EnMAP (Environmental Mapping and Analysis Program) is a German hyperspectral satellite mission to earth observation, which has been initiated and funded by the German Aerospace Center (DLR). The mission start is scheduled for 2019. The optical system consists of a telescope (Three Mirror Anastigmat / TMA) and two independent spectrometers for the visible (VNIR) and the short-wave (SWIR) infrared spectral range. The mirrors for telescope as well as spectrometer have been designed and produced in close collaboration with and to the specifications of OHB at Fraunhofer IOF. The established alloy aluminum 6061T6 was used as base material. A polishable amorphous nickel-phosphorus (NiP) layer with a phosphorous content between 10 % and 12 % was applied to the ultra-precise, diamond-finished mirrors. The subsequent shape correction by means of diamond turning ensures shape accuracies of better that 500 nm p.-v. (peak-to-valley) with a surface roughness of 5 nm rms (root-mean-square). By a subsequent polishing step, roughness values of 0.5 nm rms were obtained using White Light Interferometry (WLI) by 50-fold magnification. In a final step, the shape has been corrected by Ion Beam Finishing (IBF). For the aspheric mirrors, the shape was measured by Computer Generated Holograms (CGH) realized by electron beam lithography at Fraunhofer IOF. As a result, shape accuracies of better than 140 nm p.-v. and 18 nm rms, respectively, with a very low surface roughness (< 0.8 nm rms) have been achieved after typically two to three correction cycles by IBF. With this approach, the requirements for the optical properties (high surface shape accuracy, low surface roughness, and high reflection), the mechanical properties (high stiffness, low mass), and the thermal properties (application temperature and storage/transport) have been fulfilled. In order to obtain maximum reflection over the entire spectral range used, the mirrors were coated with protected silver layers (spectral range from 420 nm to 2450 nm) and with gold layers (spectral range from 900 nm to 2450 nm), respectively. In addition to the production of the mirrors, the coating of the prisms used in the spectrometers has been carried out at Fraunhofer IOF, too. Four different coatings were optimized and qualified for use in space. These were three broadband antireflection coatings for the prisms made of Homosil/SF6 (spectral range from 420 nm to 1000 nm each) and Infrasil (spectral range from 900 nm to 2450 nm) as well as a silver-based highly reflective layer, which has been used as a back reflector on the SF6 prisms.
Authors: Mark Schürmann, Dieter Gäbler, Stefan Schwinde, Sandra Müller, Matthias Beier, Stefan Risse, Hermann Bittner (OHB System AG), Markus Erhard (OHB System AG)