In the Optical Assembly the incident radiance and irradiance are collected and split according to wavelength and focussed on the detector modules of the two optical channels. The Optical Bench consists of a single aluminium structure in which all elements are mounted. Attached to this aluminium structure are the two Detector Modules. The Optical Assembly Thermal Hardware consists of a thermal radiator and heaters. The thermal radiator is thermally connected to the detector modules via cold-fingers and flex-links. The heaters are attached to the Optical Bench structure and the CCD detectors.
Optical Path of Earth, solar, WLS and LED measurements.
The optical paths for the Earth and solar measurements are not the same. For this reason the optical paths could degrade differently during the mission, which may produce an artificial trend in the Earth radiance to solar irradiance ratio (sun-normalised Earth reflectance).
For an Earth measurement the complete optical path of the radiation is shown in the figure for the UV-channel and in the figure for the VIS channel. The radiation enters the instrument by aperture (001), reflects from the primary mirror (103), passes the scrambler (005) and reflects from the secondary mirror (007) before entering the spectrometer by entrance slit (008). Directly behind the entrance slit the radiation is split by a dichroic mirror (009) that reflects the light of shorter wavelengths into the UV channel and transmits the light of higher wavelengths into the VIS channel.
For a solar calibration measurement, the Sun Aperture Mechanism is opened so that the sunlight can enter the instrument throught the solar mesh. This sunlight is then reflected by one of the three reflection diffusers on the Diffuser Mechanism towards the mirror C03 on the Folding Mirror Mechanism, which moves in the optical path of the telescope for this occasion in order to reflect the sunlight via the scrambler (005) towards the secondary telescope mirror (007). The solar calibration measurements therefore use a different optical path than the Earth radiation measurement, adding the mesh, the reflection diffuser and the FMM mirror C03, and missing the primary telescope mirror (103). In first approximation the folding mirror C03 and the primary telescope mirror 003, that are both made of uncoated aluminium, will have a comparable degradation.
During a White Light Source (WLS) measurement the transmission diffuser (C05) in the Diffuser Mechanism is used and the folding mirror C03 is moved into the Earth optical path of the telescope. The WLS light passes lens (C08), is reflected from mirrors (C09) and (C10), passes the transmission diffuser and is reflected by the folding mirror C03 through the polarisation scrambler (005) towards the secondary telescope mirror (007). With the WLS most of the optical path can be monitored for optical degradation.
The LEDs are directly in front of the CCD detectors with only a few optical elements in between and illuminate the CCD detectors directly, that is, without going through the complete optical system.