Cryogenic wheel mechanisms for the Mid-Infrared Instrument (MIRI) of the James Webb Space Telescope (JWST): detailed design and test results from the qualification program
2008
The Mid-Infrared Instrument (MIRI) of the James Webb Space Telescope, scheduled for launch in 2013, will provide a
variety of observing modes such as broad/narrow-band imaging, coronagraphy and low/medium resolution
spectroscopy. One filter wheel and two dichroic-grating wheel mechanisms allow to configure the instrument between
the different observing modes and wavelength ranges. The main requirements for the three mechanisms with up to 18
positions on the wheel include: (1) reliable operation at T ~ 7 K, (2) optical precision, (3) low power dissipation, (4)
high vibration capability, (5) functionality at 6 K < T < 300 K and (6) long lifetime (5-10 years). To meet these stringent
requirement, a space-proven mechanism design based on the European ISO mission and consisting of a central bearing
carrying the optical wheels, a central torque motor for wheel actuation, a ratchet system for precise and powerless
positioning and a magnetoresistive position sensor has been selected. We present here the detailed design of the flight
models and report results from the extensive component qualification.
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