here are a set of comments from mark holdaway on my review of 
stephane's memo 372.  i haven't formalized the header
on it yet - oh well...

	-bryan


> "Bryan Butler"  wrote:
>    'decorrelation estimates' are included in item 6.  If we have
>    data that is significantly decorrelated on the timescale of 
>    fast switching or WVR calibration, then we are in real trouble.
>    That is the point of having those calibrations - to avoid the
>    decorrelation, i.e., make it possible to track accurately the 
>    phase variations on short timescales and correct them in the data.

 There will be decorrelation in fast switching.  In order to select
 the details of how to fast switch, I optimize the sensitivity of
 the observations, including time lost to the calibration process and
 sensitivity lost to decorrelation (caused by residual atmospheric errors
 and to some extent thermal errors in the calibrator's phases).
 You can see a tradeoff here:
 you can calibrate like crazy to get the residual phase errors very small,
 but will then have low sensitivity because you only spend 10% of the
 time on target source -- conversely, you could spend most time on target,
 and get a poor calibration with lots of decorrelation.

 I have stated this approach for several years, and have asserted the
 need for a decorrelation correction in fast switching.  Perhaps that
 is the wrong approach, but people have not taken issue with it in the
 past.


> p.7: In discussion on quasars as primary calibrators: 'At the
>    Chajnantor site, given the variety of hour angle and declination,
>    one of them will be available at anytime for bandpass calibration.'
>    We may find that we want the bandpass calibrator near the source
>    of interest, not just anywhere in the sky.  This is certainly the
>    case at the VLA.  It will depend on the stability of the bandpass
>    with time, temperature, antenna motion, etc...

 The VLA did observe for its first 15 years not caring where on the sky
 the bandpass calibrator was located, and now only demanding 
 observations need to worry about where the bandpass calibrator is.

 At the VLA, where actual astronomers create the observation schedules,
 there is often no quantitative understanding of what is required in
 terms of calibration, and astronomers, being conservative creatures,
 will opt for over-calibration, determining the instrumental parameters
 to much higher accuracy than is required.  

 Given the more demanding calibration environment of ALMA (ie, more 
 calibrations need to be made), we likely will need to optimize each 
 calibration, and not over-calibrate by too much, in order to have 
 time on source.