The filter wheel (FW) assembly (FWA), developed by the CBK Institute, is one of the critical subsystems of the wide field imager (WFI) instrument on board the Advanced Telescope for High Energy Astrophysics—mission of the ESA Cosmic Vision 2015-25 space science program (launch scheduled around 2035). The instrument has to collect soft x-rays with very high quantum efficiency, thus WFI requires extremely thin optical blocking filter (OBF). Due to its thickness (∼150 nm) and large area (∼170 mm × 170 mm) needed to achieve a 40 ′ × 40 ′ instrument field of view, the filter is extremely vulnerable to acoustic loads generated during Ariane 6 rocket launch. On the other side, FW mechanism has to provide high overall reliability, so it is more favourable to launch the instrument in atmospheric pressure (without vacuum enclosure for filter protection). Design efforts of the FW subsystem were focused on two issues: providing maximal possible sound pressure level suppression and smallest possible differential pressure across the OBF, which should prevent filters from damaging. We describe the design of a reconfigurable acoustic-demonstrator model (DM) of WFI FWA created for purposes of acoustic testing. Also, the acoustic test campaign is described: test methodology, test criteria, and results discussion and its implication on future FWA design. In general, tests conducted with the FWA DM showed that current design of WFI is feasible and the project can be continued without introducing a vacuum enclosure, which would significantly increase system complexity and mass.
KEYWORDS: Structural design, Space operations, Imaging systems, High energy astrophysics, Galaxy groups and clusters, X-ray imaging, Imaging spectroscopy, Spectroscopes, Cameras, Interfaces
The WFI instrument is designed for high-resolution X-ray imaging and spectroscopy with a large field of view. A movable mirror focuses the X-rays onto the focal plane detectors of the WFI. For design development of the instrument, structural analysis is performed from early project phases. The WFI project has entered phase B and the preliminary design is evolving towards a more detailed design including interface definitions. The focal plane of WFI consists of two detectors: Large Detector Array (LDA) and Fast Detector (FD). The LDA assembly is used for main observation modes, whereas FD is for very bright point sources. The complexity of the LDA design is due to the fact that it has to be compliant to contradictory requirements. Its sensors have to be thermally decoupled from the structure, but with a high stiffness to avoid relative displacement and deformation of sensitive components, e.g. bond wires. The LDA has an active area that is as large as ≈14x14 cm2 but the thermal gradient across it shall be below 2.5-3K. The LDA is optimized with respect to mechanical requirements for launch loads and operational conditions in space. In order to minimize mass while withstanding thermo-mechanical, static and dynamic loads, various design studies have been carried out. With the aim to develop a design, which meets all structural and functional requirements, various structural analyses are performed. In this paper, the structural design studies and the preliminary analysis results of the WFI Large Detector Array are presented.
The Wide Field Imager (WFI) instrument for ESA’s next large X-ray mission Athena is designed for imaging and spectroscopy over a large field of view, and high count rate observations up to and beyond 1 Crab source intensity. The other focal plane instrument, the cryogenic X-IFU camera, is designed for high-spectral resolution imaging. Both cameras share alternately a mirror system based on silicon pore optics with a focal length of 12 m and unprecedented large effective area of about 1.4 m2 at 1keV. The WFI instrument employs DEPFET active pixel sensors, which are fully depleted, back-illuminated silicon devices of 450 μm thickness. The detectors provide high quantum efficiency and state-of-the art energy resolution in the 0.2 keV to 15 keV energy range with extremely fast readout speeds compared to previous generations of Si detectors for X-ray astronomy. The focal plane comprises a Large Detector Array (LDA) and a Fast Detector (FD). The LDA comprises about 1 million pixels and a time resolution in full frame mode of 5 ms. The FD optimized for bright point source observations permits a time resolution of even 80 μs with about 4000 pixels. Both detectors have a pixel size of 130 μm × 130 μm, providing oversampling of the PSF by a factor >2. The instrument development is in phase B of the project after a successful Preliminary Requirements Review and endorsement of the both instrument consortia by ESA. Critical technology developments for the WFI focal plane camera are currently investigated and finally experimentally verified with breadboard models: the detector function and performance, the real-time capability of onboard event pre-processing and the integrity of the large-area and ultra-thin optical blocking filter after environmental tests. Flight-size sensors have been produced and a flight-like detector assembly has been developed. First test results of a flight-size detector are expected in near future. Based on the thermal design and model for the camera head (CH), thermal interface requirements have been estimated.
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