Phase compensation schemes are needed for sub-mm and mm interferometry if we are to obtain high resolution images with the proposed ALMA telescope. The presence of fluctuating water vapour in the atmosphere is the main culprit as it effectively distorts the phase of the signals arriving at each element of the interferometer.
There are two possible methods of correcting for this phase distortion. The first involves fast position-switching between the astronomical source and a calibrator of known phase. The second uses a radiometer close to the antenna to measure the amount of water vapour along the line of sight. The expected phase distortion is then calculated and used to correct the astronomical signal. No matter which technique is used, it is important to know the altitude at which most of the fluctuating water vapour is to be found, so that we can set limits on the offset between the astronomical and calibration beams. If the calibration beam does not sample a similar column of water vapour as the astronomical beam, then the phase correction will be inaccurate.
We describe a method for obtaining the height at which the bulk of the turbulence occurs, using two site-testing interferometers pointing in different directions. The time-lag between the interferometers is found by cross-correlating the phase data and then converted to a height using measurements of wind speed and direction.
Our results show that much of the turbulence is close to the ground, usually within the first 500 m. This gives confidence in the use of Water Vapour Radiometers (WVR) as a means of phase compensation, as there would be a high degree of overlap between the astronomical and calibration beams.View a PDF version of ALMA Memo #345.
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