ALMA
ALMA is a major new ground-based radio telescope, comprising a large array of 12-m antennas operating at millimetre/submillimetre wavelengths. ALMA is being constructed on the Chajnantor plateau of the Atacama Desert in the Chilean Andes at an altitude of 5000 m. It is a major international collaboration and is due for its Early Science Phase at the end of 2011 before full array operations begin in 2013.
Advanced phase calibration techniques
Correcting phase fluctuations, which arise mainly from the water content of the atmosphere, will be essential for ALMA. Without such correction, ALMA will have limited sensitivity and angular resolution. A small number of people from the Cavendish Astrophysics Group work on developing phase correction strategies using data from the water vapour radiometers (WVRs), which will be installed into every 12-m ALMA antenna. The work is funded through the European Commission's Framework 6 Programme as an Enhancement to Early ALMA science.
I joined the team in April 2009 and started by looking at the effects of the dispersive path delay on phase correction (ALMA Memo 590). Supporting materials including scripts to reproduce all of the memo's results are posted on their own page. Substantial work on WVR phase correction had already been carried out before I arrived. Bojan Nikolic has developed a Bayesian framework to extract the optimal phase correction coefficients from the WVRs. This easily incorporates our prior knowledge of the atmospheric conditions and any observational data. Bojan's webpages or the new project pages are the best places to find out more information.
We're currently analysing the copious quantities of WVR test data recently taken (Jan-March 2010) at the high ALMA site.
Atmospheric temperature profiler
50% of my time is dedicated to the ALMA atmospheric temperature profiler. This will use measurements of the sky brightness, around the 60 GHz oxygen line complex, to infer the temperature profile of the atmosphere up to a ~10 km above the ALMA site. Our task is to specify, test and integrate the instrument into the ALMA observatory. The retrieved profile is likely to be useful to a number of ALMA sub-systems, not least for the phase correction procedures we're developing using the water vapour radiometers (see above). We're on our first testing run of the profiler at the ALMA site in Chile during August 2010. Bojan Nikolic delivered a talk at the ALMA operations support facility which explains a little more about the profiler (download the pdf).
ALMA Site
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