The Environmental Mapping and Analysis Program (EnMAP) is a German space borne science mission that aims at characterizing the Earth’s environment on a global scale. The single payload of the satellite is the Hyperspectral Imager (HSI). It is capable of measuring the solar radiance reflected from the Earth’s surface as a continuous spectrum in the spectral range of 420nm to 2450nm, with a spectral sampling of 6.5nm (VNIR) and 10nm (SWIR). The EnMAP swath of 30km is sampled in spatial direction with 30m GSD.
In this proceeding, we give an overview of the design and current integration status of the HSI instrument optical unit with additional focus on measured optical and electro-optical performance.
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.
Selected technologies for the integration of the TRANSMIT/RECEIVE OPTICS (TRO) are presented. One of the challenging characteristics of the TRO is its stringent requirement on opto-mechanical stability. The stability performance of the TRO must be ensured for the relevant interface environments (thermal, structural) over the 3 years mission lifetime. Comprehensive analyses have been conducted, which have confirmed the need for the development of special integration technologies. Also, dedicated test equipment has been developed to precisely verify the TRO´s optomechanical stability. Another important feature of the TRO is its exposure to the high power laser beam of the ADALIN instrument. The corresponding optical elements and their mounts must survive exposure to light intensities up to the required laser-induced damage thresholds (LIDT). Two types of adhesives for gluing of the TRO optics have been selected. Their qualification w.r.t. outgassing was necessary since LIDT´s of optical surfaces are significantly reduced when organic outgassing products are deposited there.
The EnMAP telescope is an off-axis telescope made of three aspherical mirrors and a folding mirror mounted on bipods. Following a highly precise mechanical placement process , final alignment is performed by position correction of a single compensator element. The mirror position change by shimming is demonstrated to be reproducible within 1 μm.
The Environmental Mapping and Analysis Program (EnMAP) is a German hyperspectral mission with pushbroom type imaging spectrometers covering the wavelength ranges from 420 nm to 2450 nm. The ground sampling distance is 30 m with a total swath of 30 km, while the spectral sampling distance is roughly 5 nm to 12 nm.
The Environmental Mapping and Analysis Program (EnMAP) is a German space borne science mission that aims at characterizing the Earth’s environment on a global scale. The single payload of the satellite is the hyper spectral imager (HSI). It is capable of measuring the solar radiance reflected from the Earth’s surface as a continuous spectrum in the spectral range of 420nm to 2450nm, with a spectral sampling of 6.5nm (VNIR) and 10nm (SWIR). The EnMAP swath of 30km is sampled in spatial direction with 30m.
The Environmental Mapping and Analysis Program (EnMAP) is a German hyperspectral satellite mission that aims at monitoring and characterizing the Earth’s environment on a global scale. Its hyperspectral imager (HSI) is capable of measuring the solar radiance reflected from the Earth’s surface as a continuous spectrum in the spectral range of 420 nm to 2450 nm.
The optical system of the hyperspectral imager of the Environmental Mapping and Analysis Program (EnMAP) consists of a three-mirror anastigmat (TMA) and two independent spectrometers working in the VNIR and SWIR spectral range, respectively. The VNIR spectrometer includes a spherical NiP coated Al6061 mirror that has been ultra-precisely diamond turned and finally coated with protected silver as well as four curved fused silica (FS) and flint glass (SF6) prisms, respectively, each with broadband antireflection (AR) coating, while the backs of the two outer prisms are coated with a high-reflective coating. For AR coating, plasma ion assisted deposition (PIAD) has been used; the high-reflective enhanced Ag-coating on the backside has been deposited by magnetron sputtering. The SWIR spectrometer contains four plane and spherical gold-coated mirrors, respectively, and two curved FS prisms with a broadband antireflection coating. Details about the ultra-precise manufacturing of metal mirrors and prisms as well as their coating are presented in this work.
Imaging spectrometers featuring a grating disperser allow for a compact system design. However, due to the limited
diffraction efficiency of the grating these instruments suffer from low throughput and high sensitivity to polarization.
Prism spectrometers do not have these disadvantages, but they show a low angular dispersion with noticeable non-linearity,
which is the main driver of the overall spectrometer dimensions. The envelope of a prism spectrometer can be
significantly decreased when prisms with curved surfaces are used. They allow for a reconfiguration of the concentric
Offner relay which is well known for its good imaging quality and its low distortion. In the document at hand a novel
type of compact Offner spectrometers with curved prisms is presented. As an example the optic design of EnMAP, a
German space born hyperspectral imager is given.
The Environmental Mapping and Analysis Program (EnMAP) is a German space based hyperspectral mission planned
for launch in 2012. The hyperspectral instrument covers the wavelength range from 420nm to 2450nm using a dual
spectrometer layout. Both f/3 spectrometers employ a prism disperser for maximum throughput and are linked to the
common foreoptics by a micromechanical field splitter. Together with custom designed silicon and MCT-based detector
arrays this sensor design exhibits a peak system SNR of 1000 at 495nm and of more than 300 at 2200nm. Stable and
precise in orbit performance is ensured by a multi loop thermal control system and a system calibration which relies on
onboard sources as well as a full aperture diffuser.