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.