The science:

Producing an image of an astronomical object by using an array of telescopes like the Australia Telescope Compact Array (ATCA) relies on being able to identify a radio wavelength wavefront as it reaches each of the antennae in the array. However, as cosmic radio signals pass through water vapour in the earth's atmosphere, they are delayed slightly. Moreover, the water vapour in the atmosphere is not uniformly distributed. Water vapour inhomogeneities have scale sizes ranging from centimetres to kilometres, and winds in the atmosphere mean that the distribution of water vapour is continually changing. This means that individual antennae in an array of telescopes like the ATCA view an astronomical object through different and changing amounts of water vapour, making it harder to identify a specific wavefront as it reaches each antenna. The differential, time variable, phase distortion results in a lower-quality image of the object being observed than would otherwise be possible.

The aim of the atmospheric phase correction project is to develop an instrument that will measure the amount of water vapour between each antenna and the target object being observed, to compensate for the water vapour induced delays in the received signal, and thus to produce higher-quality astronomical images from the array.

The technology:

The ATNF has already developed a successful 225 GHz water vapour radiometer (WVR) designed to sense the water vapour through which each antenna is viewing its target astronomical object. During 1997-1998 the 225 GHz WVR has been installed at the Australia Telescope Compact Array site, providing the atmospheric opacity and stability data needed for effective scheduling of high-frequency ATCA observations. Recent research (especially results from Owens Valley Radio Observatory in the US) has indicated that better phase correction may be achieved using a detector system at 22 GHz; such a system is currently being designed for the MNRF upgrade project. The signal detected around the water vapour line at 22.5 GHz will be divided into several frequency channels, allowing one to measure line emission separately from continuum emission. One can then distinguish atmospheric water vapour emission (which has the greatest effect on the phase of the signal received by the antenna) from emission from water droplets in clouds, ground emission and various other sources that are less important for atmospheric phase correction. Using the 22 GHz system, it is hoped that high-frequency observations will be possible through light cloud as well as on clear nights, thus greatly increasing the observing time available.

The 22 GHz system will make use of the astronomical receiver and feed first stages. While the 3 mm receiver system is on-axis and receiving astronomical signals, tests have shown that enough radiation will enter the off-axis 22 GHz feed horns to enable atmospheric phase correction measurements to take place simultaneously with the high-frequency observations. Separate, high-stability electronics for the phase correction system will be built and located in a temperature stabilised housing attached to the receiver dewar. The detailed design of the ATCA water line monitor at 22 GHz has begun and a prototype system is scheduled for delivery by late 1999.

The benefits for Australia:

Atmospheric phase correction is a topical area of radio science, promising to allow imaging by the putative next-generation Chilean millimetre-wave arrays at frequencies approaching 1 THz (1000 GHz). There has been considerable international pressure to develop a viable mm-wave phase correction scheme for the new and proposed sub-mm interferometers around the world. Scientists and engineers from the ATNF have been involved in several international collaborations, including one charged with the development of calibration strategies for the new arrays. Such collaborations help to maintain Australia's position as a world-leader in radio astronomy and lead to fruitful exchanges of ideas and new technologies of benefit to all parties in the collaboration.