SOIR Instrument description and data calibration A C

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) SOIR data workshop

Overview q Venus Ø Characteristics Ø Atmosphere q Venus Express q SOIR Ø Ø Solar occultation Instrument description Telemetry Calibrations v v Echelle grating AOTF Detector Optics Ø Spectrum construction Ø Geometry SOIR data workshop

The Venus orbit Credits: Celestia SOIR data workshop

Venus : Characteristics Radius Mass Sidereal day Year duration Axis inclination Distance to the Sun Surface temperature Surface pressure 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 SOIR data workshop

Venus : Atmosphere subdivisions q Troposphere Ø Poorly known region Ø High temperatures and pressures Ø Low wind q Cloud layer Ø Aerosols H 2 SO 4 Ø Wind ~ 300 km/h retrograde q Mesosphere Ø Transition zone Ø Aerosol haze q Thermosphere Ø Large differences between day and night v Temperature chemistry Ø Subsolar – antisolar circulation q Region studied by SOIR data workshop

Venus : Atmospheric composition q Main compound: carbon dioxide CO 2 Ø Ø 96. 5 % up to ~110 km Uniform Transformed by solar UV into CO (> 110 km) Quantity decreases with altitude, replaced by CO and O q Little water Ø Variable quantity Ø HDO/H 2 O fraction 140 x larger than on Earth q H 2 SO 4 in the regions close to the cloud layer Ø products SO 2, SO, OCS, H 2 CO q Halogens Ø HCl, HF SOIR data workshop

Overview q Venus Ø Characteristics Ø Atmosphere q Venus Express q SOIR Ø Ø Solar occultation Instrument description Telemetry Calibrations v v Echelle grating AOTF Detector Optics Ø Spectrum construction Ø Geometry SOIR data workshop

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

Venus Express: Payload q 7 instruments Ø Ø Ø Ø ASPERA MAG PFC SPICAV/SOIR Ve. RA VIRTIS VMC Credits: European Space Agency SOIR data workshop

Overview q Venus Ø Characteristics Ø Atmosphere q Venus Express q SOIR Ø Ø Solar occultation Instrument description Telemetry Calibrations v v Echelle grating AOTF Detector Optics Ø Spectrum construction Ø Geometry SOIR data workshop

SOIR: Solar occultation - Animation Credits: Celestia SOIR data workshop

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

$SOIR: Solar occultation – Example of measured spectra q 4 different diffraction orders measured$

SOIR: Solar occultation – Example of measured spectra q 4 different diffraction orders measured during each occultation Orbit 486 (20070820) HDO H 2 O CO 2 CO SOIR data workshop

SOIR: Solar occultations – Measurements map SOIR data workshop

Overview q Venus Ø Characteristics Ø Atmosphere q Venus Express q SOIR Ø Ø Solar occultation Instrument description Telemetry Calibrations v v Echelle grating AOTF Detector Optics Ø Spectrum construction Ø Geometry SOIR data workshop

SOIR: Optical description Credits: IASB/BIRA (1) SOIR data workshop

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

Overview q Venus Ø Characteristics Ø Atmosphere q Venus Express q SOIR Ø Ø Ø Solar occultation Instrument description Telemetry Calibrations Echelle grating v AOTF v Detector v Optics Ø Spectrum construction Ø Geometry SOIR data workshop

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

Spatial Bin 1 60 Bin 2 60 km Venus SOIR data workshop Spectral Slit position during an occultation

Overview q Venus Ø Characteristics Ø Atmosphere q Venus Express q SOIR Ø Ø Solar occultation Instrument description Telemetry Calibrations v v Echelle grating AOTF Detector Optics Ø Spectrum construction Ø Geometry SOIR data workshop

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

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

$Echelle grating: Blaze function (2) Diffraction order Mahieux, A. et al, 2008. In-flight performance$

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

Acousto Optical Tunable Filter: Characteristics q Calibrations: Ø 1. AOTF bandpass function v TAOTF = f(l, l 0, Dl. FWHM) Ø 2. Tuning function v l 0 = f(RF) Ø 3. Bandwidth v Dl. FWHM = 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. SOIR data workshop

Acousto Optical Tunable Filter: Characteristics – Bandpass function (1) q Usual transfer function for AOTF q Calibration using miniscans Ø Using deed solar lines (from Hase et al. 2009) Ø Radiofrequency of AOTF chosen to correspond to well defined solar lines Ø Different frequency steps (1 k. Hz to 20 k. Hz) around that RF q Lots of miniscans for a lot of different solar lines over the entire spectral range covered by SOIR q 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. SOIR data workshop

Acousto Optical Tunable Filter: Characteristics – Bandpass function (2) One solar line @ 2948. 7 cm-1 A B Mahieux, A. et al. 2009. A New Method for Determining the transfer function of an Acousto Optical Tunable Filter. Optics Express, 17, 2005– 2014. SOIR data workshop

Acousto Optical Tunable Filter: Characteristics – Bandpass function (3) q Sum of 5 sinc 2 q With all parameters varying linearly with n Ø = Ii, n 0 i(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. SOIR data workshop

Acousto Optical Tunable Filter: Characteristics – Tuning function q Tuning function Ø Relation between the radiofrequency applied to the crystal and the central wavenumber of the filtered spectral interval q By-product of the previous analysis q Different for the different bins Ø 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. SOIR data workshop

Acousto Optical Tunable Filter: Characteristics – Order width vs. AOTF FWHM SOIR data workshop

Detector: Flat field (1) q Pixel-to-pixel non-uniformity q Obtained: Ø In the laboratory: by illuminating the detector directly, without passing through the spectrometer, with an homogeneous light source; repeated with different exposure times Ø In-flight : v Select orders (32) with (almost) no Solar lines (T>0. 95) v Large number of repeated observations v High-pass filtering to remove the effect of AOTF, spectrometer, optics… q Depends on Ø The binning scenario (2 x 12, 2 x 16, …) Ø From bin to bin Ø Time SOIR data workshop

Detector: Flat field (2) SOIR data workshop

Detector: Sample interval SOIR data workshop

Instrumental Wavenumber calibration q Use of Solar lines in a lot of distinct orders Ø Correction for Doppler satellite (rec) – Sun (em) q Pixel – wavenumber – order relation q Wavenumber to pixel relation: SOIR data workshop

Instrumental Spectral Sensitivity (1) q Spectral dependence of the whole instrument as a function of the incoming light wavelength q Obtained from direct Sun measurements, fullscan observations SOIR data workshop

Instrumental Line Shape (ILS) (1) q From Solar lines and/or Atmospheric lines SOIR data workshop

Instrumental Line Shape (ILS) (2) SOIR data workshop

Instrumental Signal to Noise ratio (1) q From transmittance corresponding to high altitude (no absorption) SOIR data workshop

Instrumental Signal to Noise ratio (2) SOIR data workshop

Overview q Venus Ø Characteristics Ø Atmosphere q Venus Express q SOIR Ø Ø Solar occultation Instrument description Telemetry Calibrations v v Echelle grating AOTF Detector Optics Ø Spectrum construction Ø Geometry SOIR data workshop

$SPICAV/SOIR instrument description: Measurement principles – diffraction order addition q AOTF transfer function: sinc²$

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

Overview q Venus Ø Characteristics Ø Atmosphere q Venus Express q SOIR Ø Ø Solar occultation Instrument description Telemetry Calibrations v v Echelle grating AOTF Detector Optics Ø Spectrum construction Ø Geometry SOIR data workshop

Geometry: The onion peeling approach SOIR data workshop

Geometry – Tangent altitude calculation (1) q The instrument points to the Sun Ø Pointing direction displaced of 10’ above the centre of the Sun q Account for diffraction SOIR data workshop

Geometry – Tangent altitude calculation (2) q Size of the slit is 30’ x 2’ (spectral x spatial) q VEX is inertial pointing rotation of the slit SOIR data workshop

Geometry – Tangent altitude calculation (3) q Use of SPICE to calculate the tangent altitude Ø From reconstructed kernels delivered by ESOC q Pointing angle for one bin of the slit: q Tangent altitude: SOIR data workshop

Thank you for your attention SOIR data workshop