Conceptual design of a cryogenic pupil mechanism with continuous complex movements for HARMONI

In order to improve the signal-to-noise ratio of HARMONI (E-ELT first light visible and near-infrared integral field VIR spectrometer), a pupil mask has been identified to be included at the fore-optics to limit the background radiation coming into the spectrographs. This mask should rotate synchronously with the telescope pupil during observations, taking into account the combined effects of the telescope tracking and the de-rotation of the FOV. The implementation of the pupil mask functionality will require complex movements with high precision at cryogenic temperatures which implies an important technological challenge. This paper details a set of experiments completed to gain knowledge and experience in order to accomplish the design and control of cryogenic mechanisms reaching this type of pupil motion. The conceptual design of the whole mechanism started from the feedback acquired from those experiments is also described in the following sections.