HARMONI at ELT: athermal toroidal mirror mount for HARMONI pre-optics

HARMONI is the high angular resolution optical and near-IR integral field spectrograph for the Extremely Large telescope (ELT). It covers a large spectral range from 470nm to 2450nm with resolving powers from 3300 to 18000 and spatial sampling from 60mas to 4mas. The Instrument Pre-Optics (IPO), a sub-system under the responsibility of the Instituto de Astrofísica de Canarias (IAC), consists of several opto-mechanical mounts operating at cryogenic temperatures. Among these, toroidal mirrors stand out as the most sensitive and accurate elements of the IPO. An athermal design has been achieved by combining different materials for both the mount and the pads that define the position of the optical element. This design compensates the differential contractions during cooling, which allows the optical element alignment to be maintained within the tight tolerance when transitioning from warm to cryogenic operating temperatures. This paper presents the design of the athermal toroidal mirror mount, the tests carried out on its prototype and the final optomechanical mount.