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A grism (direct-vision diffraction grating) combines a diffraction grating and prism to make any order and wavelength go straight. By setting a grism into the collimating beam (necessary for an astronomical instrument to place pupil mask, filters, etc.) of an imaging instrument, it becomes possible to easily perform spectroscopic observations. The prism array is a device aimed at increasing the angular dispersion (resolving power) of a grism within the limited size of the collimating beam part of the instrument.
We adopted a 6.5-m F/1.25 light-weighted borosilicate honeycomb primary mirror and its support system that are developed by Steward Observatory Richard F. Caris Mirror Lab. An enclosure has the shape of carousel, and large ventilation windows with shutters control the wind to flush heat inside the enclosure. A support building with a control room, a mirror coating system and maintenance facilities is located at the side of the enclosure. The mirror coating system consists of a large aluminizing chamber and a mirror washing facility. The operation of the telescope will be remotely carried out from a base facility at San Pedro de Atacama, 50km away from the summit. Development of the two facility instruments has already been completed and they are transported to Hilo, Hawaii in 2017. We are going to carry out engineering observations of those instruments on the Subaru telescope for clearing up technical issues and verifying their performance. The existing summit access road from the ALMA concession area was laid in 2006, however, it is too narrow to carry large components of the telescope and the ancillary facilities such as the primary mirror, its cell, and the aluminizing chamber. The road is being expanded so that it has the width of <5m for straight portion and <7m for curved portion.. The telescope mount and the enclosure are being pre-assembled for functional and performance tests in Japan. All telescope system will be assembled at the summit and see the engineering first light early 2019.
The F/1.25 light-weighted borosilicate (Ohara E6) honeycomb primary mirror is adopted and being fabricated by the Steward Observatory Mirror Laboratory. The primary mirror is supported by 104 loadspreaders bonded to the back surface of the mirror and 6 adjustable hardpoints. The mirror is actively controlled by adjusting the actuator forces based on the realtime wavefront measurement. The actuators are optimized for operation at high altitude of the site, 5640-m above the sea level, by considering the low temperature and low air pressure. The mirror is held in the primary mirror cell which is used as a part of the vacuum chamber when the mirror surface is aluminized without being detached from the cell.
The pupil is set at the secondary mirror to minimize infrared radiation into instruments. The telescope has two Nasmyth foci for near-infrared and mid-infrared facility instruments (SWIMS and MIMIZUKU, respectively) and one folded-Caseggrain focus for carry-in instruments. At each focus, autoguider and wavefront measurement systems are attached to achieve seeing-limited image quality.
The telescope mount is designed as a tripod-disk type alt-azimuth mount. Both the azimuthal and elevation axes are supported by and run on the hydrostatic bearings. Friction drives are selected for these axis drives. The telescope mount structure and primary mirror support as well as the mirrors are under thermal control and maintained at ambient air temperature to minimize the mirror seeing.
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