[mmaimcal] ALMA Memo # 515 Calculation of integration times for WVR

[Al Wootten]

Folks,

I bring to your attention the recently published ALMA Memo entitled:
"Calculation of integration times for WVR" 
File : http://tinyurl.com/4dmby
or http://www.alma.nrao.edu/memos/html-memos/alma515/memo515.pdf
Memo Number: ALMA Memo # 515

Authors:  Alison Stirling, Mark Holdaway, Richard Hills, John Richer

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 microns, 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.

Clear skies,
Al