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The APEX telescope is an ALMA prototype designed by the VERTEX Corporation in Duisburg, Germany. It will be modified to operate as a single-dish observatory housing heterodyne receivers for all submm atmospheric windows, and several bolometer array cameras. The telescope design is a on-axis Cassegrain mounted on an elevation over azimuth mount. The primary mirror has a focal length of 4.8 meters and is 12 meters in diameter. The telescope is designed for submillimeter observations, with an antenna rms surface accuracy of 18 microns. A mirror in the optical path within the cabin is used to switch between receivers. The telescope weights roughly 77 tons.
APEX will be located at 5000 meters in the Atacama plateau in Chile. This region is one of the driest places on earth. This makes the site ideal for millimeter-wave observations, as the limiting factor for ground-based observation sensitivity is noise from atmospheric water vapor fluctuations. The Atacama site can view most of the celestial sphere with good overlap with many other ground based telescopes, which will be needed for later follow-up observations to determine the redshifts of galaxy clusters.
Design of the tertiary optics is being done by the Berkeley group in collaboration with MPIfR. The 12 m diameter primary mirror and 0.75 m secondary form a Cassegrain focus inside the cabin. The field of view of the system is limited by the 0.75 meter diameter hole in the primary to be < 0.5 deg. Initially, the focal plane array will subtend a 0.4 deg field of view. Tertiary optics reimage the focal plane with f/# ~ 1.75 to cover the bolometer array, which will be 15 cm in diameter.