Developmental work continues on various schemes to calibrate interferometer phases using measured fluctuations of atmospheric brightness due to water vapor. The effort is divided between observations of the 22 GHz line, on the one hand, and the 183 GHz line, on the other. Although the 183 GHz transition is the one chosen for ALMA (because the line is much stronger and thus better suited to the dry conditions on the site), the 22 GHz experiments on established interferometers are of great value in establishing the phase correction technique.
A report from the OVRO program described phase correction measurements based on a three channel spectrometer centered on the 22 GHz line. Typical data showed residual errors of 160 microns RMS in delay leading to improved correlation and successful correction of astronomical data obtained concurrently. Regular use of the technique for improving observations is presently limited somewhat by awkward software. Better accuracy seems to be limited by the effects of gain fluctuations on the high receiver temperatures of the ambient amplifiers. Cooled amplifiers are being installed, and the accuracy is expected to improve by a factor of two to three. The system has been demonstrated to operate even in cloudy weather. Another future improvement may be enabled by an addition from the Maryland BIMA group of a multichannel correlation system.
A Canadian group led by McMaster University and the JCMT group at NRC has built a clone of the 183 GHz radiometer systems installed at the JCMT and the CSO. This system is uncooled and uses a flip mirror to switch between sky, warm, and hot loads. This radiometer and the radiometer currently at the CSO will be installed on two of the SMA telescopes in the spring of 2001, where they will be used to extend the tests of the capabilities of the 183 GHz system to a wider range of weather conditions and frequencies than was possible with the JCMT-CSO interferometer. A second Canadian group led by the University of Lethbridge and NRC has designed a novel system to operate on the water lines in the 20 micron region. The prototype device has now been installed at the JCMT for a long-term campaign of tests in comparison with radiosonde probes of the atmosphere. A second device is under construction, which should permit interferometric tests similar to those planned with the 183 GHz systems within the next two years.
A joint proposal from Chalmers and the Cavendish has been submitted to the ALMA management for the development of a new 183 GHz radiometer system. This project will test two options; one is a chopping system, and the other is a correlation detector. The goal is to carry out laboratory tests at the outset, with field tests possible in the future.
In connection with the discussion of the problems of pointing errors early in the meeting, the question was raised whether the WVR system might be able to produce on-the-fly pointing corrections. Pointing errors due to the atmosphere (``(a)nomalous refraction'') will limit sensitivity and dynamic range, particularly during the day, and water vapor radiometry is a way to eventually correct for this. At its March 2000 meeting in Leiden, the ASAC recommended that the project press ahead with the basic phase correction scheme using the 183 GHz line, and did not recommend work on the anomalous refraction correction; this was both due to the extra complexity such a system entails, and to the fact that this is only a significant problem for mosaiced observations of extended sources. At the present meeting, because of the greater emphasis on high fidelity imaging of extended sources, the opinion was voiced that this issue should be re-evaluated by the project. The project should thus continue with development of 183 GHz prototypes, but should also attempt to quantify the scientific impact of not correcting for anomalous refraction. There was also a suggestion that the Cosmic Background Interferometer currently operating on the Chajnantor site might already have the necessary data to address this option, and an effort will be made to obtain and study the relevant CBI observations.
The Committee felt that all of these activities were important and urged the various investigators to press on. Accurate phase correction schemes will be the only way to realize the full resolution capabilities of the array. More resources need to be pushed toward all of the schemes under way or planned above and take advantage of any opportunities to test the systems and their ability to correct phase under the appropriate conditions. For the 183 GHz systems, the SMA is clearly a good choice for tests.