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SOIR Instrument description and data calibration

SOIR Instrument description and data calibration. A.C. Vandaele, R. Drummond, A. Mahieux, S. Robert, V. Wilquet SOIR Team @ Belgian Institute for Space Aeronomy (IASB-BIRA). Overview. Venus Characteristics Atmosphere Venus Express SOIR Solar occultation Instrument description Telemetry

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SOIR Instrument description and data calibration

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  1. SOIRInstrument description and data calibration A.C. Vandaele, R. Drummond, A. Mahieux, S. Robert, V. Wilquet SOIR Team @ Belgian Institute for Space Aeronomy (IASB-BIRA)

  2. Overview • Venus • Characteristics • Atmosphere • Venus Express • SOIR • Solar occultation • Instrument description • Telemetry • Calibrations • Echelle grating • AOTF • Detector • Optics • Spectrum construction • Geometry

  3. Overview • Venus • Characteristics • Atmosphere • Venus Express • SOIR • Solar occultation • Instrument description • Telemetry • Calibrations • Echelle grating • AOTF • Detector • Optics • Spectrum construction • Geometry

  4. The Venus orbit Credits: Celestia

  5. Venus : Characteristics Venus 6051.8 km (0.95) 4.87 1024 kg (0.82) -243 days 224.65 days 177.3° 0.723 AU 730 K (457°C) 92 atm Earth 6371.0 km (1.00) 5.97 1024 kg (1.00) 1 day 365.15 days 23.44° 1 AU 287 K (14°C) 1 atm Characteristics Radius Mass Sidereal day Year duration Axis inclination Distance to the Sun Surface temperature Surface pressure

  6. Venus : Atmosphere subdivisions • Troposphere • Poorly known region • High temperatures and pressures • Low wind • Cloud layer • Aerosols H2SO4 • Wind ~ 300 km/h retrograde • Mesosphere • Transition zone • Aerosol haze • Thermosphere • Large differences between day and night • Temperature  chemistry • Subsolar – antisolar circulation • Region studied by SOIR

  7. Venus : Atmospheric composition • Main compound: carbon dioxide CO2 • 96.5 % up to ~110 km • Uniform • Transformed by solar UV into CO (> 110 km) • Quantity decreases with altitude, replaced by CO and O • Little water • Variable quantity • HDO/H2O fraction 140 x larger than on Earth • H2SO4 in the regions close to the cloud layer • products SO2, SO, OCS, H2CO • Halogens • HCl, HF

  8. Overview • Venus • Characteristics • Atmosphere • Venus Express • SOIR • Solar occultation • Instrument description • Telemetry • Calibrations • Echelle grating • AOTF • Detector • Optics • Spectrum construction • Geometry

  9. Venus Express: Mission description N • Launched from Baïkonour in November 2005 • Reached Venus in May 2006 • Apoapsis • North pole • Distance ~ 250 km • Periapsis • South pole • Distance ~ 65 000 km • Already two mission extensions • Should end in December 2012 • Maybe until 2014? Sun

  10. Venus Express: Payload • 7 instruments • ASPERA • MAG • PFC • SPICAV/SOIR • VeRA • VIRTIS • VMC Credits: European Space Agency

  11. Overview • Venus • Characteristics • Atmosphere • Venus Express • SOIR • Solar occultation • Instrument description • Telemetry • Calibrations • Echelle grating • AOTF • Detector • Optics • Spectrum construction • Geometry

  12. SOIR: Solar occultation - Animation Credits: Celestia

  13. SOIR: solar occultation – Measurement principle Orbit 232 – Order 129 Transmittance Side view To Sun VEX N View from Venus Express Venus Atmosphere

  14. SOIR: solar occultation – Measurement principle Orbit 232 – Order 129 Transmittance Side view To Sun VEX N View from Venus Express Venus Atmosphere

  15. HDO H2O CO2 CO SOIR: Solar occultation – Example of measured spectra • 4 different diffraction orders measured during each occultation Orbit 486 (20070820)

  16. SOIR: Solar occultations – Measurements map

  17. Overview • Venus • Characteristics • Atmosphere • Venus Express • SOIR • Solar occultation • Instrument description • Telemetry • Calibrations • Echelle grating • AOTF • Detector • Optics • Spectrum construction • Geometry

  18. SOIR: Optical description (1) Credits: IASB/BIRA

  19. Acousto-optic filter Echelle grating 250 µm Crystal Reflective surfaces Infrared detector Spatial direction: 256 pixels Spectral direction: 320 pixels SOIR: Optical description (2)

  20. Overview • Venus • Characteristics • Atmosphere • Venus Express • SOIR • Solar occultation • Instrument description • Telemetry • Calibrations • Echelle grating • AOTF • Detector • Optics • Spectrum construction • Geometry

  21. Spatial Spectral SOIR telemetry – Constraints on the combination of detector lines • Telemetry = equivalent of 8 spectra/second • If 4 orders/second  2 spectra/order = 2 ‘bins’ Detector: 320 x 256 pixels 32 illuminated rows

  22. Slit position during an occultation Spatial Spectral Bin 1 Bin 2 60 km 60 Venus

  23. Overview • Venus • Characteristics • Atmosphere • Venus Express • SOIR • Solar occultation • Instrument description • Telemetry • Calibrations • Echelle grating • AOTF • Detector • Optics • Spectrum construction • Geometry

  24. SOIR: Calibrations • Need to obtain different calibrations • In flight calibration of almost all characteristics • Echelle grating • Blaze function • Acousto-optic filter • Transfer function • Tuning relation wavenumber – acousto-optic frequency • Detector • Non-uniformity of the detector pixels • Pixel to wavenumber relation • Sample interval • Instrument • Sensitivity • Resolution • Signal to noise ratio

  25. Echelle grating: Blaze function (1) • The efficiency of the grating in terms of refracted angle • Is maximum when the refracted angle = incident angle Pyo, Tae-Soo. 2003. Blaze Function and the Groove Shadowing Effect.

  26. Diffraction order Echelle grating: Blaze function (2) Mahieux, A. et al, 2008. In-flight performance and calibration of SPICAV/SOIR on-board Venus Express. Applied Optics, 47(13), 2252–65.

  27. Acousto Optical Tunable Filter: Characteristics • Calibrations: • 1. AOTF bandpass function • TAOTF = f(l, l0, DlFWHM) • 2. Tuning function • l0 = f(RF) • 3. Bandwidth • DlFWHM = f(l) Mahieux, A. et al, 2008. In-flight performance and calibration of SPICAV/SOIR on-board Venus Express. Applied Optics, 47(13), 2252–65.

  28. Acousto Optical Tunable Filter: Characteristics –Bandpass function (1) • Usual transfer function for AOTF • Calibration using miniscans • Using deed solar lines (from Hase et al. 2009) • Radiofrequency of AOTF chosen to correspond to well defined solar lines • Different frequency steps (1 kHz to 20 kHz) around that RF • Lots of miniscans for a lot of different solar lines over the entire spectral range covered by SOIR • Performed routinely to follow aging of the crystal Mahieux, A. et al. 2009. A New Method for Determining the transfer function of an Acousto Optical Tunable Filter. Optics Express, 17, 2005–2014.

  29. A B Acousto Optical Tunable Filter: Characteristics –Bandpass function (2) One solar line @ 2948.7 cm-1 Mahieux, A. et al. 2009. A New Method for Determining the transfer function of an Acousto Optical Tunable Filter. Optics Express, 17, 2005–2014.

  30. Acousto Optical Tunable Filter: Characteristics –Bandpass function (3) • Sum of 5 sinc2 • With all parameters varying linearly with n • = Ii, n0i(i≠0), FWHMi Mahieux, A. et al. 2009. A New Method for Determining the transfer function of an Acousto Optical Tunable Filter. Optics Express, 17, 2005–2014.

  31. Acousto Optical Tunable Filter: Characteristics –Tuning function • Tuning function • Relation between the radiofrequency applied to the crystal and the central wavenumber of the filtered spectral interval • By-product of the previous analysis • Different for the different bins • Different parts of the crystal Mahieux, A. et al, 2008. In-flight performance and calibration of SPICAV/SOIR on-board Venus Express. Applied Optics, 47(13), 2252–65.

  32. Acousto Optical Tunable Filter: Characteristics – Order width vs. AOTF FWHM

  33. Detector: Flat field (1) • Pixel-to-pixel non-uniformity • Obtained: • In the laboratory: by illuminating the detector directly, without passing through the spectrometer, with an homogeneous light source; repeated with different exposure times • In-flight : • Select orders (32) with (almost) no Solar lines (T>0.95) • Large number of repeated observations • High-pass filtering to remove the effect of AOTF, spectrometer, optics… • Depends on • The binning scenario (2x12, 2x16, …) • From bin to bin • Time

  34. Detector: Flat field (2)

  35. Detector: Sample interval

  36. Instrumental Wavenumber calibration • Use of Solar lines in a lot of distinct orders • Correction for Doppler satellite (rec) – Sun (em) • Pixel – wavenumber – order relation • Wavenumber to pixel relation:

  37. Instrumental Spectral Sensitivity (1) • Spectral dependence of the whole instrument as a function of the incoming light wavelength • Obtained from direct Sun measurements, fullscan observations

  38. Instrumental Line Shape (ILS) (1) • From Solar lines and/or Atmospheric lines

  39. Instrumental Line Shape (ILS) (2)

  40. Instrumental Signal to Noise ratio (1) • From transmittance corresponding to high altitude (no absorption)

  41. Instrumental Signal to Noise ratio (2)

  42. Overview • Venus • Characteristics • Atmosphere • Venus Express • SOIR • Solar occultation • Instrument description • Telemetry • Calibrations • Echelle grating • AOTF • Detector • Optics • Spectrum construction • Geometry

  43. Measured spectrum SPICAV/SOIR instrument description:Measurement principles – diffraction order addition • AOTF transfer function: sinc² like • AOTF transfer function shape determination is critical • 7 diffraction orders have to be taken into account to correctly reconstruct measurement spectra AOTF transfer function Central order Mahieux, A. et al, 2008. In-flight performance and calibration of SPICAV/SOIR on-board Venus Express. Applied Optics, 47(13), 2252–65.

  44. Overview • Venus • Characteristics • Atmosphere • Venus Express • SOIR • Solar occultation • Instrument description • Telemetry • Calibrations • Echelle grating • AOTF • Detector • Optics • Spectrum construction • Geometry

  45. Geometry: The onion peeling approach

  46. Geometry – Tangent altitude calculation (1) • The instrument points to the Sun • Pointing direction displaced of 10’ above the centre of the Sun • Account for diffraction

  47. Geometry – Tangent altitude calculation (2) • Size of the slit is 30’ x 2’ (spectral x spatial) • VEX is inertial pointing  rotation of the slit

  48. Geometry – Tangent altitude calculation (3) • Use of SPICE to calculate the tangent altitude • From reconstructed kernels delivered by ESOC • Pointing angle for one bin of the slit: • Tangent altitude:

  49. Thank you for your attention

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