Atmospheric phase correction for ALMA with 183 GHz water vapour radiometers

One of the great challenges for ALMA is overcoming the natural limits set by the turbulence in the atmosphere to achieve resolutions as fine as ten milli-arcseconds. A critical component in the strategy to achieve this are mm-wave radiometers on each of the 12 m diameter telescopes that observe the emission from the atmospheric water vapour line at 183 GHz. The information from these radiometers can be used to compute the fluctuations in total water vapour along the line of sight of each telescope, and from this, the fluctuation in effective path to each antenna. The estimates of path fluctuations are then used to phase-rotate recorded visibilities leading to much increased coherence. In this paper we briefly review the design of the radiometers, describe the software processing steps to derive phase corrections and show some of the first results from the ALMA site in Chile.