EM IR is a Nasmyth focus non-rotating instrument, designed to be attached directly to the Nasmyth platform. Both containers will be handled as independent units and will be translated separately to the Nasmyth position. A common mechanical structure will be used to support both cryostats. It will also perform as the mechanical interface between the instrument and the Nasmyth floor. The optical rotator is made of three flat mirrors situated in a K configuration and mounted inside the first vacuum vessel.
Mirrors rotate around the entrance optical axis.
The field lens is used as the main entrance window. A flat optical bench is used to support the main spectrograph cold subassemblies. Multislit masks shall be mounted in a cassette, using a cryogenic mechanism to extract each mask from the cassette and to insert it in the focal plane. The cassette can be removed, changed and inserted again in the cryostat on a daily basis using an auxiliary exchange dewar. Other mechanisms will allow to change the observing grating, to conmute between spectroscopic and imaging modes, to focus the detector and to insert the pupil imaging lenses in the optical beam.