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1INTRODUCTIONThe EUMETSAT Polar System – Second Generation (EPS-SG) shall provide global observations from which information on variables of the atmosphere and the ocean and land surfaces can be derived. The observation data shall cover a broad spectral range (from UV to MW), are related to different spatial coverage (global and regional) and are characterised by a variety of different time scales, in order to continue and enhance the services offered by the EPS system. The EPS-SG mission encompasses various observation missions and consists of space and ground-based elements. The Meteorological Operational Satellite – Second Generation (MetOp-SG) is the space segment of the EPS-SG mission. It is composed of two separate satellites, each carrying a different payload instruments complement (Figure 1). These satellites are operating in a low-earth, near-polar, sun-synchronous orbit with a midmorning mean local solar time descending node. They are 3-axis stabilised and Nadir-pointing with a yaw steering mode. METimage is embarked on MetOp-SG satellite A. METimage [2,3] is implemented as passive imaging spectro-radiometer, capable of measuring thermal radiance emitted by the Earth and solar backscattered radiation in 20 spectral bands from 443 nm to 13.345 μm. The instrument achieves global coverage with 500 m square pixels by continuous scanning orthogonal to the flight direction. It employs in-field separation of the spectral channels. Due to the scan motion, the image moves sequentially over the detector channels. By proper timing of the sampling, a certain pixel in the image is measured sequentially by different spectral channels. The definition of the spectral range for the spectral bands is performed by filters in front of the detectors. The instrument is implemented as in-beam scanner with static telescope and synchronous field de-rotation. Calibration is performed during each scan with different calibration sources without interrupting the scientific observation. The observation principle is depicted in Figure 2. At the entrance of the optical instrument a continuously rotating scan mirror is redirecting the light to the telescope, where the light either is coming from the Earth view or from the calibration sources. A de-rotator assembly, which is synchronized with the scanner and rotates at exactly half of the scan speed, follows the telescope and ensures a regular imaging geometry by correcting the image rotation in the focal plane. Two beam splitters split the observational wavelength range into three bands, each supported by a separate detector. The VNIR FPA is located in the telescope’s focal plane. The 6 spectral bands are realised by filters. While the instrument and the visible focal plane operate at ambient temperature, the infrared optics and focal planes operate within a cryostat at 60K. Field masks within the optical paths of the infrared bands ensure proper spatial co-registration between the bands. The relay optics, needed to reduce the spot size at the infrared detectors, are implemented by lens optics. The infrared focal planes (SMWIR and LVWIR bands) are actively cooled by a pulse tube cooler. Details on the optical design of the METimage instrument can be found in [5]. 2DESIGN DESCRIPTION2.1Derotator OverviewThe optical system consists of five flat mirrors (referred to as M1, M2, M3, M4 and M5), which form an optical derotator as originally presented in the US Patent 4,929,040 [1]. The derotator optics is mounted on a rotating mechanism (Figure 3) provided by Airbus Defence & Space (ADS). During operation, the image of the object through the derotator rotates twice with the rotation of the derotator optics. The Derotator mechanism being half-speed synchronized with the scanner compensates the image rotation in the focal plane. The orientation of the five mirrors with respect to each other is optimized to minimize polarization sensitivity. The five mirrors are made of BOOSTEC® SiC material (SiC) and mounted on a baseplate from the same material. The baseplate is bonded to an Invar ring that is mounted to the rotor of the mechanism. The Derotator Optics is shown in Figure 4. Its mass is 2.260kg (calculated) with a maximum dimension of 254mm. It consists of:
2.2BaseplateThe Baseplate is a monolithic piece of SiC (0.8kg) that provides four patterns of mounting holes for the individual mirrors, a circular entrance port on the mechanism side, and a pattern of three holes for interfacing the mechanism rotor on its bottom face (Figure 5). An invar counterweight is added to the baseplate for mass balancing of the derotator around the rotation axis. The counterweight supports removable alignment references like e.g. CMM reference balls. 2.3Mirrors and CoatingDetails of the mirror constructions are shown in Figure 6. M2 and M4 mount to the baseplate walls using a horseshoe shaped ring. Because of room restrictions, M3 rather uses a bracket to mount onto the baseplate bottom. M1 and M5 are arranged back-to-back, with a small prism angle, in a monolithic SiC piece. M1/M5 duplex mirror is mounted on a tilted plane. The optical surfaces are coated with a polishing layer of silicon carbide by Chemical Vapour Deposition (CVD). Per process, the CVD SiC is free of voids and can be optically polished. After polishing, a specific optical coating is applied on the mirror surfaces to improve the reflectivity and limit the polarization sensitivities. A protected silver coating with superior performance over the full wavelength range of METimage has been designed and space qualified for this programme. Each mirror is fastened to the baseplate with three bolts. Except for M1/M5, shims are inserted between the mirror and the baseplate. The shims are adjusted for aligning the mirrors (see below). 2.4Optical PerformanceThe derotator optical performance requirements can be split in 3 categories
LoS and image quality performance requirements are broken down by error source, and by mirror, as shown in next figure. One of the tightest requirements is the LoS/pointing performance, which allows only very small tilt/decentre on each mirror (a few arcsec). Hopefully, the main contributors (manufacturing & AIV) can be corrected by proper alignment between the derotator optics and the mechanism. Exit pupil wobbling can also partially be corrected in the same way. Image quality is mainly specified in terms of roughness and wave front error (WFE). The WFE is specified to 50 nm RMS, which is challenging for a 5-mirror configuration. The major contributor to these errors is obviously the polishing errors. The mechanical design of the mirrors, with integrated flexures, allows to keep the impact of integration errors to an acceptable level, and to reach the requested performance. Transmission and polarization requirements are mainly dependent on coating design and manufacturing, and reflectivity measurement accuracy. The proposed optical coating has been optimised to approach at best the mission requirements. Its end-of-life performance was assessed through a severe qualification programme. 3DEVELOPMENT & VERIFICATION APPROACH3.1Model PhilosophyTable 1.Derotator model philosophy summary.
The STM is based on the flight design but the optical surfaces are not optically finished. i.e. no CVD SiC, no mirror figuring, no mirror polishing, no optical coating, no alignment. The STM is intended for AIT training, interface fit-checks and design qualification. The STM has been submitted to full mechanical & thermal qualifications of the assembly including the glued interface ring. More details can be found in [6]. The EQM reuses the STM hardware with refurbished mirrors polished to roughly 5nm RMS, 2 fringes and coated (without CVD). The Derotator Optics EQM is aligned to flight standard and shall be submitted to thermal and mechanical acceptance (1-axis). At the time of this paper, it has been delivered to the customer. The Derotator Optics flight models (FM1, FM2, FM3) use qualified materials, parts, and processes according to all configuration control and product assurance provisions. The Derotator Optics FM2 & FM3 shall be identical copies of the FM1. The FM1, FM2 and FM3 shall be submitted to the full sequence of acceptance testing prior to delivery. The FS is intended to be a one-to-one replacement unit in case of accidental failure of a flight model. 3.2Assembly, Integration and Verification OverviewA discussion of the AIV activities and GSE was presented in [4] and [6]. The AIV sequence is recalled in the following chart for reference. 3.3Derotator optical ground support equipment (OGSE) descriptionBeside the derotator itself, the derotator OGSE is one of the main equipment manufactured within this activity. This OGSE is able to:
Two version of this OGSE have been developed:
The architecture of the OGSE, including the difference between OGSE#1 and OGSE #2, is shown on Figure 11. Figure 12 depicts the mechanical design of the OGSE. Its overall dimensions are 2.8x1.7x2m. It is composed of:
The derotator is installed inside the tower, with the rotation axis vertical. The OGSE operation is automated to allow fast and precise measurement of the derotator performances. One measurement sequence consists in rotating the derotator step by step over 360 deg while taking pictures of the pattern plate through the derotator at each rotation step. The processing of the acquired images allows to estimate the derotator pointing performance and the alignment to be performed to improve this performance. Alternatively, the OGSE can be set in pupil mode by installing a lens on the camera. The pupil wobbling (which is another specified performance) can then be measured with a similar sequence. 4FIRST FLIGHT MODEL RESULTSThis chapter presents the main results achieved on the first flight model (FM1) of the derotator optics. 4.1Mirrors optical qualityThe FM1 mirrors have been polished at AMOS using different techniques (grinding, polishing, mechanical figuring and ion beam figuring) to achieve the stringent requested optical quality. The achieved surface figure error is below 10 nm RMS on each optical aperture (see Figure 13) and compliant to the requirement. The microroughness of each optical surface is measured by White Light Interferometry on several locations distributed over the optical surface. The achieved roughness is between 0.3 and 0.5 nm RMS on the 5 mirrors (for a requirement of 0.8 nm RMS). These measurements show the excellent optical quality of the manufactured mirrors. 4.2Integration and AlignmentAfter polishing, the derotator mirrors are integrated with the baseplate and aligned. The alignment is done in several steps:
Figure 14 shows the FM1 derotator completely mounted and aligned with 3D CMM. 4.3Derotator fine alignmentThe derotator fine alignment is performed based on measurements made with the OGSE. A measurement of the derotator FM1 before and after fine alignment is shown in Figure 15. The graphs show the evolution of the centroid on the camera plane during a complete rotation of the rotator. Before alignment, the pointing error is around 700μm PtV. From this error, the processing script computes the alignment to be performed at the interface between the derotator optics and the rotation table. Once this correction has been applied (using micrometric screws), the pointing error is dramatically improved, down to 50 μm PtV, which is compliant to the specification. A similar approach is used to improve the alignment using pupil wobbling measurement. 4.4Verification campaignFollowing the successful alignment of the derotator, the system was dismounted and the mirrors were sent to coating. The system was then reassembled in ISO5 clean room and realigned (see Figure 16). The FM1 was then submitted to the following test sequence:
These tests were performed successfully and the derotator optics was delivered to the customer (ADS), where it was integrated and aligned with respect to the derotator mechanism (using the second version of the OGSE). It was then submitted to another successful verification campaign:
The pointing/co-registration and WFE tests are detailed below. 4.5Pointing / co-registration performanceThe derotator was retested with the OGSE after coating and after thermal vacuum tests. The measured pointing performance is similar to the one achieved earlier, compliant to the requirement and these tests are thus successful. The co-registration performance was also tested. The co-registration error is defined as the relative pointing error variation between two points located in the same line in the image plane. This performance is assessed by measuring the differential distortion of several couple of points located on the same line in the OGSE pattern plate while rotating the derotator. The co-registration requirements are severe: for example the average distortion over 1 scan (~60 deg rotation) shall be less than 7.5μm in each direction, while the size of the pixels in the OGSE camera is 6μm. Figure 17 shows the measured co-registration performance. The graphs show the distribution of the co-registration in the image space. The X and Y axis of each graph are the image coordinates (in mm). Each point represents the worst co-registration measured at this position in the image. The co-registration measurement has been performed by scanning the whole derotator angular range (360 deg). The measured mean co-registration over scan is compliant to the requirement. 4.6Wave front error measurementThe derotator Wave front error (WFE) is measured by interferometry. The test setup is presented on Figure 18. The interferometer is equipped with a reference sphere to create a spherical wavefront, which aperture is larger than the derotator aperture. A pupil mask is used to give the expected shape to the laser beam. A reference spherical mirror is located behind the derotator. The WFE of this mirror is calibrated before the measurement of the derotator to subtract its contribution to the derotator WFE. The derotator is positioned so that the interferometer focal point is in its image focal plane. In order to address different field of views, the derotator is tilted using an hexapod, and the interferometer is moved accordingly. The measured WFE of the FM1 derotator in the central field of view is shown in Figure 19. The other field of view show similar performance. This is compliant to the specified requirement (50 nm RMS). The performance is maintained after thermal vacuum tests, which demonstrates the excellent thermal stability of the system. 5CONCLUSIONSThis paper presents the design and test results of the first flight mode (FM1) of a full-SiC five-mirror derotator for the METimage instrument. The SiC design has been preferred over other options because of its compactness, mass, and robustness. The proposed design fulfils the accommodation constraints in terms of mounting interface, mass, envelope, and stiffness. The performed test campaign on FM1 demonstrated the good optical quality of the system (excellent micro-roughness and WFE, compliant to the requirements). The precise alignment of the optics lead to pointing and co-registration performances better than specified. Finally, thermal vacuum and vibration tests confirmed the robustness of the derotator against space environment. This success was achieved thanks to a detailed preparation and anticipation of the different integration, alignment and tests (including a detailed design of a dedicated OGSE), and by capitalizing efficiently on the lessons learned from the early test models (STM and EQM). ACKNOWLEDGEMENTSThe METimage Instrument described herein will be developed by an industrial team led by Airbus Defence and Space GmbH on behalf of the German Space Administration DLR with funds from the German Federal Ministry of Transport and Digital Infrastructure and co-funded by EUMETSAT under DLR Contract No. 50EW1521. The Derotator Optics are developed by AMOS s.a. for Airbus Defence & Space GmbH under R&D contract No. F.45706/G01000-6593. REFERENCEST. Pagano, R. Turtle,
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