ALMA long baseline phase calibration using phase referencing
| Autor: | Stuartt Corder, Edward B. Fomalont, Anthony J. Remijan, Neil M. Philips, Todd R. Hunter, William R. F. Dent, Satoki Matsushita, Baltasar Vila-Vilaro, Yoshiharu Asaki, Catherine Vlahakis, Lars-Åke Nyman, Akihiko Hirota, Satoko Takahashi, Bojan Nikolic |
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| Rok vydání: | 2016 |
| Předmět: |
Physics
Radiometer 010504 meteorology & atmospheric sciences business.industry Phase (waves) atmospheric phase error phase referencing 01 natural sciences Submillimeter Array Troposphere Optics water vapor 0103 physical sciences Calibration ALMA Millimeter Antenna (radio) business 010303 astronomy & astrophysics Water vapor 0105 earth and related environmental sciences Remote sensing |
| Zdroj: | SPIE Proceedings. |
| ISSN: | 0277-786X |
| Popis: | © 2016 SPIE.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 capa- bilities, 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, es- pecially, 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. |
| Databáze: | OpenAIRE |
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