Within Infra-Red large wavelength bandwidth instruments the use of mechanisms for selection of observation modes,
filters, dispersing elements, pinholes or slits is inevitable. The cryogenic operating environment poses several challenges
to these cryogenic mechanisms; like differential thermal shrinkage, physical property change of materials, limited use of
lubrication, high feature density, limited space etc.
MATISSE the mid-infrared interferometric spectrograph and imager for ESO's VLT interferometer (VLTI) at Paranal in
Chile coherently combines the light from 4 telescopes. Within the Cold Optics Bench (COB) of MATISSE two concepts
of selection mechanisms can be distinguished based on the same design principles: linear selection mechanisms (sliders)
and rotating selection mechanisms (wheels).Both sliders and wheels are used at a temperature of 38 Kelvin.
The selection mechanisms have to provide high accuracy and repeatability. The sliders/wheels have integrated tracks
that run on small, accurately located, spring loaded precision bearings. Special indents are used for selection of the
slider/wheel position. For maximum accuracy/repeatability the guiding/selection system is separated from the actuation
in this case a cryogenic actuator inside the cryostat.
The paper discusses the detailed design of the mechanisms and the final realization for the MATISSE COB. Limited
lifetime and performance tests determine accuracy, warm and cold and the reliability/wear during life of the instrument.
The test results and further improvements to the mechanisms are discussed.