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Paper: The New JCMT Holographic Surface Mapping System - Implementation
Volume: 238, Astronomical Data Analysis Software and Systems X
Page: 93
Authors: Smith, I. A.; Baas, F.; Olivera, F.; Rees, N. P.; Dabrowski, Y.; Hills, R.; Richer, J.; Smith, H.; Ellison, B.
Abstract: The James Clerk Maxwell Telescope (JCMT) on the summit of Mauna Kea in Hawaii is a 15 meter telescope which operates in the sub-millimeter region of the spectrum (in practice from about 2 mm to 350 microns). Operation at such short wavelength requires the dish to have a surface accuracy of ~20 microns RMS. The primary reflector surface consists of 276 panels each of which is positioned by 3 stepper motors. In order to ensure the highest possible surface accuracy we are embarking upon a project to improve the mapping of the surface. The mapping of the primary reflector surface is achieved by "holographic" techniques. Currently a 94 GHz radiation source, located on a nearby Telescope (United Kingdom Infra-Red Telescope) is used to illuminate the JCMT surface. The orientation of the telescope is scanned in two coordinates to measure the beam pattern. From this it is possible to determine the path length of the radiation to, and hence the position of, each panel. A new instrument has been deployed in order to achieve greater accuracy and produce maps at a significantly faster rate. The instrument uses two main frequencies, 80 and 160 GHz, and is frequency agile around these frequencies to allow for the elimination of spurious signal paths.
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