Elevation angle dependence of the SMA antenna focus position

Details Elevation angle dependence of the SMA antenna focus position Elevation angle dependence of the SMA antenna focus position

2006-06-21

arxiv.org/abs/astro-ph/0606514v1

Proc. SPIE, 6275, 62751W (2006)

Astronomy and Astrophysics

Astrophysics

10 pages, 6 figures; to appear in the Proceedings of SPIE Vol. 6275 "Millimeter and Submillimeter Detectors and Instrumentatio

Scientific paper

10.1117/12.670642

We report the measurement results and compensation of the antenna elevation angle dependences of the Sub-millimeter Array (SMA) antenna characteristics. Without optimizing the subreflector (focus) positions as a function of the antenna elevation angle, antenna beam patterns show lopsided sidelobes, and antenna efficiencies show degradations. The sidelobe level increases and the antenna efficiencies decrease about 1% and a few %, respectively, for every 10 degrees change in the elevation angle at the measured frequency of 237 GHz. We therefore obtained the optimized subreflector positions for X (azimuth), Y (elevation), and Z (radio optics) focus axes at various elevation angles for all the eight SMA antennas. The X axis position does not depend on the elevation angle. The Y and Z axes positions depend on the elevation angles, and are well fitted with a simple function for each axis with including a gravity term (cosine and sine of elevation, respectively). In the optimized subreflector positions, the antenna beam patterns show low level symmetric sidelobe of at most a few %, and the antenna efficiencies stay constant at any antenna elevation angles. Using one set of fitted functions for all antennas, the SMA is now operating with real-time focusing, and showing constant antenna characteristics at any given elevation angle.

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