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I.INTRODUCTIONToday, optical instruments with FOV of ±30 degrees or more can be achieved by combining several optical modules thus increasing the complexity at instrument level in the design, integration, validation and calibration phases. For instance the MERIS instrument on the ENVIronment SATellite (ENVISAT) has a 68.5 degree FOV shared between 5 identical modules. Each one of those modules needs to be calibrated independently and the modules require to be co-registrated, which leads to stringent constraints at system level in terms of stability. Reducing the number of modules to achieve large FOVs will have potentially beneficial impact on the mass and volume of the instrument and will ease the calibration and co-registration process. Benefiting from a longstanding experience in Earth Observation missions, and especially in the field of dispersive spectrometers with Ocean and Land Colour Imager instruments (MERIS and OLCI), Thales Alenia Space has studied for ESA different concepts of advanced large FOV UV/VIS/NIR/SWIR spectrometers able to be implemented in the frame of Sentinel-3 Second Generation and GMES II programs. The targeted application is an ocean/land colour measurement instrument from low Earth orbit, with at least the same FoV (70°) and spectral range (340-1040 nm) than OLCI, but a Ground Sampling Distance nearly two times better (150 m), and a reduced volume. The main objectives of the study were to assess the limits of the currently available spectrometer architectures and to identify the technologies needed to achieve the requirements, then to propose optomechanical designs for a spectrometer based on these technologies, and at last to define an instrument development roadmap including the technology development roadmap. To tackle these needs, the following tasks were performed by TAS and consultants:
The present paper focuses mainly on the optical design activities. The sizing of the instrument was optimized to fulfill all the requirements. A major constraint being to stay within a small volume, the collimator and imager functions of the spectrometer were proposed to be achieved with a single optics used in double-pass. The significant heritage in design and realization of spectrometers at Thales Alenia Space allowed a comprehensive approach of potential solutions. Several types of spectrometers were assessed, among which the Offner concept, the Dyson concept, and the double-pass TMA II.OVERVIEW OF THE REQUIREMENTSTwo sciences cases are identified for advanced LFOV missions. The first case foresees measurements of ocean color in the UV, visible and infrared with a GSD of 150m with a volume reduction compared to past and spectrometers in development. The second case includes the first case but with addition of a SWIR channel for atmospheric correction. To fulfill the sciences cases, the proposed spectrometer should be compliant with the following main characteristics:
III.INSTRUMENT CANDIDATE CONCEPTS AND SELECTIONA.Families of conceptDuring the study several families of concepts have been identified, using the following rules:
Chosen family of concepts is based on a different separation between the UV/VIS/NIR channels and the SWIR. The UV/VIS/NIR channels are separated with the help of a spectrometer whereas the SWIR channel is directly imaged on a second adapted focal plane. This separation was found to be the only way to keep the SWIR option in the race. The number of modules in this family of concepts is one or two. B.Rationale for concepts selectionAt the end of the first phase of the Study, the Concepts Review led to the selection of two instrumental concepts to be investigated in more details during the second phase. The rationale of the method proposed for assessing the best candidates was as follows:
C.Possible optical conceptsThe instrument can be of two kinds: The constraint on instrument volume strongly favors the double-pass concept. D.Design characteristics and constraintsDesign drivers are as follows:
Also, in double-pass design, the dual-pass optic is used as collimator and imager, so that the collimator focal length is equal to the imager focal length. Furthermore, the telescope focal length is equal to the instrument focal length. At last, the design of the instrument is constrained by the position of the different pupils (cf Fig. 2Erreur ! Source du renvoi introuvable.). Indeed, at slit level, the telescope Exit Pupil shall be equal to the collimator Entrance Pupil, and the collimator Exit Pupil and imager Entrance Pupil shall be real and shall be on the disperser. These are strong constraints on the optical design. E.Telescope designMany telescopes designs were studied (reflectives solutions : TMA, Schwarzschild, Walrus,… but also refractive designs). For each concept, a set of performance and characteristics was assessed :
Among them, few telescopes were able to meet simultaneously all the instrument needs (compact / few modules / accessible entrance pupil, and compatibility with spectrometer entrance pupil). None of them was based on a single module (70° FoV). In the following table the telescope designs are divided in 4 groups depending on their assets. In each group the best candidate is highlighted. F.Spectrometer designAs explained above, priority was given to double-pass spectrometers in order to reduce the overall instrument volume. We have thus studied the telescopes listed above in dual-pass configuration. In order to reduce volume, the grating was put in pupil plane. Optics with no real exit pupil have thus not been studied because they led to large volumes (ex: tma4, tma5,…). Some telescopes were too compact and could not be used as double-pass spectrometers (ex: ma4_2, tma10, walrus, 5ma) because of lack of space to accommodate them. Beside these double-pass spectrometers, we also studied Dyson/Offner spectrometers, and two examples of classical spectrometers (collimator+imager). For each of the spectrometers the following characteristics and performances were assessed :
Among them, a few was able to meet simultaneously all the instrument needs (compact / few modules / accessible Entrance Pupil, and the constraint with telescope exit pupil) and none was based on a single module. In the following table the spectrometer designs are divided in 5 groups depending on their assets. In each group the best candidate is highlighted. G.Outcomes of the concepts review: solutions at instrument levelThe combination of telescopes and spectrometers led to 20 possible instrument concepts. From the telescopes and spectrometers listed above, the following best candidates were discussed at the Concepts Review: The following two concepts were selected for further design in the second part of the Study, emphasizing compactness versus the preference of having a single-module instrument: MA4_4 (four mirrors astigmatic) – TMA16DB : 2 modules, flat grating, compact. Later on, the telescope has been replaced by a TMA. FlatSchwarz6 (Schwarzchild) – Dyson5: 2 modules, concave grating, very compact. IV.INSTRUMENT OPTO-MECHANICAL DESIGNThe outcome from the concept review allowed to identify two spectrometers concepts that have been further assessed in the second part of the study. On these two concepts, requirement apportionments to sub-systems and performance estimations have been made. Mechanical design and accommodation have been realized as well. A.Concept 1In the first concept, represented in Fig. 8, the scrambling window is placed in the entrance pupil of the instrument, and a folding mirror enables to accommodate the telescope and the double-pass TMA in a small volume. The telescope images the Earth on the slit, and the double-pass TMA images the slit to infinity. The grating disperses light, and the double-pass TMA images this dispersed light on the detector. The instrument has a focal of 81.4mm, f# of 3.3 and FOV of 70° (35° for each module). B.Concept 2In the second concept, represented in Fig. 9, the scrambling window is placed in front of the telescope, but not in a pupil. The telescope images the Earth on the slit, and the Dyson spectrometer disperses and images the slit on the detector. The instrument has a focal length of Focal=81.4mm, a f# of 3.3 and a FOV of 70° (35° for each module). C.Estimated performance of both conceptsFor the two concepts, requirements apportionment toward sub-systems was made. Mirrors reflectivity and grating efficiency are the main drivers for the optical components. The grating efficiency is particularly stringent for Concept 1, whereas it is relaxed for the Concept 2 and is marginally compliant with classical gratings (35% in the UV instead of the 50% required. Gratings based on new technologies could achieve the required level of performance. Grating solutions able to meet the requirement were studied extensively by Thales Research and Technology in the frame of this Study. Candidate gratings are based on sub-wavelength binary structures. The nano-structured patterns are made of SiO2 or Si3N4 and the effective index depends on the fill factor (i.e. the width-to-period ratio). These type of gratings have already been developed but not for a design in reflection and with a curved surface. Some solutions in silica have been tested in transmission on small samples and evaluated numerically for a transitive design. They present high efficiency (up to 60-80%), a flexibility to adapt the shape of the spectral efficiency profile and a good polarization behavior. However specific development are clearly needed, in particular regarding the expected size and shape of the component to be realized. The performance of both instrument concepts have been estimated, and main outcomes are listed in Fig. 10. Concept 1 main non-compliance concerns spectral co-registration. Such concept would therefore need to implement smile and keystone correction by processing. Further optimization of these performance at optical level was not further investigated. Concept 2 meets all the requirements. In order to be fully aware of gains obtained with these new designs, we have tuned Concept 2 so that it be compliant to the OLCI requirements only (i.e. twice relaxed versus the LFOV requirements). Thanks to that we could estimate the gain on volume for two equivalent designs. The volume of one optical module compliant to the LFOV requirements is 5.3L. The volume of one optical module compliant to the OLCI requirements only is 1.7L. Fig. 11 provides with the same scale the volume of the Concept 2 OLCI-tuned (2 modules), compared to the OLCI concept (5 modules). V.CONCLUSIONThe Advanced Large Field Of View Spectrometers study identified concepts for future EO missions. The requirement review pointed out the main stringent, but achievable, requirements for such mission : large spectral range with extension toward UV and, as an option, toward SWIR, small volume, small GSD (150m) and high optical transmission (>30%). Families of concepts have then been identified: single module concepts, with only one telescope covering the whole FoV, two modules concepts, with the FoV split in two, and also alternatives with one or two modules in which the separation between the UV/VIR/NIR and the SWIR occurs right after the telescope before the spectrometer. This last family enables a SWIR option in multispectral mode. More than 20 instrument concepts have been studied. Among them, two concepts have been pushed forward: the first one based on a double pass spectrometer and the second one based on a Dyson spectrometer. Both concepts use a plane reflective grating. With better image quality and a volume equivalent to 75L, the concept based on a Dyson spectrometer proved to be the most promising one, fulfilling all the requirements with an instrument having two modules. Two technological areas have been identified as critical and needing dedicated developments to achieved the required level of performance: anti-reflective coatings in the UV band and gratings efficiency on a large spectral band. Preliminary performance have been assessed, involving coatings based on dielectric multilayers and gratings with two types of promising technologies (nanostructured gratings and achromatic blazed gratings). The need to secure performance by dedicated instrument breadboarding activities has been pointed out. The technological roadmap and the instrument roadmap have been elaborated. |