A filter wheel mechanism for the Euclid near-infrared imaging photometer

The Euclid mission is currently being developed within the European Space Agency's Cosmic Vision Program. The five year mission will survey the entire extragalactic sky (~ 20 000 deg²) with the aim of constraining the nature of dark energy and dark matter. The spacecraft's payload consists of two instruments: one imaging instrument, which has both a visible and a near-infrared channel, and one spectroscopic instrument operating in the near-infrared wavelength regime. The two channels of the imaging instrument, the Visible Imaging Channel (VIS) and the Near-Infrared Imaging Photometer Channel (NIP), will focus on the weak lensing science probe. The large survey area and the need to not only image each patch of sky in multiple bands, but also in multiple dithers, requires over 640 000 operations of the NIP channel's filter wheel mechanism. With a 127 mm diameter and a mass of ~ 330 g per element, these brittle infrared filters dictate highly demanding requirements on this single-point-failure mechanism. To accommodate the large filters the wheel must have an outer diameter of ~ 400 mm, which will result in significant loads being applied to the bearing assembly during launch. The centrally driven titanium filter wheel will house the infrared filters in specially designed mounts. Both stepper motor and brushless DC drive systems are being considered and tested for this mechanism. This paper presents the design considerations and details the first prototyping campaign of this mechanism. The design and finite element analysis of the filter mounting concept are also presented.