To PLAnetary Transit or not? An extremely large field of view camera with a CaF₂component tested in thermo-vacuum

Abstract

Because of its nicely chromatic behavior, Calcium Fluoride (CaF 2 ) is a nice choice for an optical designer as it can easily solve a number of issues, giving the right extra degree of freedom in the optical design tuning. However, switching from tablet screens to real life, the scarcity of information -and sometimes the bad reputation in term of fragility-about this material makes an overall test much more than a "display determination" experiment. We describe the extensive tests performed in ambient temperature and in thermo-vacuum of a prototype, consistent with flight CTEs, of a 200mm class camera envisaged for the PLATO (PLAnetary Transit and Oscillations of Stars) mission. We show how the CaF 2 lens uneventfully succeeded to all the tests and handling procedures, and discuss the main results of the very intensive test campaign of the PLATO Telescope Optical Unit prototype.

The PLATO camera

“…A TOU prototype BreadBoard (BB) [2] has been designed and produced during the phase A of the project, to validate assembly, integration and test procedures, verify their timecompatibility with respect to industry framework (since the realization of the TOUs is intended to be assigned to industry), and test the on-axis system performance in ambient (20 °C, 1 atm) and thermal vacuum conditions (-80°C, 0 atm). The BB consists of a set of 6 custom made lenses as close as possible to final design and a mechanical structure equivalent to TOU in terms of thermal behaviour.…”

“…They have an extremely large sensitive area of 4510x4510 pixels of 18 μm pitch, yielding a sensitive area of 81.18x81.18=6590 mm 2 . In all, the sensitive area of the complete instrument is 0.9 m 2 .…”

“…• The overall field of view of the instrument with such an accommodation is higher than 2200 deg 2 , with a pupil diameter larger than 340 mm.…”

“…CaF 2 is a material alleged to show brittleness and high susceptibility to temperature changes. Dedicated tests [2] have been done on CaF 2 blanks (non-polished parallel-plane disks of thickness 26 mm and diameter 120 mm) to observe the response of this material when subjected to thermal and shock stress comparable to a launch through a Soyuz-Fregat type vehicle. One blank was tested under vibrations at Bern University and then tested in thermo-vacuum in at CNES Toulouse facility, with temperatures cycling from 4°C to -100°C, and a gradient of 0.2°C/min.…”

The PLATO Mission

“…The optical design of each camera (Magrin et al, 2010) consists of a six lens system with a central Calcium Fluoride lens (Bergomi et al, 2012) close to the stop, and with a first aspheric lens protected, for thermal and radiation hardness purposes, by a Suprasil window. All the optical materials have been the subject of specific radhard studies (Corso et al, 2018) and the whole design is a fine tuning balance between radiation, thermal and mass budgets (Magrin et al, 2016).…”