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The Submillimeter Array - Antennas and Receivers

Authors:
Raymond Blundell
Abstract:
The Submillimeter Array (SMA) was conceived at the Smithsonian Astrophysical Observatory in 1984 as a six element interferometer to operate in the major atmospheric windows from about 200 to 900 GHz. In 1996, the Acadernica Sinica Institute of Astronomy and Astrophysics of Taiwan joined the project and agreed to provide additional hardware to expand the interferometer to eight elements. All eight antennas are now operating at the observatory site on Mauna Kea, and astronomical observations have been made in the 230, 345, and 650 GHz bands. The SMA antennas have a diameter of 6 m, a surface accuracy of better than 25 µm rms, and can be reconfigured to provide spatial resolutions down to about 0.5" at 200 GHz and eventually, 0.1" at 850 GHz. Coupling to the receiver package within each antenna is achieved via a beam waveguide, in a bent Nasmyth configuration, comprised of a flat tertiary mirror and two ellipsoidal mirrors that form a secondary pupil used for receiver calibration. An additional fixed mirror and a rotating wire grid polarizer are then used for receiver selection. Each antenna houses a single cryostat, with an integrated cryocooler capable of cooling up to eight receivers to 4 K. In the current configuration only three receiver bands are available: 175 — 255 GHz, 250 — 350 Gliz, and 600 — 720 GHz, and simultaneous operation of the 650 GHz receiver with either of the lower frequency receivers is possible. Eventually dual polarization will be available from 325 — 350 GHz, and dual frequency operation will be possible, pairing either of the lower frequency receivers with any of the high frequency units: 325 — 425 GHz, 425 — 510 GHz, 600 — 720 GHz, and 800 — 900 GHz. Each receiver currently uses a single superconductor-insulator-superconductor junction as the mixing element and has first stage intermediate frequency amplification at 4 K with an instantaneous bandwidth of 2.5 GHz, centered at 5 GHz. The mixers are of a fixed-tuned waveguide design, are inherently broad band, typically 80 — 100 GHz, and provide for relatively low receiver noise, typically 5 — 10 hv/k for the low frequency bands, and 10 — 15 hv/k for the 650 GHz receivers. Local oscillator power to each receiver is provided by a mechanically tunable Gunn oscillator followed by the appropriate diode frequency multiplier or multiplier combination.
Categories:
Invited Talk
Year:
2004
Session:
Inv
Full-text:
Download a PDF of this paper.
Page Number(s):
3-15