The Challenge Spot-5 lifetime expires in 2012. The next French satellite, Pleiades, is solely dedicated to HiRes. The Belgian Federal Science Policy Office (BELSPO) declared their interest to develop an instrument to ensure continuity of Végétation products provided that it could be flown on a Proba type satellite, a small satellite developed by the Belgian Verhaert Space. SPOT Vegetation Proba Vegetation miniaturization The SPOT Vegetation Camera (1000x1000x700 mm 3 150kg) must be miniaturized to fit the Proba platform (800 x 600 x 200 mm 3 30 kg) This requires : Reduction in mass Reduction in volume Proposed solution: Miniaturization by switching from refractive to reflective optics by using a Three Mirror Anastigmat (TMA) Technology assessment: Criticalities: M1 & M3 beyond Manufacturability M3 cannot be tested Alignment is critical Challenges to be demonstrated: Design meeting performance needs Manufacturability of mirrors Testability of mirrors Alignment of the TMA Meeting the required performance
The Requirements Optical system Focal Length F/# Along track FOV Across track FOV Wavelength range 2s MTF Distortion Telecentric Lightweighted Compact Three Mirror Anastigmatic Telescope 110mm 6 5.5 34.6 from 400 to 1700nm > 55% < 2% in image plane < 1.5 kg (optomechanics only) < 260 x 254 x 150 mm 3 The Results: Mirror Manufacturing The TMA is a full aluminium instrument, 6061T6 special alloy has been chosen for its ultra fine and homogeneous microstructures. Mirror M2 After milling and specific stabilisation thermal process, the optical surfaces of the TMA mirrors were machined by Single Point Diamond Turning (SPDT). The resulting surface roughness ( < 3nm RMS) is compliant with the instrument applications in visible, without need of post-polishing. The mirror surface error has been measured by null test with dedicated Computer Generated Holograms (CGH). A surface accuracy better than 35 nm RMS was achieved on the largest mirrors. The structure offers interfaces whose severe position accuracy were made possible by diamond machining on an ultra precise indexing table. Mirror M3 Interferometric measurement of Mirror M3
The Results: Alignment & Performance The mirrors are mounted from the external face of the structure, in such a way that the interfaces are oriented in the same direction as the optical surface. Interfaces were machined by Single Point Diamond Turning along with the optical surface. The relative precision on position is about 1 µm with respect to the mirror vertex. Mirror assembly These manufacturing tolerances ensure the good positioning of the mirrors by mechanical assembly. The inplane position and orientation of the mirrors are fixed by pins inserted in the structure and can be adjusted by shims for fine alignment. Final alignment of the TMA has been completed in less than one week. The assembled TMA TMA under test For the four spectral channels the Modulation Transfer Function of the TMA is higher than the specified 0.55 at Nyquist frequency, and the wavefront error is less than 100 nm RMS at focal plane for VNIR channels. The focal length is within 0.5% of the nominal value and the distortion is less than 2% at the edge field of view. Interferometric testing of of the TMA
Proba-V: The first Mission of the TMA Telescope A remote sensing mission for global vegetation monitoring Mission: Circular Sun-Synchronous orbit, 820km altitude Data and service continuity of Vegetation data Spectral and radiometric performance identical to SPOT-5 Daily global coverage for latitudes above 35 Mission lifetime 5 years: 2012-2017 Four wavelength bands: Blue, Red, Near-IR, SWIR Instrument consisting of 3 spectral imagers, each viewing 34 ; total swath width: 102 (2250km @ 820km flight height) Ground sampling distance Blue, Red, Near-IR: 100m centre 360m edge Ground sampling distance Short-Wave IR: 200m centre 685m edge Absolute geolocation accuracy < 300m Interband Geolocation Accuracy < 100m MTF at Nyquist (38lp/mm) > 55% over complete FOV and all wavelengths Mission prime Instrument Prime Principle Investigator
The Partners Prime Contractor; TMA Optical and Mechanical Design Mirror and Telescope Manufacturing, Assembly, Alignment and Verification TMA Stray Light Analysis Requirement Definition and Analysis OIP Sensor Systems Westerring 21 B-9700 Oudenaarde Belgium Phone: +32 (0) 55 333 811 Fax: +32 (0) 55 333 802 E-mail: sales@oip.be Website: www.oip.be