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AITA 9, MERTIS Team – MERTIS Calibration, Oct 2007 Folie 1 Development of a calibration concept for the MErcury Thermal Infrared Spectrometer Thomas Saeuberlich, Eckehard Lorenz, Wolfgang Skrbek, Ingo Walter, Carsten Paproth, Joern Helbert German Aerospace Center (DLR) Optical Information Systems Rutherfordstr. 2, 12489 Berlin, Germany

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AITA 9, MERTIS Team – MERTIS Calibration, Oct 2007 Folie 2 MERTIS Calibration (a)determination of coefficients for the conversion of the digital number output DN ij of each spectro-spatial channel (i,j) into radiance values L ij (b) characterization of the optical, thermal and electronical status of the instrument (c) determination of the accuracy of the acquired values (d) characterization of system stability CalibrationOn-GroundIn-Flight spectral wavelength assignment, SSD, spectral resolution - radiometric sensitivity, offset, noiseoffset, sensitivity, noise geometric line of sight of pixels, geometric resolution - Goals:

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AITA 9, MERTIS Team – MERTIS Calibration, Oct 2007 Folie 3 MERTIS Calibration – Experimental Setup & Procedure 1.200 object-images (shutter open) for each blackbody temperature T BB in [50°C….1000°C] with T BB = 50 K and an additional measurement of dark-images (shutter closed) 2.calculation of the difference image from the averaged images for object- and dark-measurements 1.assignment of CWL to each channel (i,j) using narrow band passes (FWHM = 100nm) in the spectral range [7 µm…14 µm] and = 0.5 µm 2.calculation of the spectral sampling distance SSD = l j+1 -l j = 88 nm Spectral Calibration: Radiometric Measurement: MERTIS optics including TMA, Shutter, Slit, Offner- Spectrometer, Bolometer Detector Array

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AITA 9, MERTIS Team – MERTIS Calibration, Oct 2007 Folie 4 MERTIS Calibration – Radiation Signal Transformation I (i,j)-single channel; i corresponds to the spatial and j to the spectral direction U ij,opened -measured and averaged voltage for channel (i,j) with shutter open U ij,closed -measured and averaged voltage for channel (i,j) with shutter closed U ij,diff -difference voltage calculated from the measured voltages with opened and closed shutter U 0,ij -residual offset r ij -channel sensitivity L-spectral radiance according to the Planck Law T BB -blackbody temperature SSD-spectral sampling distance (SSD=88 nm for current breadboard) j -spectral interval that corresponds to a single channel (i,j) linear approach model does currently not include the spectral and spatial smearing

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AITA 9, MERTIS Team – MERTIS Calibration, Oct 2007 Folie 5 MERTIS Calibration – Radiation Signal Transformation II spatial direction (index i) spectral direction (index j) object temperature blackbody @ 50°C blackbody @ 1000°C The cross marks channel (i,j) = (88,88) that corresponds to a spectral position of 9µm within the area illuminated by the blackbody verification of linear dependence between the incoming (integrated) radiance and the measured voltages for channel (88,88) calculation of the calibration coefficients (residual offset U 0 and sensitivity r ij ) by a linear fit 6.3µm 16.8 µm 37 < i < 137 slit height

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AITA 9, MERTIS Team – MERTIS Calibration, Oct 2007 Folie 6 MERTIS Calibration – Calibration Coefficients spectral distribution of the sensitivity r 88,j spectral distribution of the residual offset U 0,88,j

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AITA 9, MERTIS Team – MERTIS Calibration, Oct 2007 Folie 7 MERTIS Calibration – Application of Calibration Coefficients I r ij and U 0,ij can be applied to dark-corrected raw images in order to calculate the (integrated) radiances L int,ij having an error e ij :

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AITA 9, MERTIS Team – MERTIS Calibration, Oct 2007 Folie 8 MERTIS Calibration – Application of Calibration Coefficients II The radiances L int,ij calculated from a dark-corrected raw measurement can be used to calculate temperatures T ij using Planck´s law and assuming a small SSD (which actually is only 88 nm): +/- 3 K +/- 7 K +/- 15 K

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AITA 9, MERTIS Team – MERTIS Calibration, Oct 2007 Folie 9 a spectro-radiometric calibration procedure for 1st MERTIS prototype has been developed 2-step background correction used linear approach for modeling the signal transformation from an incoming radiance into a measured voltage approach verified by calculating the temperature of the reference radiator for each spectral channel in the spectral interval [7 µm…14 µm] the temperature error is ± 3 K…± 15 K depending on the object temperature (1000°C – 100°C) topics of the following investigations: model has to be extended by the spectral and spatial smearing effects caused by the optics determination of the spectral and spatial resolution MERTIS Calibration – Conclusion

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