[mmaimcal] Re: phase calibration

[Al Wootten]

Sorry to be tardy in response; I have been away.  I agree with Stephan's
comments in response to Wolfgang's question.  Some related items came up
in various email discussions this week, which I circulate here.

As long as the 90 GHz channel is in use, it might be useful to have
it continuously tuned.  How often will one want to use it (or the WVR system)?
We will want to use it whenever the instrument is not 'phase stable'.
We have defined at one point 'phase stable' as 
better than a radian, and at another, in the goals listed in the project 
book, 0.15 radian at 230 GHz.  D'Addario has proposed updating these:

  1.  Greater than 90% interferometric coherence at 950 GHz (77 fsec
  rms=23 microns), after all calibrations and corrections, on all time scales
  from 1s to 1e4 sec.

  2.  Absolute visibility calibration to 0.1 radian at 950 GHz (16.8
  fsec=5 microns).

Darrel pointed out that 1) is very hard but probably the right goal.
2) comes to 5 microns of path length, which is probably not possible.
Perhaps the goal might be 0.3 radian; but that is still very difficult.
We will discuss this at the 13 Sept. Imaging and Calibration phone meeting.
For the moment, let's take the more stringent project book specification
(acknowledging that Larry's is more stringent yet).  What do we know of the 
site?

For the project book goal, we reach 99% of theoretical sensitivity.  Without
any corrections, we achieve this 30% of the time on 150m baselines, and 1%
of the time on 1.5km baselines, according to MMA Memo No. 169.  If we only
want 50% sensitivity, this is achieved 85% of the time on the shorter
baseline and 43% on the longer baseline.  For 90% sensitivity, the percentages
are 60% and 12%.  If we assume that we are thinking of 230 GHz, I would 
say that fast switching/WVR system will NOT make a significant 
improvement only 1% of the time at kilometer baselines, and will be used for
all observations on those scales.  In the compact array, I would say that
the WVR system would NOT make a significant improvement perhaps 40% of the
time, and would be used just over half of the time.  I would conclude that
for the average observation at 230 GHz or above, in whatever array, the
WVR system or fast switching will be in use.  Below 230 GHz in the compact 
arrays, it might not make a significant improvement in most cases.

If we were to assume that the compact configuration will be used say 25% of the
time, this suggests that we will use fast switching or WVR almost all of
the time.  The 3mm system will nearly always be in
use for fast switching, unless the WVR system works so well it is not needed.
At the moment, I don't think we can say.  Th most conservative course of
action, it seems, would be to plan to have the 90 GHz channel on almost
all of the time.

In the LO engineering meeting where Larry suggested the above, Darrel
reports:
in fast switching to a calibrator, we would probably observe the calibrator
at a different frequency from the main source.   In that
case, the instrumental phase (or rather the changes in the differential
phase between two receivers at different frequencies) wouldn't be
calibrated out. If we fast switched to a different frequency on the
calibrator, then would we cross-calibrate the phase difference between the
different receivers every, say, 10 minutes? Whether we need to hold to 0.1
radians at 950 GHz for a few seconds, or tens of minutes, of course makes
a big difference to the engineering (like the difference between possible
and impossible). 

The strawman model we have been using supposes cross-calibration on ten minute
like scales, as Darrel suggests.

Al