Toward more realistic quantification of ionizing radiation effects on ZERODUR spaceborne mirrors

Dimensional changes of mirrors under spaceborne irradiation must be considered in the context of increasingly narrow tolerances. Among those factors impacting the optical figure, thermal transients are one of the main reason accounting for deformation, thus this is the reason why low CTE material (ZERODUR®) and/or heat diffusive material are exclusively considered for these applications. Radiation induced compaction can play a role in dimensional stability. This compaction effect is due to the local change of density induced by the collision of energetic particles (<i>e.g.</i> protons and electrons trapped in the Van Allen belt) with the mirror substrate. To address this perceived problem, we performed numerical evaluation of the irradiation levels expected over four different orbits (LEO, GEO, Sun Synchronous, L2) as seen by a ZERODUR® primary mirror. From these orbits, we then selected the harshest Earth-Orbiting conditions, namely those at GEO, and performed an equivalent lab-based e- and H+ irradiation. The effects of radiation induced deformation (compaction) onto the coupons have been then measured interferometrically for change. Finally, using our laboratory irradiation results and following a rigorous mechanical approach, we propose a upper bound for the expected deformation that would be expected with a baffled spaceborne lightweighted telescope. The negligible deformation derived in this study are consistent with decades of successful missions using ZERODUR® mirrors.