Dichroic/ Grating Wheel
The wheel comprises three working positions to move gratings and dichroics simultaneously. Each is located on separate wheel discs. The two wheels feed light in to the four spectrometer channels inside MIRI. The system has to operate in the cryovacuum of 7k (as well as ambient conditions) during a ground-test of 3 years on Earth and a mission period of up to 10 years in space.
The spectrometer wheel assembly is composed of two distinct wheel discs for gratings and dichroics, respectively. Rotation is realized by a central two-phase torque motor allowing for bi-directional movements. Positioning is via a ratchet-cam system comprising a spring loaded detent arm with one ball bearing at its end and cam disc of the wheel’s axis. Fixing a certain position with the desired optical element is realized by accurately machined grooves on the cam disc representing a local minimum of the potential energy in the system. The wheel is suspended by an integrated ball bearing on the wheel’s axis between disc and mechanism structure.
The system is electrically supplied by the warm Instrument Control Electronics (ICE) to drive the motor and to read one position indicator and one temperature sensor for housekeeping purposes.
The Dichroic and Grating wheel fit into the spectrograph wheel as shown in the picture below.
The following links lead to very high resolution pictures of a Dichroic Wheels and open in a new window.
picture of a Dichroic Wheel alongside a ruler
Dichroic Wheels are made at the ATC
Grating Wheels are made by Astron
Spectrograph Wheels are made by MPIA