Design update of the central wheel mechanism

We present an update of the design of the Central Wheel Mechanism (CWM), a subsystem of the MICADO instrument for the Extremely Large Telescope (ELT)1. The CWM consists of a Filter Wheel Mechanism (FWM) with two Filter wheels, the Pupil Wheel Mechanism (PWM) with one Pupil wheel and the Atmospheric Distortion Corrector (ADC). The CWM has a diameter of about 1.5 meter. Challenges in the mechanical design work are focused on fitting all the components in the available design volume and ensuring that the accuracy and repeatability requirements are met. The mechanisms should work in a cryogenic environment for an intended lifetime of 10 years and survive transport and seismic conditions. Additionally the system has to cool and warm-up properly. The most noticeable update since the PDR2 is a self-contained ring assembly of a connected FWM and PWM that is suspended to the MICADO instrument using flexible supports. The arrangement of these supports creates a point of shrinkage at the optical position which is off-axis from the cryostat center. An alignment tool will guarantee a position accuracy within 50 μm for both the ADC as well as the Pupil masks that are mounted after the system is integrated into the instrument. The most sensitive bearings are protected from shock loads by a system that limits their load carrying capability by transferring the excess load to bumpers. These bumpers do not affect the repeatability of the system.