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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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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)
Overview • Venus • Characteristics • Atmosphere • Venus Express • SOIR • Solar occultation • Instrument description • Telemetry • Calibrations • Echelle grating • AOTF • Detector • Optics • Spectrum construction • Geometry
Overview • Venus • Characteristics • Atmosphere • Venus Express • SOIR • Solar occultation • Instrument description • Telemetry • Calibrations • Echelle grating • AOTF • Detector • Optics • Spectrum construction • Geometry
The Venus orbit Credits: Celestia
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
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
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
Overview • Venus • Characteristics • Atmosphere • Venus Express • SOIR • Solar occultation • Instrument description • Telemetry • Calibrations • Echelle grating • AOTF • Detector • Optics • Spectrum construction • Geometry
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
Venus Express: Payload • 7 instruments • ASPERA • MAG • PFC • SPICAV/SOIR • VeRA • VIRTIS • VMC Credits: European Space Agency
Overview • Venus • Characteristics • Atmosphere • Venus Express • SOIR • Solar occultation • Instrument description • Telemetry • Calibrations • Echelle grating • AOTF • Detector • Optics • Spectrum construction • Geometry
SOIR: Solar occultation - Animation Credits: Celestia
SOIR: solar occultation – Measurement principle Orbit 232 – Order 129 Transmittance Side view To Sun VEX N View from Venus Express Venus Atmosphere
SOIR: solar occultation – Measurement principle Orbit 232 – Order 129 Transmittance Side view To Sun VEX N View from Venus Express Venus Atmosphere
HDO H2O CO2 CO SOIR: Solar occultation – Example of measured spectra • 4 different diffraction orders measured during each occultation Orbit 486 (20070820)
Overview • Venus • Characteristics • Atmosphere • Venus Express • SOIR • Solar occultation • Instrument description • Telemetry • Calibrations • Echelle grating • AOTF • Detector • Optics • Spectrum construction • Geometry
SOIR: Optical description (1) Credits: IASB/BIRA
Acousto-optic filter Echelle grating 250 µm Crystal Reflective surfaces Infrared detector Spatial direction: 256 pixels Spectral direction: 320 pixels SOIR: Optical description (2)
Overview • Venus • Characteristics • Atmosphere • Venus Express • SOIR • Solar occultation • Instrument description • Telemetry • Calibrations • Echelle grating • AOTF • Detector • Optics • Spectrum construction • Geometry
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
Slit position during an occultation Spatial Spectral Bin 1 Bin 2 60 km 60 Venus
Overview • Venus • Characteristics • Atmosphere • Venus Express • SOIR • Solar occultation • Instrument description • Telemetry • Calibrations • Echelle grating • AOTF • Detector • Optics • Spectrum construction • Geometry
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
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.
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.
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.
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.
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.
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.
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.
Acousto Optical Tunable Filter: Characteristics – Order width vs. AOTF FWHM
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
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:
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
Instrumental Line Shape (ILS) (1) • From Solar lines and/or Atmospheric lines
Instrumental Signal to Noise ratio (1) • From transmittance corresponding to high altitude (no absorption)
Overview • Venus • Characteristics • Atmosphere • Venus Express • SOIR • Solar occultation • Instrument description • Telemetry • Calibrations • Echelle grating • AOTF • Detector • Optics • Spectrum construction • Geometry
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.
Overview • Venus • Characteristics • Atmosphere • Venus Express • SOIR • Solar occultation • Instrument description • Telemetry • Calibrations • Echelle grating • AOTF • Detector • Optics • Spectrum construction • Geometry
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
Geometry – Tangent altitude calculation (2) • Size of the slit is 30’ x 2’ (spectral x spatial) • VEX is inertial pointing rotation of the slit
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: