29 September 2004 Holographic surface setting of the Sub-millimeter Array antennas
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Abstract
The holography program to measure and set the surfaces of the antennas of the Submillmeter Array (SMA) has been very successful, with the best antenna meeting the stringent 12 μm rms specification. The surfaces of the 6-meter diameter antennas of the 8 element array have been set to accuracies of 12-25 μm, and are under constant improvement. This allows efficient operation in the 660 GHz band, currently the highest frequency band of observations. The system used to make routine near-field holographic measurements at 232.4 GHz -- the primary method of obtaining surface error maps -- is now fully integrated into the SMA. The measurements are carried out remotely from Cambridge. A sequence of upto 4 rounds of measurements and adjustments is needed to achieve the design specification of 12 μm rms starting typically from 65 μm rms. The last sets of adjustments incorporate corrections for panel flexures, allowed by the 4 points of adjustment for most of the panels, and the high spatial resolution (~ 8 cm) of the surface error maps. Repeat measurements indicate a surface stability time scale of ~ 1 year including antenna transport between stations. Celestial holography to characterize gravitational deformations and careful efficiency measurements to validate the holographic measurements are in progress.
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Tirupati K. Sridharan, Masao Saito, Nimesh A. Patel, Robert D. Christensen, "Holographic surface setting of the Sub-millimeter Array antennas", Proc. SPIE 5495, Astronomical Structures and Mechanisms Technology, (29 September 2004); doi: 10.1117/12.550939; https://doi.org/10.1117/12.550939
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