Thermal and dynamic qualification test results on PLATO optical mount groups based on lenses made of brittle materials (CaF2, S-FPL51)

Thermal and dynamic qualification loads on spacecraft are usually very high and the design of space components requires to use strong material to withstand them. However optical payloads usually mount brittle materials for optics and lenses. Two of them are the crystal CaF2 and the OHARA glass S-FPL51. Allowable design values for this kind of materials are hard to define, also considering that the numerical values for strength are low and the safety factor to use for design of brittle materials are really high. These two optical glass/crystal shall be used for three of the s ix lens es mounted on each one of the 26 Telescope Optical Units (TOU) of PLATO (PLAnetary Transits and Oscillation of stars), an ESA satellite that will be launched in 2026 to discover exoplanets. These brittle lenses, together with the mounts on which they are bonded, have been tested on a breadboards campaign checking their resistance to cryogenic temperatures (down to -115 °C), random loads up to failure and their behavior under shock loads. The results presented in this article show an unexpected and very high performance of each lens and its mount considering both thermal and dynamical behavior.