no co-alignment of two separate lenses is needed.

The instrument line profile full width at half maximum (FWHM) is determined by the slit width and is equivalent to 2 detector pixels (60 μm). SOIR is a low power (17 W average) and a light weight instrument (7 kg), that can easily be accommodated on board of constraining planetary missions.

A periscope-like device was added in front of SOIR’s entrance aperture because VEX’s solar pointing direction is at an angle of 60° with respect to the optical axis of SOIR. As seen from Venus, the solar diameter is 40 arcmin; the entrance slit of the spectrometer corresponds to a FOV of 2x32 arcmin, while 1 pixel of the 2D detector corresponds to 1x1 arcmin. In this spectro-imager, the spectrum of each point of the slit is aligned along the lines of the 2D detector, while the slit is aligned with the columns.

The detector system is a modified SOFRADIR Integrated Detector Dewar Cooler Assembly, sensitive to radiation in the 1.7 to 4.3 µm spectral region, consisting of a two-dimensional array of photovoltaic HgCdTe pixels (320 columns x 256 rows) cooled by a Stirling type cooler (RICOR) to about 88 K.

SOIR optical components

Newton type, focal length 180 mm, 35 mm × 50 mm parabolic mirror

0.5 mm × 4 mm, nickel

Tellurium oxide, 10 mm × 10 mm

Zinc selenide, focal length 35 mm, diameter 15 mm, R 1 = 31.5  mm, R 2 = 79.8 mm

Acousto-Optic Tunable Filter

Zinc selenide , focal length 35 mm, diameter 15 mm, R 1 = 79.8 mm, R 2 = 31.5 mm

Tellurium oxide, 10 mm × 10 mm

0.06 mm × 3 mm, nickel

focal length 375 mm, 70 mm × 100 mm, off-axis 8° parabolic

blaze angle 63.42°



More info


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