Abstract:In this memo we address the issue of how to apply water vapour radiometer estimates of atmospheric phase to visibility data. We consider the impact of smoothing the radiometer data over a period of time to reduce the noise in the w.v.r. estimate, and applying a multiplicative factor to decrease the impact of the w.v.r. estimate when the phase fluctuation amplitude is small compared with the radiometer noise.
We find that when fast-switching is taken into account, for fully three-dimensional turbulence, and r.m.s. path length fluctuations of order 75 micros', the optimal smoothing timescale is 11 seconds. This timescale decreases to around 3 seconds as the thickness of the turbulence layer becomes small compared with the baseline length. These values are found to decrease as the r.m.s. fluctuations increase. A multiplicative factor is required to modify the w.v.r. correction term for fluctuations less than ~ 50 microns', where the noise in the radiometer becomes comparable to fluctuations at the site.
View a pdf version of ALMA Memo #515.
Last modified: 2005-03-03