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Engineering of a cryogenic filter mount for MICADO

Submitter: Niels Tromp
Description: The past six months Luuk Tigelaar worked as an intern on the development of a fixture for retaining an optical filter in a filter exchange mechanism. This mechanism is developed and build by the NOVA Optical InfraRed Instrumentation group and will be part of the MICADO instrument developed by MPE in Germany. MICADO is selected as a first light instrument on the E-ELT in Chili.

Part of the challenge was to engineer a mechanical mount which would hold the glass element with minimum force (i.e. minimum deformation) while at the same time maximizing thermal contact for rapid cooling from room temperature to below -200degC. A key parameter is the thermal resistance of the glass-aluminium interface. These values are hard to find in literature and highly depend on a range of different characteristics like temperature, clamping force, surface roughness, filler material etc.

Pictured is Luuk during his final presentation at NOVA-ASTRON with on the table, covered in insulating foil, his cryogenic test setup of the glass-aluminium interface.

The second image shows its internals in detail. The test setup is optimized to accurately measure the actual thermal contact resistance between glass and aluminium in a cryogenic environment (i.e. vacuum & at low temperature) while maintaining a constant clamping force. The latter is not trivial as the used materials behave differently in the cold (shrinkage, stiffness).

Thanks to Luuk his eagerness to learn, his creativity and determination, we now have a viable cooling concept and an accurate thermal simulation model based on the obtained parameter values. This allows us to optimize the mechanical design and thermal performance of the filter mount and exchange mechanism for MICADO.
Copyright: NOVA
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