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.
Behind the entrance slit (008) a dichroic mirror (009) reflects the spectral range of the UV channels to folding mirror (101) and transmits the VIS spectral range to a flat mirror (201), that reflects the light at 90 degrees out of the plane of drawing.
The UV radiation is reflected by mirror (101) to a plano convex fused silica lens (102), that collimates the beam from the entrance slit in the direction of the grating (103). The function of the collimating / imaging lens (102) is twofold: it creates a parallel incoming beam on the grating (103) and forms an intermediate spectrum of the diffracted beams close to the field mirror (104). The grating (103) is used in the first order.
The layout of the UV channel is determined to a large extent by the stray light requirements, especially for the UV-1 spectral area (264 - 311 nm). Within the UV spectral range the variation in radiance from the Earth between the shortest wavelength (< 290 nm) and the longer wavelength (> 320 nm) varies by more than three orders of magnitude. Without proper measures the stray light at wavelengths below 290 nm would exceed the signal itself. This stray light originates from wavelengths between 310 to 380 nm. To avoid this unwanted situation, the UV channel is split into to sub-channels: UV-1 and UV-2.
An intermediate UV spectrum is created, close to a (split) field mirror (104). This mirror has a coating with a wavelength (= position) dependent variable reflectance in order to suppress the stray light of larger wavelengths at the shorter wavelengths. Depending on the bandwidth of reflection coating (at each position), a stray light light suppression of one order of magnitude is obtained.
Moreover, by splitting the spectral range in 2 parts the stray light that is caused by internal reflections in the UV-2 imaging objective, has no effect on the UV-1 spectrum at the detector surface.
The UV-1 image is scaled down by a factor of 2 on the CCD detector as compared to the UV-2 image. This is done in order to improve the signal-to-noise performance of the UV-1 channel by a factor of sqrt(2), at the cost of doubling the groundpixel size in the swath direction.