Investigation of a focal plane array for a spaceborne wide-field Schmidt telescope

The Ultraviolet Transient Astronomy Satellite (ULTRASAT) is a scientific UV space mission that will operate in geostationary orbit (GEO) in 2028. Its Schmidt telescope provides an unprecedented field of view (FoV) of 204 deg2, captured by a focal plane array (FPA) with a total sensitive area of ≈ 90mm x 90mm, comprising four backside-illuminated (BSI) complementary metal oxide-semiconductor (CMOS) image sensors. This research focuses on the development of the FPA, concentrating on the associated mechanical and thermal design challenges.Due to the Schmidt telescope design, the detector is located within the optical path, limiting the overall size of the FPA. The ULTRASAT telescope requires accurate positioning of the sensors within ±50 μm along the optical axis, with a flatness tolerance of ±10 μm. During operations, the telescope will be at an approximate temperature of 295K. However, the sensors must be operated at 200K for dark current reduction, with a thermal gradient stability of less than 1K for at least 300 s during imaging. In addition, the design should support heating the sensors to 343K for decontamination purposes.The design discussed in this thesis addresses these stringent requirements. The four CMOS sensors are packaged into an innovative silicon-aluminium alloy and mounted on a carrier plate to form a 2x2 mosaic assembly. Within the mosaic integration, each sensor can be individually adjusted for focus, ensuring the required coplanarity. The mosaic assembly is thermally isolated from the rest of the FPA thanks to highly insulating plastic flexures, enabling cooling of the sensors down to 200K by means of a thermal link consisting of copper wire thermal straps and propylene heat pipes, which transport excessive heat towards a cold interface provided by the spacecraft. The spacecraft will precisely regulate the sensors’ temperature using a combination of heaters and temperature sensors embedded in the sensor packages.The design was verified through analysis using mechanical, thermal, and thermoelastic simulations with the finite element method (FEM) to ensure compliance with the individual requirements. The simulations demonstrate that the proposed solution can withstand the harsh launch conditions, maintain the focus on the assembly within the telescope’s required range, and realise operational and decontamination temperatures, ensuring the feasibility of the FPA for the ULTRASAT mission. Furthermore, this investigation provides a verified framework and a scalable solution for future developments of FPAs in Schmidt telescopes.