Minimize SCIAMACHY Product Handbook

Wavelength Calibration

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fig. 4-3:

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Non-Linearity in channel 8 for different pixel regions (indicated by colours). 'Low' pixels are those with pixel numbers below 512. The pixel numbering starts at 0. (Graphics: SRON)
 

For channels 6+ and 8 the quantum efficiency changes with the detector temperature Tdet, whereas the first part of channel 6 and channel 7 shows no significant temperature dependence. The thermal background is caused by the thermal radiation of the instrument and is the dominant part of the dark signal (about 4000 BU/sec) in channel 8. It depends on the orbit phase φ because the temperature gradients in the instrument are not completely stable but vary over one orbit due to the changing angle of solar irradiation. The variation of the dark signal over the orbit can reach up to 60 BU/sec which has significant impact on the retrievals of trace gases. In-flight the orbital variation is measured once a month during a special calibration orbit in which only dark signal measurements are performed by looking to deep space at a tangent height of 250 km in limb mode. The variation of the transmission makes the dark signal correction time dependent meaning that for channels 7 and 8 a dark signal correction, calculated from measurements in the same orbit, must be used.

The final detector related correction is the Pixel-to-Pixel Gain (PPG) correction. The pixels in the SWIR channels do not show the same response to incoming light. Variations of a few percent can be observed. The PPG is derived by first smoothing a WLS measurement, assuming the spectrum is flat. Then the original spectrum is divided by the smoothed measurement, leaving only the high frequent variations that are caused by the different pixel gains in the result. The PPG is strictly an effect caused by the electronics and the detector and is thus associated to the individual pixels but not to the wavelength.

4.3 Wavelength Calibration

In-flight spectral calibration of SCIAMACHY data uses the internal SLS measurements with the exception of channels 7 and 8 (see below). For selected lines the Falk algorithm determines the pixel positions (Falk 1984). These are then fitted to theoretical line positions provided with the calibration data. From the polynomial coefficients of the fit the wavelength for each pixel can be calculated. Measurements of solar Fraunhofer lines serve as a quality check. In channels 7 and 8 a calibration with the internal SLS lamp is impossible because in these channels not enough useful lines are available to calculate the wavelength calibration with sufficient accuracy. In channel 8 this is caused by bad pixels interfering with the determination of the line position. Channel 7 only contains two strong doublet lines preventing an accurate determination of line positions over the whole channel. In both channels data from on-ground gas cell absorption measurements establish the wavelength calibration.
An additional effect discovered during the on-ground calibration is the so-called Blocking Shift: During the spectral calibration on-ground, the internal SLS and an external SLS were used. A comparison of the measurements done with the two lamps revealed a wavelength shift of up to 0.07 nm. The reason is a partial blocking of the light path during internal SLS measurements. The blocking shift was characterised and is part of the calibration data. Verification of the spectral calibration in-flight has proven that SCIAMACHY is spectrally very stable.

 

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