HARMONI at ELT: High Precision Cryogenic Mechanisms for HARMONI Spectrographs

HARMONI is the first light visible and near-IR integral field spectrograph for the ELT. It covers a large spectral range from 450nm to 2450nm with resolving powers from 3500 to 18000 and spatial sampling from 60mas to 4mas. It can operate in two Adaptive Optics modes - SCAO (including a High Contrast capability) and LTAO - or with NOAO. The project is preparing for Final Design Reviews. The spectrographs of HARMONI collimate and direct the output beam at 3 different incidence angles on the holographic gratings, one angle per family of gratings (low, medium and high resolution). To maximize the observing efficiency of the ELT, the operation of HARMONI envisages the calibration exposures to occur during day time. This requires the collimated beam to be positioned at the grating with a repeatability of 1.4 arc seconds in a 24-hour period along the spectral direction and 10 times smaller along the slit, independently of the resolution used. This work presents the design, analysis and prototyping performed on the Folding Mirror Mechanism in order to comply with the repeatability requirements under cryogenic conditions, highlighting the improvements incorporated into the final design.