Getting a clear view through the atmosphere - Radiometric phase
correction at 183 GHz
Martina C. WIEDNER
Harvard-Smithsonian Center for Astrophysics
Résumé :
The angular resolution that can be obtained by ground-based aperture
synthesis telescopes at millimeter and sub-millimeter wavelengths is
limited by phase fluctuations caused by water vapor in the Earth's
atmosphere, an effect very similar to "seeing" in optical astronomy.
Unless corrections are applied, these fluctuations will render
impossible observations of faint objects with an angular resolution of
better than a few tenths of an arc second, even on a good site, such
as Chajnantor. In the talk I will explain the cause of these phase
fluctuations and methods of phase correction. I will describe the
first water vapor monitors (WVMs) designed to allow phase correction
by measuring the atmospheric emission from the 183~GHz water vapor
line. The two radiometers were tested on Mauna Kea, Hawaii, at the
CSO-JCMT interferometer and the Sub-Millimeter Array (SMA). In fair
weather (2.2mm PWV) phase correction reduced the rms phase
fluctuations from 60 deg (140 microns) to 26 deg (60 microns) over 30
minutes, i.e. 75% of the intensity of the decorrelated signal was
retrieved. Besides phase correction I will also present a few
measurements of the atmospheric turbulence (temporal power spectrum)
taken with the radiometers, which underline the importance of phase
correction for interferometery on long baselines. Encouraged by our
results, both the Sub-Millimeter Array (SMA) and the Atacama Large
Millimeter Array (ALMA) are planning to use 183GHz radiometers for
phase correction.
