The unique field-of-view and focusing budgets of PLATO

Kurzfassung

The PLAnetary Transits and Oscillations of stars mission (PLATO) is the M3 mission in ESA’s Cosmic Vision 2015-2025 Programme, see Rauer et al. (2014).1 The PLATO mission aims at detecting and characterizing extrasolar planetary systems, including terrestrial exoplanets around bright solar-type stars in the habitable zone. In order to achieve its scientific objectives, PLATO must perform uninterrupted high precision photometric monitoring of large samples of stars during long periods to detect and characterize planetary transits. The scientific payload of PLATO, developed and provided by the PLATO Mission Consortium (PMC) and ESA, is based on a multi-telescope configuration consisting of 24 “Normal” (N) cameras and 2 “Fast” (F) cameras, so as to provide simultaneously a large field of view and a large collecting aperture. The optical design is identical for all cameras and consists of a 6-lens dioptric design with a 120 mm entrance pupil and an effective field of view of more than 1000 deg2. This concept results in an overall field-of-view of more than 2000 deg², spread over 104 CCDs of 20 mega-pixels each. Associated to very accurate pointing and alignment requirements, this is a real challenge to define and breakdown precise specifications to several sub-systems in order to ensure that this overall field of view budget is achieved and verified. We propose to go through the budget that was performed for the PLATO camera and to describe how we intend to satisfy this scientific requirement. To make it more challenging, it has to be taken into account that the PLATO spacecraft will have to rotate of 90° every three months without changing its field of view (due to its orbit in L2 and the sun illumination limitations). This has to be considered in the breakdown of the budget and design of all sub-systems. A consequence of this large field of view is the difficulty to reach very good and harmonious optical performances across the field, and in a realistic depth of focus. Therefore, the focusing budget is also very challenging for the development of the PLATO cameras. We will describe the way the PLATO’s camera focusing budget has been broken down into allocations and how it is planned to be verified. To ensure optimal performances in-flight, the PLATO cameras have the extraordinary capabilities to perform re-focusing using a high precision Thermal Control System (TCS). Each individual camera on the payload can be thermally controlled independently from its neighbor to reach its own optimal operational temperature. The different consequences of this concept into the budget allocations and sub-system development will be discussed.