The Camera Head of the Wide Field Imager (WFI), which is to fly on the X-ray telescope NewAthena, is a compact sub-system requiring the integration of three separate temperature regions. One will be the environment temperature of the instruments support structure. The other two temperature environments are governed by the requirements of the detector front-end electronics and the sensor itself. Due to the limited space available and high heat fluxes that the system needs to remove at temperatures as low as 160 K, graphene thermal straps are used. Although already in use in some space applications, space qualified graphene thermal straps are not available on the market, therefore a qualification program, specific for the environmental conditions and requirements of the WFI system, has been carried out. The thermal straps are tested for their thermal performance, structural stability and particle shedding, thermally cycled and outgassed.
The Wide Field Imager (WFI) is one of two focal plane instruments for NewAthena, ESA’s next large X-ray observatory, planned for launch in 2037. It will study the growth mechanisms of supermassive black holes, the origin of their violent outflows and their impact on the evolution of their hosting galaxies, and the processes that lead to the assembly of luminous matter into the cosmic web [1]. WFI is a Si-based detector using DEPFET Active Pixel Sensor (APS) technology designed for sensitive wide-field imaging, spectroscopy, and high-count capability. The instrument's focal plane consists of a Large Detector Array (LDA) composed of four sensors with 512x512 pixels each and a smaller Fast Detector (FD) with 64x64 pixels, optimised for high-count rate applications.
The WFI instrument had its System Requirements Review in 2022, after which the Athena mission underwent a design-to-cost exercise to reduce the estimated Cost-at-Completion to restart the mission with a Delta Phase A-B from 2024. In the meantime, the WFI design has continued its development, including the verification of the most critical aspects to improve the maturity of the technologies used. This paper presents WFI's preliminary design based on its structural, thermal, and functional requirements. It includes the results of the numerical and experimental activities aiming at design verification and demonstration of technology readiness.
The Wide Field Imager (WFI) is one of two instruments on the ATHENA (Advanced Telescope for High-ENergy Astrophysics) X-ray space telescope. The WFI features a camera head comprising a large detector array with a readout time of less than 5 ms and a fast detector with a readout time of 80 μs. The large detector array consists of four quadrants and each quadrant houses a sensor with a matrix of 512 × 512 pixels of Depleted P-channel Field Effect Transistors (DEPFET) with a pixel size of 130 μm. The fast detector comprises a sensor with 64×64 pixels, using the same technology and pixel size as the large detector. Both sensors are controlled and read out by the frontend electronics (FEE), which consists of two types of application specific integrated circuits (ASIC) mounted close to the DEPFET sensors.
The DEPFET sensors operate at temperatures between 193 K and 213 K, dissipating less than 1 W per LD. In contrast, the frontend electronics operate at a higher temperature, requiring approximately 10 W per large detector. The proximity of the ASICs and the DEPFET sensor for electrical performance necessitates a complex thermal design, decoupling the cooling chains for the DEPFET and the FEE.
The Wide Field Imager (WFI) is one of two focal plane instruments of the Advanced Telescope for High-Energy Astrophysics (Athena), ESA’s next large x-ray observatory, planned for launch in the early 2030s. The current baseline halo orbit is around L2, and the second Lagrangian point of the Sun-Earth system L1 is under consideration. For both potential halo orbits, the radiation environment, solar and cosmic protons, electrons, and He-ions will affect the performance of the instruments. A further critical contribution to the instrument background arises from the unfocused cosmic hard x-ray background. It is important to understand and estimate the expected instrumental background and to investigate measures, such as design modifications or analysis methods, which could improve the expected background level to achieve the challenging scientific requirement (<5 × 10 − 3 counts / cm2 / keV / s at 2 to 7 keV). Previous WFI background simulations done in Geant4 have been improved by taking into account new information about the proton flux at L2. In addition, the simulation model of the WFI instrument and its surroundings employed in Geant4 simulations has been refined to follow the technological development of the WFI camera.
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) is one of two focal plane instruments of the Advanced Telescope for High-Energy Astrophysics (Athena), ESA’s next large X-ray observatory, planned for launch in the early 2030’s. The current baseline halo orbit is around L2, the first Lagrangian point of the Sun-Earth system, L1 is under consideration. For both potential halo orbits the radiation environment, solar and cosmic protons, electrons and He-ions will affect the performance of the instruments. A further critical contribution to the instrument background arises from the unfocused cosmic hard X-ray background. It is important to understand and estimate the expected instrumental background and to investigate measures, like design modifications or analysis methods, which could improve the expected background level in order to achieve the challenging scientific requirement (< 5 × 10−3 cts/cm2/keV/s at 2 - 7 keV). Previous WFI background simulations1 done in Geant4 have been improved by taking into account new information about the proton flux at L2. In addition, the simulation model of the WFI instrument and its surroundings employed in GEANT4 simulations has been refined to follow the technological development of the WFI camera.
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