[alma-config] short spacing

[John Conway]

Hi,

 This is a very good point I think. All of our complex images 
have very extended diffuse structure in them. I think it would 
be very  useful to have some image  in the opposite extreme, 
consisting of many compact  sources.-  after all 
one  major science goal of  ALMA is very deep imaging of 
continnum emission  from high z  galaxies. 

It would be very useful to have this as a test image - to 
see that when we  improve short spacing converage for the diffuse
source we don't throw out too much imaging quality for other 
images which are collections of compact sources . My wager 
is that the cost is worth it - in that the peak sidelobe 
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.

  Cheers
   John

P.S Such deep field imaging would probably be single pointing
- although the influence of bright sources outside the 
primary beam  is of course a concern, because  the universe is 
isotropic  one would select to look at field away from known bright 
millimeter sources - just as deep VLA surveys are done.


On Thu, 4 Jan 2001, [Leonia Kogan] wrote:

> The short baselines measurments are important for expanded sources with 
> much information at the low spatial frequencies. The sources used at the 
> current simulation (SYG, M51..) are the sources of such a type.
> 
> If the source does not have much contribution at the low spatial frequencies,
> then short baselines are not so important for such sources.
> 
> The clusters of compact sourses is an example of the source with small
> contribution at the low spatial frequencies.
> 
> Is such type of the sources on the line to make a simulation?
> 
> Leonia
>