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Lang-Bardl, F.; Monna, A.; Grupp, F.; Bender, R.; Haeuser, M.; Hess, H. -J; Hopp, U.; Kravcar, H.; Richter, J.; Schlichter, J. (2018): The MICADO Main Selection Mechanism (MSM) - An operational mode selector for the MICADO instrument. In: Ground-Based and Airborne Instrumentation for Astronomy Vii, Vol. 10702
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Abstract

MICADO, the Multi AO Imaging Camera for Deep Observations, is one of the first light instruments for the ELT, currently under construction by the European Southern Observatory (ESO) on Cerro Armazones in Chile. It is built by a huge consortium with partners from the Netherlands, Austria, France, Italy, Finland and Germany under the lead of the Max-Planck-Institute for extraterrestrial Physics in Garching. The instrument will operate in the NIR wavelength range, thus is developed as a cryogenic instrument to work under vacuum conditions. It can be used as an imaging camera in a high and low resolution mode, a spectrometer and also as a coronagraph. For calibration purposes a so called "pupil imager" mode will also be implemented. To switch between the operational modes MICADO will use the MSM to insert different optical modules to the fixed components of the High Resolution Imager (HRI) inside the cryostat. All moving parts have to operate under vacuum and at cryogenic temperatures. The MSM consists of a rotating platform, where the optical modules are mounted on. To lower the friction inside the mechanism we decided to use several small bearings to support the platform instead of a central big one. The small bearings are placed in a way, that the movement of the platform is limited to a rotation. Some of the bearings will be preloaded by springs to take also CTE differences or temperature gradients during the cool down and warm up phases into account. The mechanism will be driven by a cryogenic Phytron stepper motor with an integrated planetary gear box. Switches will be used to limit the rotation of the platform to the necessary range. Because of the challenging requirements on repositioning of the optical modules inside the science beam, we will use an indent mechanism. We are still investigating if the indent mechanism has to be actively driven or can be implemented as a passive version. The necessary optics to switch between the operational modes are designed as individual pre-aligned modules, each with a defined mechanical and thermal interface to the rotating platform. The Low Resolution Imager (LRI) consists of two flat mirrors, blocking some of the fixed components of the HRI. The spectrometer will use two reflective gratings, one acting as the main and one as a cross disperser. The cross disperser separates the overlaying orders on the focal plane array. The pupil viewer consists like the LRI module of two flat mirrors and an additional lens imaging the pupil to the focal plane. In this paper we will present the current mechanical design and first results of the structural and thermal FEM analyses we performed. We will also highlight first ideas on integration and alignment. A second paper (A. Monna et al., same proceedings) concentrates on the cryogenic setups we perform inside a cryostat to proof proper functionality of the chosen components and designs.