[mmaimcal] Re: [alma-sw-ssr] question about frequency/band switching speeds and rates

[Ed Fomalont]

Hi Mark,

    I think you have thought of most everying - in only 10 years.
What model do you use for the phase correlation as a function of
calibrator-target separation?  To first order the rms phase fluctuations
are a linear function of target-calibrator separation, but when the
residual phase fluctuations become larger than about 50 deg, significant
decorrelation occurs.  Also, we find with the VLBA that there are
systematic phase gradients in the sky which persist over many minutes.
These can be determined and removed by observing more than one
calibrator source.  However, the VLBA baselines are much longer than those
for ALMA, so that the phase fluctuations should be dominated by the
short-term, small-angle phase cloudlets over the array.

    Also fitting the antenna-based phase fluctuations for the calibrator 
over
the whole array may be useful, especially for weaker calibrators when 
fitting
a smaller number of parameters to fit the atmosphere delay wedge (and maybe
higher order terms) rather than one value for each antenna, would
give better SNR.  You might be able to see relatively big blobs of 
refraction
moving over the array, and hence apply their effect better to the target
source. 

    If I seem to have a fixation of very weak calibrators, it is
because they do give better phase referencing results.  Also, you might have
to position nod more frequently than 25 seconds so that the interpolated
phase between calibrator observations really does apply to the target.
Of course, finding weak calibrators will be time consuming.

    Ed