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Acronyms/AbbreviationsCTE: Coefficient of Thermal Expansion CVD: Chemical vapour deposition FFI: Forsvarets Forskningsinstitutt GSD: Ground Sampling Distance IBF: Ion beam figuring MERLIN: Methane Remote Sensing LIDAR Mission MMLD: Metalic Multi Layer Dielectric coating MTF: Modulation Transfer Function RMS: Root Mean Square. RX: MERLIN receiving channel SEEING: Small satEllite instrument for Earth ImagiNG SiC: Silicon Carbide SNR: Signal to Noice Ratio TX: MERLIN transmitting channel 1.IntroductionDuring the recent years, the introduction of free from mirrors and lenses in optical design has demonstrated their benefit, allowing achieving more compact system, use less optical components as well as better image quality. The optical manufacturing technology has always been constrained since its origin by the limitation that only spherical or flat surfaces could be produced with the sub-micron surface quality required for vision optical imagery. This is due to the fact optical surfaces were traditionally produced by a full aperture mutual rubbing process between the part and the tool with abrasive grains or polishing slurry inserted in between. The sphere (the flat being a particular sphere with infinite radius) is the only mathematical surface enabling such full aperture intimate contact during the relative stroke motions. Thus conventional polishing converges naturally, and only, towards spherical surfaces. This is valid during loose abrasive grinding in the first steps of optical fabrication or during pitch polishing in the final steps. Master opticians could finely tune this art-like process and produced many high quality optics in this way. On the other side, due to its inherent simplicity, the process could be well industrialized for mass production of consumer optics. The introduction of rotationally symmetric aspheric surfaces like the Schmidt plate for the well-known Schmidt telescope constituted a first improvement step. The use of such surfaces will be illustrated in section 4, with example of the NIORD imager. The second improvement step in optical instrumentation appears with the use of off-axis segments of a rotationally symmetric asphere. Tilts and decenters offer additional degrees of freedom very useful to gain in performance, especially for rectangular image formats. The use of such surfaces will be illustrated in section 2, with example of the MERLIN primary mirror of the receiving channel. However, in such off-axis optics, there is still an inherent constraint remaining with the intrinsic link between coma and astigmatic profiles originating out from the parent optics aspheric profile. What is surprizing is that, at the same time, the optical manufacturing technology evolved quickly with robotics machines that was completely unconstrained. But the designers were still reluctant to use really freeform optics with total liberty in term of mathematical definition, within their instrument design. Today, this barrier has been definitively broken and freeform optical surfaces are now more and more used in advanced optical instrumentation. The sag of the surface is now a fully free function of the distance ρ from the centre and the azimuth angle θ. This is the third evolution step in non spherical optics (See Fig. 1). Freeform surfaces offer more degrees of freedom to the designer and its optimization software and obtain:
The use of these free form optics will be illustrated by the optics manufactured during the recent years by Safran Reosc for the MicroCarb instrument (see section 3.) 2.MERLIN2.1MERLIN MissionThe joint French-German cooperation Methane Remote Sensing LIDAR Mission (MERLIN) employs an Integrated Path Differential Absorption to measure the spatial and temporal gradients of atmospheric CH4 columns [1], [2] on a global scale The satellite is being developed and operated by both countries in a joint partnership between the French Space Agency CNES and the German Space Administration DLR. A general overview on the MERLIN mission and a detailed description of the overall instrument architecture is given in [3] and [4] respectively. The instrument measures the laser signal absorption by atmospheric methane at two wavelengths around 1,645 nm, reflected either by the Earth surface or by cloud tops. Safran Reosc has been selected by Airbus Defence and Space to design and manufacture the optical components of the both the receiving channel (RX optics) and the transmitting channel (TX optics) and this paper will focus on the primary mirror of the receiving channel (M1 RX) 2.2Optical designThe receiver telescope RX is an afocal design with a magnification ratio of 50x. It consists of two conical mirrors and an achromatized ocular lens, which generates an image of the entrance pupil. A major design driver has been the need for a very compact envelope allowing a maximal M1-M2 mirror distance of 470mm (see Fig. 2). 2.3M1RX descriptionThe need for a very short distance between the primary and the secondary mirrors results in the need of a very fast primary mirror. This will lead to a very challenging and complex mirror to manufacture due to the aspherical profile that departs from the best fit sphere by several millimetres.. The main parameters and requirements of the mirror are summarized below:
2.4M1RX manufacturingIn order to achieve very stable optical performances over an extended thermal range, Zerodur material (which has a very low CTE has been selected) The mirror mass requirement requires the lightweighting of the mirror by drilling the glass on a computer controlled machine in order to generate the triangular pattern of the mirror back surface. After the lightweithed process is completed, the mirror is stressed relieved by acid etching to remove all the micro cracks left by the previous manufacturing steps. After lightweighting, only 15% of the initial mass remains (see Fig 3). At this stage, the mirror has an outer diameter extension that eases the polishing process and avoids the creation of a possible edge effect that could affect the mirror surface quality. This outer extension is removed by machining before the final polishing steps of the mirror. The polishing of the mirror is challenging do to off-axis aspherical profile of the mirror. The off axis distance and the short radius of curvature of the mirrors leads to an aspherical departure with respect to the best fit sphere of 7,5mm and a maximum local slope of the aspherical shape with respect to the best fit sphere of 50mrad. Because the mirror is off axis, the aspherical shape has a non-symmetrical profile (see Fig. 4) that makes this a truly free form mirror. The grinding and the computer controlled polishing process have been used to smooth the surface and improve the surface accuracy to better than a micron with respect to the specification. The mirror fixations in invar are then bonded to the mirror back surface on dedicated interfaces located at three interfaces location at 120° apart from each other. After bonding of the mirror fixation and removal of the polishing extension, a final surface figure correction is performed by IBF process, a highly deterministic process, used to achieve the final polishing requirement of 40nm RMS. After polishing, the assembled mirror is then mechanically (see Fig. 5) and thermally tested in order to demonstrate that the mirror will be stable after launch and during the thermal conditions the mirror will see during the operations. The next step consist in the mirror integration on the composite flight structure provided by Airbus Defence and Space to Safran Reosc and final verification of the optical surface quality in flight mounted conditions. After integration and final testing, the optical surface is coated with a high reflectivity MMLD coating (see Fig. 6), that has been specifically developed and qualified for this application. The values of reflectivity achieved are the following: 3.MICROCARB3.1MicroCarb MissionThe objective of MicroCarb is to create a global map of the sources and sinks of the main greenhouse gas, carbon dioxide. The mission aims to determine how the main carbon sinks of our planet –the oceans and forests –function, and to map them. At the same time, MicroCarb will measure how many tonnes of CO2are emitted by all sources (especially human activities and vegetation) region by region. CNES is the prime contractor for the system and the satellite, which are based on the Myriade platform. The payload consists of a passive spectrometer in the near infrared developed by Airbus Defence and Space. 3.2Optical conceptThe concept is a purely reflective (all mirror design) optical system made of a three mirror telescope, a three mirror spectrometer, a scanning mirror at the entrance of the telescope and several folding mirrors (see Fig. 7). The use of free form mirror allows fitting into a very low volume and to reduce the optical distortion (especially the smile effect), while achieving at the same time a very good image quality. The major challenge is the polishing of these free form mirrors (see section 3.3) 3.3MicroCarb mirrorsThe free form mirrors are specified by Zernike polynomials (up to polynomial 36) and the aspherical departure from the best fist sphere ranges from 100μm to 2000μm (including a free form contribution up to 1000μm) and a maximum aspherical slope departure of 30mrad. The optical surface requirement of these mirrors is 10nm RMS and a micro-roughness better than 1nm RMS. The substrate of the mirrors as well as the assembly structure is made of Silicon carbide provided by Boostec. In order to allow the polishing of the SiC substrates a dedicated thin layer is deposited on the mirror surface. This layer, made of R-SiC, developed by Safran Reosc, has several advantages compared to standard polishing layers (for instance CVD SiC): it is softer than CVD SiC and so easier and faster to polish and it can be removed without the need of grinding again the substrate. After successful qualification of this new polishing layer, the MicroCard are the first flight mirrors to benefit from this new process. The mirrors have been polished using computer controlled nano-station robots (see Fig. 8) developed by Safran Reosc for addressing relatively small sizes free form mirrors such as those of MicroCarb (typically smaller than 300mm). This type of polishing equipment is also designed to address the case of mirrors having a non-conventional outer contour (see Fig. 9), which is often a complex issue if edge effects are to be avoided. The small polishing tools used by the nano-station robots allows to address the specific problems linked to these unconventional outer shaped The final figure correction have been performed by IBF and the surface error achieved on the mirror are similar to those that are traditionally obtained on aspheric mirrors (as shown in Table 1). Table 1.Mirror final performances
3.4ConclusionsIn the frame of the MicroCarb project Safran Reosc has introduced two major innovations in space optical instruments. Highly aspherical free form mirrors have been polished with optical quality ranging from 10 to 20nm RMS, paving the way for the more extended use of such optics in future instrument The use of the R-SIC layer on silicon carbide substrates has been successfully used on flight mirrors. 4.NIORD4.1NIORD InstrumentThe NIORD imager will be used by the Norwegian government on the Nor-Sat4 satellite dedicated to surveillance of the seas and costal traffic. Compared to the previous generation of instruments, such as AISSat-1 & AISSat-2 launched in 2010and 2014 respectively, as well as the previous suite of Nor-Sat satellites, the imager will provide extended low light imaging capabilities. FFI has selected the SEEING 130 imager developed by Safran Reosc for the Nor-Sat4 project. The imager will be integrated on a satellite platform developed by the University of Toronto (UTIAS) 4.2Imager Specification & DesignThe imager will provide a GSD of 8-m at an orbit altitude of 500km. The main optical requirement is to achieve an in orbit MTF better than 0,1 over the entire field of view of 60x40 km with SNR larger than 400. Moreover a compact design is required to fit into a small volume of 200x200x200 mm3 (without the outer baffle) requiring several highly aspherical surfaces in the optical design. The total mass of the imager is less than 8kg and the required power 25W. The optical concept is based on a catadioptric system combing reflective and refractive surfaces. This allows achieving diffraction limited performances over the entire field of view. The optical components are assembled in individual barrel using a space qualified bonding process and the assembled lenses ate then fitted into a metallic structure that interfaces to the platform via three bipods (see Fig. 10) 4.2Imager performances and next stepsAfter manufacturing and coating of the optical components, the imager has been fully integrated (see Fig. 11) and the wavefront error and the MTF have been measured. The measured MTF is in line with the optical simulation including the specified tolerances as well as the as built parameters of the optics (see Fig. 12) The following step was the integration and the alignment of the focal plane assembly (see Fig. 13) The next steps will consists in the qualification of the imager through a sequence of mechanical tests (vibrations) and thermal test (thermal cycling). The fully qualified imager is planned to be deliver beginning of 2023. 5.ConclusionsIn this paper, we have illustrated how the use of highly aspheric or the newly introduced free form mirrors allow achieving more compact and more powerful optical system. The development of improved polishing and testing techniques have allowed producing such complex surfaces with the same accuracy as the accuracy currently achieved for more conventional optical surfaces. It is expected that future optical systems, especially those intended for space instruments for which volume and mass are amongst the key requirements will use more commonly free form surfaces. ReferencesC. Stephan et al.,
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