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Proceedings Paper

ALMA long baseline phase calibration using phase referencing
Author(s): Yoshiharu Asaki; Satoki Matsushita; Edward B. Fomalont; Stuartt A. Corder; Lars-Åke Nyman; William R. F. Dent; Neil M. Philips; Akihiko Hirota; Satoko Takahashi; Baltasar Vila-Vilaro; Bojan Nikolic; Todd R. Hunter; Anthony Remijan; Catherine Vlahakis
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Paper Abstract

The Atacama Large Millimeter/submillimeter Array (ALMA) is the world's largest millimeter/submillimeter telescope and provides unprecedented sensitivities and spatial resolutions. To achieve the highest imaging capabilities, interferometric phase calibration for the long baselines is one of the most important subjects: The longer the baselines, the worse the phase stability becomes because of turbulent motions of the Earth's atmosphere, especially, the water vapor in the troposphere. To overcome this subject, ALMA adopts a phase correction scheme using a Water Vapor Radiometer (WVR) to estimate the amount of water vapor content along the antenna line of sight. An additional technique is phase referencing, in which a science target and a nearby calibrator are observed by turn by quickly changing the antenna pointing. We conducted feasibility studies of the hybrid technique with the WVR phase correction and the antenna Fast Switching (FS) phase referencing (WVR+FS phase correction) for the ALMA 16 km longest baselines in cases that (1) the same observing frequency both for a target and calibrator is used, and (2) higher and lower frequencies for a target and calibrator, respectively, with a typical switching cycle time of 20 s. It was found that the phase correction performance of the hybrid technique is promising where a nearby calibrator is located within roughly 3◦ from a science target, and that the phase correction with 20 s switching cycle time significantly improves the performance with the above separation angle criterion comparing to the 120 s switching cycle time. The currently trial phase calibration method shows the same performance independent of the observing frequencies. This result is especially important for the higher frequency observations because it becomes difficult to find a bright calibrator close to an arbitrary sky position. In the series of our experiments, it is also found that phase errors affecting the image quality come from not only the water vapor content in the lower troposphere but also a large structure of the atmosphere with a typical cell scale of a few tens of kilometers.

Paper Details

Date Published: 8 August 2016
PDF: 18 pages
Proc. SPIE 9906, Ground-based and Airborne Telescopes VI, 99065U (8 August 2016); doi: 10.1117/12.2232301
Show Author Affiliations
Yoshiharu Asaki, National Astronomical Observatory of Japan (Japan)
Joint ALMA Observatory (Chile)
Satoki Matsushita, Academia Sinica Institute of Astronomy and Astrophysics (Taiwan)
Edward B. Fomalont, National Radio Astronomy Observatory (United States)
Joint ALMA Observatory (Chile)
Stuartt A. Corder, Joint ALMA Observatory (Chile)
Lars-Åke Nyman, Joint ALMA Observatory (Chile)
William R. F. Dent, Joint ALMA Observatory (Chile)
Neil M. Philips, Joint ALMA Observatory (Chile)
Akihiko Hirota, National Astronomical Observatory of Japan (Japan)
Joint ALMA Observatory (Japan)
Satoko Takahashi, National Astronomical Observatory of Japan (Japan)
Joint ALMA Observatory (Chile)
Baltasar Vila-Vilaro, Joint ALMA Observatory (Chile)
Bojan Nikolic, Univ. of Cambridge (United Kingdom)
Todd R. Hunter, National Radio Astronomy Observatory (United States)
Anthony Remijan, National Radio Astronomy Observatory (United States)
Catherine Vlahakis, National Radio Astronomy Observatory (United States)

Published in SPIE Proceedings Vol. 9906:
Ground-based and Airborne Telescopes VI
Helen J. Hall; Roberto Gilmozzi; Heather K. Marshall, Editor(s)

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