[mmaimcal]Re: Instrumental Delay offsets

[Mark Holdaway]

Just a few isolated comments which address several emails
over the last few days (but before looking at John's most 
recent e-mail):

* Concerning Dick's calculations about the effects of baseline
* errors after fast switching, which will reduce the effect of
* the baseline errors on the phase, but that phase error will
* be slowly varying:

The residual phase errors after fast switching should be mostly
(ie, 90%) random, and hence will average down in a reasonable 
way.  This was considered in setting the 300s drift specification.
The "slowly varying" residuals due to baseline errors won't average
down, so we need to "count" them differently -- ie, we'd like them
to be MUCH smaller than the atmospheric errors.

* Concerning Larry's point about additional measurements to
* help average down the errors:

I think that we should make a paradigm shift based on this comment: we
need to do the 1-hour Conway run to get the baselines determined well
enough to begin observations, but we then use all subsequent high SNR fast
switching calibrator observations to check/update/average-down the
position solutions.  (The SSR people will not smile when we tell this to
them.)

There are TWO Things that go into a specification for antenna
position determination:
   (1) If the baselines are really far off, you get decorrelation over
	the bandpass, you lose sensitivity.  This is not nearly
	as stringent as the specification that John is working
	towards.
   (2) If the baselines are in error, you make a slowly changing 
	phase error after fast switching, as noted by Dick.
	
Thing (1) is permanent, we have to meet that spec before any astronomical
observations take place -- I don't have any equations or numbers at this
moment (at home), but I bet that John's scheme will get the positions much
better than the requirements of Thing (1).

Thing (2) can be corrected after the fact (and is routinely corrected 
after the fact at the VLA).  SO: after reconfiguration, there may be a
delay in the pipeline imaging as we use hours of fast switching
data to refine the position determinations.  ONE PROBLEM: most
observations will be close to transit, making it difficult to
determine some components of the baselines.  BUT: as most observations
are close to transit, we sort of don't have to get those components
that are hard to get.

   -Mark