Status of current 183 GHz phase correction experiments

The JCMT-CSO single-baseline interferometer was the first to demonstrate phase calibration using the 183 GHz line, using equipment built by Martina Wiedner, Richard Hills and colleagues. Only a limited quantity of data were gathered but the results (ALMA memo 252) were encouraging and suggested that even an uncooled system could provide effective phase calibration at submillimetre wavelengths. Single baseline interferometry at JCMT-CSO is no longer a supported mode of operation, so further observations would be difficult though not perhaps impossible to arrange. It is however possible that two SMA antennas can be equipped with 183 GHz systems, using the radiometer currently at CSO plus a clone of it being built in Canada by Christine Wilson and the HIA. It is unclear when this experiment might produce results on Mauna Kea, but access to a large set of data in a variety of atmospheric conditions would certainly be useful in establishing the capabilities of the technique. On the Chajnantor site, two further 183 GHz radiometers are in operation; these were built as a collaboration between Onsala and Cambridge and are very similar to the Mauna Kea systems, again using uncooled DSB mixers and three roughly 1-GHz wide filters. These two independent systems are aligned with the twin 11-GHz site testing interferometers, with their beams matched as well as possible using newly designed mirrors. The intention is to see how effectively and for what fraction of the time it is possible to use the 183 GHz systems to correct the 11GHz atmospheric phase measurements. It is possible to estimate the height of the turbulence from the lag between the two 11 GHz phase measurements (which are obtained by looking at different satellites) together with information on wind speed and direction. This will be important in establishing how strongly the quality of radiometric phase calibration depends on the turbulent scale height, both in practice and through models. Initial results for both the lag estimates and radiometric phase calibration have been obtained in the past 2 months, although operational difficulties (principally power outages, and the difficulties in performing system upgrades and receiver tests on site) have restricted the quantity of data so far obtained. Work on analysing the existing data and on improving the measurements will continue as a high priority, with the goal of producing a report in about 6 months. Although the results from these more detailed studies will be needed in order to answer some of the questions, we need to have an initial set of specifications for the ALMA radiometer system and a baseline design for inclusion in the plans and cost estimates. We do in fact have sufficient information to provide much of this information already. The following sections summarise our current thinking on the requirements and the design choices.