We describe the mechanical, optomechanical and thermal design and development of the IRIS2, an infrared imager and spectrograph for operation at the Cassegrain focus of the Anglo Australian Telescope. IRIS2 is reconfigured by four encoded worm driven wheels which carry slits and slit masks, filters, cold stops, and grisms, and a pupil imager. A detector translator provides fine focus.
The instrument is housed in a split, or dual, vacuum vessel. Helium cryo-coolers provide operational cooling, but to reduce turn around time during commissioning and maintenance a liquid nitrogen pre-cooling system has been implemented in the main vessel. The slit wheel is housed in a separate, smaller vessel, which may be thermally cycled when new slit masks are installed, while the rest of the instrument remains at operational temperature. The common plate between the vessels serves as the structural base on which the instrument is assembled. Matched trusses on opposite sides of the plate minimize the relative deflection between the slit wheel assembly and the spectrograph optics.