With the increasing demand on compact space instruments, using multispectral filters instead of complex spectrometer setups, the butcher-block filter technology is pushed to larger number of channels, more extreme length to width ratios of the individual filter bars. The resulting requirements on center wavelength and bandwidth and their tolerances are more and more demanding and require not only improved production technology but also a dedicated spectral characterization setup to verify the performance of the individual filter channels within such a butcher block filter assembly. The setup presented here operates in the spectral regime between 800nm and 2,5μm. Due to the geometry of the individual filter channels of a butcher block filter, the setup is designed for a measurement spot size down to 50μm and can operate at F-Numbers between 2,5 and 4. An absolute spectral resolution of 0,2nm is demonstrated, with a relative resolution of 0,1nm for homogeneity measurements along the individual filter stripes. The out of band blocking can be characterized better than OD3 over the full spectral range. A precision x/y position stage allows to position the small metrology spot (50μm) on individual filter channels and to scan along these channels to map their performance. This metrology setup allows to verify the final performance of multispectral filters assemblies, especially butcher block filters.
We present the measurement results of the pre-flight characterization and calibration of the EnMAP HyperSpectral Imager (HSI) and draw conclusions on the performance of the instrument itself as well as the optical ground support equipment (OGSE) deployed. The HSI is capable of measuring the solar irradiance reflected from the Earth’s surface as a continuous spectrum in the spectral range of 420 nm to 2450 nm, with an average spectral sampling of 6.5nm (VNIR) and 10nm (SWIR). The EnMAP swath of 30 km is sampled with 30 m in along and across track direction. The on-ground characterization was performed in air comprising a spectral, geometric, radiometric, polarimetric, and stray light characterization. For spectral calibration, a monochromator setup was used to characterize the spectral response function of the instrument yielding a wavelength and spectral resolution calibration including spectral smile. Geometric calibration was performed using a knife edge in the focal plane of a collimator allowing to determine the line of sight of each pixel. Additionally, the keystone and MTF performance of the instrument was confirmed. A large integrating sphere was used for radiometric characterization of the instrument. This provides an on-ground baseline calibration of radiometric coefficients and response non-uniformity to be updated in orbit. Additionally, the noise performance of the instrument was validated. A polarizer was used to measure polarization sensitivity of the instrument which was found to be in excellent agreement with expectations. Finally a stray light characterization of the instrument was conducted.
CHIME, the Copernicus Hyperspectral Imaging Mission for the Environment, is one of the six High Priority Candidate Missions (HPCM) of the evolution in the Copernicus Space Component (CSC) foreseen in the mid-2020s that is proposed for further analysis. In this paper we summarize the results as retrieved by OHB (D) as part of the Phase A/B1. The contract was kicked off in 2018 and concluded in 2020 after finalisation of the Pre-development activities. The proposed instrument is a hyperspectral imager instrument with reflective telescope and grating-based spectrometer. The selected orbit is in the range of 625 ± 30 km, LTDN 10:45 – 11:15 am with a repeat cycle of 20 to 25 days for a single satellite and 10-12.5 days revisit for 2 satellites. The payload of each satellite records at a Spatial Sampling Distance (SSD) of 30m the full spectral range from 400 to 2500nm at a Spectral Sampling interval < 10nm with Low Keystone/Smile. On the front end a high performance TMA with wide-band coated optics collects the light from ground and feeds it to a highly linear almost distortion free spectrometer assembly attaining very good spectral stability. All units are integrated in an optical bench structure that offers excellent AIT access and provides a highly stable LOS. The electro-optical backend contains low-noise cold MCT detectors creating margin in the predicted NEDL performance. The instrument can be calibrated via on-board devices or using reference targets outside the spacecraft. We present the functional decomposition and the physical instrument architecture: the optical design and opto-mechanical layout, the electro-optical imaging chain ant thermal control system.
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 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 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 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 [1], 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 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.
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