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Calibration Unit

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Calibration Unit

The requirements to maintain high spectral stability and relative radiometric accuracy over the mission’s lifetime are verified via an on-board calibration unit. It consists of two calibration lamps, one for white light and one for spectral lines. The White Light Source (WLS) is a 5 Watt UV-optimized Tungsten-Halogen lamp with an equivalent blackbody temperature of about 3000 K. Its signal is used to verify the pixel-to-pixel signal stability and to monitor the Etalon effect. The Spectral Line Source (SLS) is a Neon filled hollow Pt/Cr cathode lamp. Its operation allows the determination of the pixel-to-wavelength relation. The calibration unit is located close to the ESM. By rotating the ESM mirror into specific positions it is possible to reflect light from the WLS respectively the SLS towards the telescope mirror and thus onto the entrance slit.

An extra calibration mirror near the ESM can be used to provide for an additional reflection of the incoming light on the ESM mirror. Due to its protected position well within the instrument it is assumed that this extra mirror will not degrade throughout the mission. Thus it can be used as a further means of monitoring optical performance.

 

Channel  Spectral Range (nm)  Detector Temperature ( C) 
310- 365  -18 
455 - 515  -18 
610 -690  -18 
800 - 900  -18 
1500 - 1635  -18 
2280 - 2400  -18 
45   800 - 900  -18 

Table 2-6: SCIAMACHY PMD. The wavelength ranges are defined so that they contain 80% of the total signal given in the PMD channels.


Polarization Measurement Device

The measurement sensitivity of the spectrometer depends on the polarization state of the incoming light. Therefore SCIAMACHY is equipped with a Polarization Measurement Device. Six of its channels (PMD A-F) measure light polarized perpendicular to the SCIAMACHY optical plane, generated by a Brewster angle reflection at the second face of the pre-dispersing prism. This polarized beam is split into six different spectral bands. The spectral bands are quite broad and overlap with spectral regions of channels 2, 3, 4, 5, 6, and 8 (table 2-6).

The PMD and the light path to the array detectors, including the detectors, have different polarization responses. Consequently, with the appropriate combination of PMD data, array detector data and on-ground polarization calibration data the polarization of the incoming light from the nadir measurements can be determined. For atmospheric limb measurements, where both limb and nadir mirrors are used, the light is outside the optical plane of the spectrometer. This requires measurements of additional polarization information of the incoming light. A seventh PMD channel measures the 45° component of the light extracted from the channels 3-6 light path. All PMD channels are non-integrating and read out every 1/40 sec. They observe the same atmospheric volume as science channels 1-8.

 

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