[alma-config]misc config comments

[Mark Holdaway]

Sorry I missed the telecon.

I am not at all sure that one wants to have (as Dave Woody suggested), on
average, positive sidelobes over some region of the PSF (like the inner
part of the PSF).

If we observe a source which is just a bunch of low resolution blobs
(or similarly, if we taper the (u,v) coverage or smooth the dirty
image to the low resolution), any region which has, on average,
positive or negative sidelobes will show up as a large sidelobe at
this low resolution.  I think that If we mandate that the sidelobes
all be positive somewhere in the PSF, we will end up with a huge low
resolution sidelobe there and large errors of extended structure will
result.

So, while the approach of iteratively moving out to more distant sidelobes
sounds very good to me, that approach seems to be missing something:
looking at various spacings in the (u,v) plane as well.

Frederic said:

> Thus, in interferometry, a given distribution of
> Fourier samples yielding high sidelobes may allow to
> reconstruct the brightness distribution with high
> precision provided that the uv-plane is sampled with 
> the required accuracy. 

I think this statement is not necesarily true, as images are
constructed via concrete heuristic algorithms.  These algorithms do
not take advantage of any knowledge of the completeness of the (u,v)
coverage, though they make minor use of the PSF sidelobe level.  CLEAN
is more susceptable to sidelobe level than MEM-type algorithms which
change many pixel fluxes simultaneously.  This speaks to the problem:
we may imagine a future imaging algorithm which makes explicit use of
the completeness of coverage of the Fourier plane, and this algorithm
will speak to our desire to have a completely sampled Fourier plane,
but that algorithm doesn't yet exist in the radio-astronomical lexicon.


	-Mark