Positioning of optical elements in the cryogenically cooled mid-infrared instrument MIRI for the James Webb Space Telescope

MIRI is one of three focal plane instruments for the JWST covering the wavelengths region 5...28 μm. It is jointly developed by US and European institutes with the latter ones being responsible for the complete optical bench assembly, cryomechanisms, calibration source and the related electronics. MIRI is the combination of an imager with coronographic and low-resolution spectroscopic capabilities and a high-resolution integral-field spectrometer. These diverse options require several mechanisms to select a specific observing mode: (1) a filter wheel with bandpass filters, coronographic masks and a prism, (2) two grating/dichroic wheels with dispersing and order-sorting elements and (3) a flip mirror to direct the beam of an internal black body source into the spectrometer section. All mechanisms are required to operate under laboratory conditions (warm launch) as well as in the cryovacuum in space. The heat dissipation has to be small and the reliability and precision very high. Our low risk approach is the application of successfully qualified and flown components of the ISOPHOT (ISO) and PACS (HERSCHEL) instruments. We will report on the concept developed in phase B.