An all-silicon, high precision double-slit device for hyperspectral imager EnMAP

Modern space-born spectrometer applications more and more rely on highest-precision slit devices defining the spectrometer entrance aperture. Reason for this is the increasing demand for broadband and high-resolving spectrometer or imaging spectrometer applications. High-NA optics necessitate very thin (microns) and accurate slit structures, whose manufacturing is demanding or impossible with common technology. In addition, they must withstand harsh environmental loads like shock, vibration and thermal cycling. The hyperspectral imager of the Environmental Mapping and Analysis Program (EnMAP) satellite mission comprises two spectrometers whose entrance slits are realized by an all-silicon, highly integrated double slit device. It is manufactured by use of micro-machining and lithographic processes, reaching sub-micron geometric precision. Each slit aperture is 24 μm x 24 mm large, corresponding to an aspect ratio of 1:1000. In some critical respects – such as planarity or coalignment – the technology excells established manufacturing technologies like metal electroforming, milling and others. In addition to slit aperture definition, the double slit device achieves field separation for the two imaging spectrometers. One of the two transmitted light beams is deflected by a flat mirror. The minute silicon mirror is completely integrated into the device. The EnMAP slit assembly has undergone an intensive qualification test program. Included were vibrational, shock and thermal load tests as well as a more specific sun intrusion test. The results of these tests are briefly presented and discussed.