[alma-config] telecon this Wednesday

[Mark Holdaway]

> 
> Mark,  I am not disagreeing with the fact that earth rotation does little
> to solve the near-in sidelobe problem.  It certainly suppresses the 
> far-sidelobes.  Deep imaging studies with the VLA certainly suggests
> far-sidelobes cause serious imaging problems (ask Frazer). 

Just because far out sidelobes are a problem doesn't mean that near-in
ones AREN'T.  The Briggs problem with marginally resolved sources
was greatly exacerbated by near-in sidelobes, leading to on-source
errors.  These on-source errors are then scattered off-source via the
far out sidelobes.  If you had just near-in sidelobes, and no far out
sidelobes, you would always make beautiful incorrect images with no
off-source errors, ie, you would always be wrong, but you would never know
it.
 
> My point is that the strawperson donut arrays have SMALLER near-in 
> sidelobes than the spiral array Morita-san used for his study -- because
> donut arrays also have tapered uv coverage and the particular sprial
> array used was not optimized -- and it will make an interesting
> comparison.  I would like to shatter this incorrect notion some people
> have that the donut arrays have a much worse near-in sidelobe problem.
> The near-in sidelobe problem is a general one and is not inherent to 
> any particular style of array.

I think the near-in sidelobe levels reported by Morita for the
spiral array are in error -- I believe they are the main beam's
wings rather than sidelobes -- this may be relevant or irrelevant,
I don't know.  I suppose it would make more sense to look at the
deviation from a Gaussian.  While it is true that the near-in sidelobes
of the spiral configuration will not be reduced by earth rotation,
I contend that they start out much smaller than the ring or donut
configurations near in sidelobes.

Similarly, I contend that if Leonia did an optimiazation over a long
integration, the outer sidelobes would be small due to earth rotation
averaging down, so the algorithm would concentrate more on reducing the
inner sidelobes via structural means, ie, by moving antennas.

> No one has yet to offer a concrete reason why Leonia's snapshot 
> optimization is flawed.  While earth rotation clearly has an impact
> on the imaging quality due to changes in the far-sidelobe levels,
> I don't know of any telescope TAC that has yet to grant extra observing
> time so the imaging quantity can be improved by a factor of 2.  

Not in so many words, but I think long tracks are often granted at the
VLA, not for sensitivity, but for imaging quality.  But it is done with a
wave of hands, not by looking in tables or on graphs.

> In other words, earth rotation synthesis is a relevant imaging issue
> (and we should warn the users about this as much as other
> critical issues such as the thermal and atmospheric noise), but it
> is also a science issue that is outside our considerations.  We are 
> not going to impose a 4 hr earth rotation synthesis on any project
> that does not require it (from the raw sensitivity requirement)
> only because the configuration designer
> determined this mode produces the best imaging characteristics.
> Personally I like what you have done in the project book, namely
> offering the amount of HA coverage that is required to sample
> some fraction of the uv cells.  No matter what the configuration scheme
> we adopt in the end, we should provide this information (i.e.
> what HA coverage is needed to sample 40 or 50% of uv cells for
> "good" imaging), just the same way we we inform people of the 
> sensitivity and resolution achievable.

OK, there are three issues for hour angle range:
	1) fraction of filled cells, which translates to image quality
	2) sidelobe level, which translates to image quality
	3) noise as a function of elevation angle, which translates
		to sensitivity as a function of observing time.

Memo 201 did not say anything about fraction of occupied cells, it looked
at hour angle ranges as a function of declination and observing frequency
(I think...), considering the increase in noise due to atmospheric
opacity.  So, that would be the relevant MAXIMUM hour angle range for
very long integrations (set by SENSITIVITY requirements...ie, multiple
transits).  For the smaller arrays, we may CHOOSE to optimize for
less than the maximum hour angle range, as we may say that snapshots
are more important.


> 
> Since we apparently we both missed the telecon, you will have to make
> your point at our next telecon.  What Morita-san has shown is that
> point error can severely impact imaging.  The same is true for a number
> of other things such as atmospheric phase noise and gain fluctuations.
> Perhaps you or some other brilliant person will someday
> produce a software solution so that pointing errors are properly
> accounted for and thus does not limiting the imaging.
> 
> 

I thought the telecon is NOW.  Well, I'm more out of it than I thought!

	-Mark