ESA Science & Technology - Publication Archive
Beyond their intrinsic interest, ground-based observations have proven their usefulness in supporting spacecraft observations of Solar System bodies. Probably the most spectacular illustration ever was provided during the descent of the Huygens Probe on Titan, when the radio astronomy segment detected the "channel A" carrier signal from Huygens and allowed the recovery of the Doppler Wind Experiment that had been compromised by the failure of the corresponding Cassini channel (Lebreton et al., 2005). Furthermore, ground-based science observations performed during or around the Huygens mission provided new, complementary information on Titan's atmosphere and surface, helping to put the Huygens observations into context (Witasse et al., 2006). Another example of a successful ground-based campaign is the Deep Impact event, when numerous Earth-based and Earth-orbiting observatories monitored comet 9P/Tempel 1 when it was hit by the impactor (Meech et al., 2005).
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The NIRSpec OA (optical assembly) design largely relies on SiC components. The properties of the SiC material and very tight stability budgets required a dedicated development process. Starting from validation of design principles by breadboard testing, this paper describes the development process up to the SM test of the NIRSpec optical assembly. From breadboard testing the design of the mounting interface was established. The test programme also included gluing processes, torque free mounting of mirrors and verification of stability of friction joints. The basic design rules for the mirrors to cope with distortion of mirror surfaces due to bi-metallic bending effects and flatness deficiencies were derived. A modular design using 3 TMAs (Three Mirror Anastigmats) was followed for the OA. From the overall design, budget allocations and design loads for the TMAs were determined. The detailed design process was then driven by distortion budget allocations derived from optical analysis. Due to stringent stability requirements and high mechanical loads, most elements needed several design iterations to meet the budget allocations. Finally, distortions and displacements of the optical elements under the predictable in-orbit conditions were calculated and used in the optical model. The effects can be partially compensated by adjustment. The budget allocation was then revised to account for non-predictable effects only.
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The JWST Mid-Infrared Instrument (MIRI) is designed to meet the JWST science requirements for mid-IR capabilities and includes an Imager MIRIM provided by CEA (France). A double-prism assembly (DPA) allows MIRIM to perform low-resolution spectroscopy. The MIRIM DPA shall meet a number of challenging requirements in terms of optical and mechanical constraints, especially severe optical tolerances, limited envelope and very high vibration loads. The University of Cologne (Germany) and the Centre Spatial de Liege (Belgium) are responsible for design, manufacturing, integration, and testing of the prism assembly. A companion paper (Fischer et al. 2008) is presenting the science drivers and mechanical design of the DPA, while this paper is focusing on optical manufacturing and overall verification processes. The first part of this paper describes the manufacturing of Zinc-sulphide and Germanium prisms and techniques to ensure an accurate positioning of the prisms in their holder. (1) The delicate manufacturing of Ge and ZnS materials and (2) the severe specifications on the bearing and optical surfaces flatness and the tolerance on the prism optical angles make this process innovating. The specifications verification is carried out using mechanical and optical measurements; the implemented techniques are described in this paper. The second part concerns the qualification program of the double-prism assembly, including the prisms, the holder and the prisms anti-reflective coatings qualification. Both predictions and actual test results are shown.
The Mid-Infrared Instrument (MIRI) is a 5 to 28 micron imager and spectrometer that is slated to fly aboard the JWST in 2013. Each of the flight arrays is a 1024x1024 pixel Si:As impurity band conductor detector array, developed by RaytheonVision Systems. JPL, in conjunction with the MIRI science team, has selected the three flight arrays along with their spares. We briefly summarize the development of these devices, then describe the measured performance of the flight arrays along with supplemental data from sister flight-like parts.