Rapid Phase Calibration and the Atmospheric Phase Interferometer (API)

For some objects, and under suitable weather conditions, the phase calibration can be considerably improved by rapidly switching between the source and calibrator. An observing technique denoted `fast switching' has been developed to more conveniently permit the user to implement this methodology. Source-Cal observing cycles as short as 40 seconds can now be used - such a short cycle is impossible with traditional VLA observing techniques.

This method is not for everyone! Considerable integration time is lost with very short cycle times, so it is important to balance this certain loss against a realistic estimate of the possible gain. Experience has shown that cycle times of 100 to 150 seconds at high frequencies have been effective for source-calibrator separations of less than 10 degrees. The fast switching technique `stops' tropospheric phase variations at an effective baseline length of $\sim v_a t/2$ where $v_a$ is the atmospheric wind velocity aloft (typically 10 to 15 m/sec), and $t$ is the total switching time. This technique has been demonstrated to result in images of faint sources with diffraction-limited spatial resolution on the longest VLA baselines. Under average weather conditions, and using a 120 second cycle time, the residual phase at 43 GHz should be reduced to $\le$ 30 degrees. Further details can be found in VLA Scientific Memos # 169 and 173. These memos, and other useful information, can be obtained from Reference 12 in Section 6.

The fast switching system has been implemented in the current version of the JObserve program. Note that the technique will not work in bad weather (such as rain showers, or when there are well-developed convection cells - most notably, thunderstorms). Contact Claire Chandler or Chris Carilli for details and advice, and see the high-frequency observing guide at
http://www.vla.nrao.edu/astro/guides/highfreq/ .

The NRAO Atmospheric Phase Interferometer (API) is now operational at the VLA, and software has been installed for real time monitoring of the phase stability through the web. A detailed description of the API, and instructions for accessing its data, may be found at
http://www.vla.nrao.edu/astro/guides/api/ . The API continuously measures the tropospheric contribution to the interferometric phase using an interferometer comprised of two 1.5 meter antennas separated by 300 meters, observing an 11.3 GHz beacon from a geostationary satellite. The API data can be used to estimate the required calibration cycle times when using fast switching phase calibration, and in the worst case, to indicate to the observer that high frequency observing may not be possible with current weather conditions. Contact Chris Carilli for further details.