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

[Mark Holdaway]

I think there may be a misconception.

Consider a circularly symmetyric primary beam.  If we
slew while observing (as in OTF mosaicing), then during
an integration time, that circular beam will get convolved
by a boxcar in the direction of the antenna motion.
(Or at least the measured visibilities will be the same
as if we had been using such a perverse primary beam.)
IF we are actually moving at constant velocity for each integration,
then we have a single stretched-out primary beam that we can use
for imaging all the data.  An important thing is that
we are better than Nyquist sampled in the beam, or that the
antenna motion be not more than about 0.3 of a beam.

   -Mark



> Hi Bryan,
> 
>     Of course you are correct.  Maybe the limiting time for phase center 
> updating is
> governed by the change of primary beam on the sky.  So, the phase center 
> update
> should occur less than the change of the pointing by 0.1 beam (probably 
> smaller
> for high dynamic range work).
> 
> Ed
> 
> Bryan Butler wrote:
> 
> >
> > i don't follow this.  why do you care what the magnitude of the phase 
> > center change is between discrete pointings, as long as the instrument 
> > is stable for each "pointing"?  it's just like "point-and-shoot" 
> > mosaicing, just moving the primary beams of the antennas while you do 
> > it, and going fairly fast...  now, if there is an unknown phase shift 
> > across the PB, then there would be trouble, but last i knew mark had 
> > worked out a way to calibrate that out...
> >
> >     -bryan
> >
> >
> > On 2/4/05 13:08, Ed Fomalont wrote:
> >
> >> Hello all,
> >>
> >> 1.  Discrete changing of the phasing while doing an OTF is okay, as
> >> long as the phase change for a source in the beam doesn't change by
> >> more than about 10 deg between phase center updates - whatever
> >> time increment this is for the highest frequency and longest baseline.
> >
> 
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