Executing various LoS trajectories and pointing to the sun or moon requires elaborated scanner control functions, particularly when the movement of both mechanisms has to be synchronized. The scanner control tasks are programmed in on-board software with supporting information being generated by the Sun Follower (SF) in the case of solar and lunar observations. Each scanner is operated separately in feedback control using readings of the rotation angle by an incremental optical encoder. Angular scan trajectories are assembled from pre-programmed basic and relative scan profiles for offset and motion generation. Since precise LoS steering to the Earth’s limb or celestial targets depends on the in-orbit platform attitude and state vector, the selected trajectory can be corrected accordingly. In limb measurements the horizontal scans through the atmosphere maintain a constant altitude by applying a correction which takes into account the varying curvature of the Earth (WGS84 model) along the orbit. Further corrections provide for the yaw steering attitude mode of the ENVISAT platform and the known misalignment of the instrument reference frame relative to the spacecraft frame. Sun and moon observations require one or both axes to be centred onto the target. Analytical control algorithms cannot always ensure this. Therefore, information derived from the readout of the four quadrants of the SF is fed into the control loop to steer the scanner motors such that the mirrors lock onto the weighted centre of the intensity distribution and follow the trajectory of sun or moon after successful acquisition. The SF receives light which is reflected from the blades of the spectrometer entrance slit. It has a 2.2° × 2.2° wide Field-of-View (FoV) which is reduced for sun tracking to 0.72° × 2.2° by the small aperture stop (see below).
For obtaining the solar irradiance, the sun has to be measured via a diffuser. Two aluminium diffusers are mounted on SCIAMACHY: one on the backside of the ESM mirror, one on the backside of the ASM mirror. Originally the ESM diffuser was the only one in the instrument. During calibration it turned out that this type of diffuser exhibits spectral features which would have endangered successful retrieval of some trace gas species. Thus very late in the development of phase C/D, when SCIAMACHY was already mounted on the spacecraft, an ASM fitted with the additional diffuser was integrated. Its surface was ground in a different way to yield optimized diffuser properties.
Telescope and Spectrometer
The ESM reflects light towards the telescope mirror, which has a diameter of 32 mm. From the telescope mirror the light path continues to the spectrometer entrance slit. With linear dimensions of 7.7 mm × 0.19 mm (cross-dispersion × dispersion) the entrance slit defines an Instantaneous Field of View of 1.8° × 0.045°. This corresponds to a ground pixel size of 25 km × 0.6 km at the sub-satellite point (nadir) and of 105 km × 2.5 km at the Earth’s horizon (limb). For solar observations, the IFoV can be reduced to 0.72° × 0.045° by operating the Aperture Stop Mechanism (APSM) which is located between the ESM and telescope mirror. The APSM reduces the aperture area and thus the intensity level. Channel dependent effects lead to a reduction by a factor of more than 5000 for channels 1-5 and about 2500 for channels 6-8.
The overall spectrometer design is based on a two stage dispersion concept: First, the incoming light is pre-dispersed and projected onto a spectral image. Subsequently, this spectral image is dissected into eight spectral intervals that are diverted into eight spectral channels for further dispersion. The selected approach has the advantage of reducing stray light in the channels covering low light intensity in the UV and NIR-SWIR part of the spectrum. It also effectively prevents the various spectral orders from one grating overlapping with the other parts of the spectrum (Goede et al. 1991).
The pre-disperser prism, located behind the entrance slit, serves two purposes. It weakly disperses the light and directs fully polarized light for further processing to the Polarization Measurement Device (PMD). Small pick-off prisms and subsequent di-chroic mirrors direct the intermediate spectrum to the 8 science channels where the light is further dispersed by individual gratings. In the light path routed to channels 3-6 the Neutral Density Filter Mechanism (NDFM) can move a filter into the beam. With a filter transmission of 25% it can be used, in conjunction with the APSM, to even further reduce light levels during solar measurements.