Opto-mechanical design for transmission optics in cryogenic IR instrumentation

ASTRON is involved in the development and realization of various optical astronomical instruments for ground-based as well as space telescopes, with a focus on near- and mid-infrared instrumentation. ASTRON has developed, among others, cryogenic optics for the first generation ESO VLT and VLTI instruments VISIR, MIDI and the SPIFFI 2K-camera for SINFONI. Currently under construction are MIRI for the James Webb Space Telescope and X-shooter for the second generation ESO VLT instrumentation, while the initial design of several ELT instruments has started. Mounting optics is always a compromise between firmly fixing the optics and preventing stresses within the optics. The fixing should ensure mechanical stability and thus accurate positioning in various gravity orientations, temperature ranges, during launch, transport or earthquake. On the other hand, the fixings can induce deformations and sometimes birefringence in the optics and thus cause optical errors. Even cracking or breaking of the optics is a risk, especially at the cryogenic temperatures required in instruments for infrared astronomy, where differential expansion of various materials amounts easily to several millimetres per meter. Special kinematic mounts are therefore needed to ensure both accurate positioning and low stress. Though ASTRON is involved in the full realization of instruments from initial design to commissioning, this paper concentrates on the opto-mechanical design of optics mountings, especially for large transmission optics in cryogenic circumstances. It describes the development of temperature-invariant ("a-thermal"), kinematic designs and how they are implemented in instruments such as SPIFFI and X-shooter.