[alma-config] short spacing

[Mark Holdaway]

> level only increases moderately as the short spacing 
> coverage is improved  - but it would be nice to see this
> demonstrated  (or not as the case may be).
> 
> 
> Tommy Wiklind here at Onsala has done some simulations of what SCUBA
> fields would look like at ALMA resolution at 1mm. I guess others
> must have made such simulations (I know Min is an expert in this 
> field perhaps he has also something to hand?).  If not I have a
> simple model callled 10000DOTS which is just a random  distribution
> of 10000 point sources drawn from a N \propto S^-3/2 cumlative
> distribution down to some limit. We could easiliy include this in the 
> simulations that Steven is doing. Since its just a CC file its 
> very quick to create dummy data using UVCON and CLEAN it. I doubt 
> from the imaging point of view the exact details of the flux 
> cutoff or power law exponent are vitally  important  - the main 
> point is to include -something- as an antidote to all the other 
> diffuse emission dominated sources - and to represent a imaging
> area of ALMA science which is not at all  yet represented in the 
> test set.

The exponent may actually be of concern.  A flat exponent
will basically drive most of the weak sources to a level below the
deconvolution error limit, effectively reducing the number of imagable
sources and the source complexity.  A steep exponent will put that
"sea of weak sources" at a higher flux level, and at 10,000 sources,
there will be one in each beam (for a 256 x 256 image), so this would
qualify as a complex source.

I don't have a strong opinion about what is more appropriate, so I
would advocate the s^-3/2.

	-M