Cryostat design and fabrication for the Gemini NIRI instrument

ABSTRACT The Gemini Near Infrared Imager (NIRI) is a cryogenic instrument cooled by two closed-cycle cryo-coolers. The vacuumjacket is a hexagon shaped vacuum vessel made of three sections. Each section is forged out of aluminum 606 1 . All theinternal structural components are made of aluminum 6061T6 except the supporting trusses, which are made of titanium.All the internal structural members are stress relieved to maintain dimensional stability and good optical alignment. Thethennal insulation includes floating shields and cold shields. Two closed-cycle coolers are mounted opposite to each otherand electronically synchronized in order to cancel the vibration caused by the oscillating expansion valve. Several differentfabrication methods and stress relief methods are discussed.KEYWORD: infrared, instrument, cryogenic, cooler, vacuum, cryostat, radiation, shield, aluminum, titanium 1. Introduction The Gemini Near Infrared Imager (NIRI) [11 will be the main infrared imaging instrument on the 8-meter Gemini NorthTelescope. It will be used for the commissioning of the telescope and the characterization of its performance. NIRI is beingdesigned and fabricated by the University ofHawaii Institute for Astronomy under contract from AURA.The ciyostat consists of the following major components: the vacuum jacket (vacuum vessel), main work surface, imageroptical table, on-instrument-wave4ront-sensor (OIWFS) optical table, internal mounting trusses, closed cycle coolers,floating radiation shields, cold shields, and photon shields.The vacuumjacket is the enclosure of the instrument. It provides a high vacuum and ciyogemc environment for the infraredarray and all the optics inside the vacuum jacket. The main work surface is a 40-mm thick aluminum plate. It is attachedto the vacuum jacket through three pairs of titanium trusses. All the rest of the internal components and assemblies aremounted on the main work surface. The main work surface is like a partition, dividing the instrument into two sections:Imager section and WFS section. The imager optical table is mounted on one side of the main work surface, the WFSoptical table is mounted on the other side of the main work surface.Two closed-cycle coolers are used to cool the instrument and maintain the operating temperature inside the vacuum jacketat about 60 °