SOIR Instrument description and data calibration

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

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

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

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

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

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

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

10. 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

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

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

13. Diffraction order Echelle grating: Blaze function • The efficiency of the grating in terms of refracted angle • Is maximum when the refracted angle = incident angle Mahieux, A. et al, 2008. In-flight performance and calibration of SPICAV/SOIR on-board Venus Express. Applied Optics, 47(13), 2252–65.

14. 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.

15. Acousto Optical Tunable Filter: Characteristics –Bandpass function Mahieux, A. et al. 2009. A New Method for Determining the transfer function of an Acousto Optical Tunable Filter. Optics Express, 17, 2005–2014.

16. 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.

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

18. Detector: Sample interval

19. 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: Should be considered with care!

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

21. Instrumental Line Shape (ILS)

22. Instrumental Signal to Noise ratio

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

24. Measured spectrum SPICAV/SOIR instrument description:Measurement principles – diffraction order addition (1) • 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.

25. SPICAV/SOIR instrument description:Measurement principles – diffraction order addition (2) Mahieux, A. et al, 2008. In-flight performance and calibration of SPICAV/SOIR on-board Venus Express. Applied Optics, 47(13), 2252–65.

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

27. 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

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

29. 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:

30. Thank you for your attention