[alma-config] simulated imaging test

[John Conway]

Hi,

 I think that doing it the way you have proposed, with a single
image for all reolutions will not be a useful exercise. In your 
C-array simulation the model image is too simple, i.e. has too few
beam areas across the image, to distinguish between the imaging 
properties of the different array designs. I predict that at this 
level of complexity the two array designs will give very similar results.

I think that in order to test the different arrays one wants to use
the most complex images consistent with single pointing imaging. Is
the proposed C-array image of Cygnus A on your web page really the most 
complex image one can imagine coming out of non-mosiaiced imaging 
in C-array with ALMA? Surely that is what we want to have?

I believe its essential to have complex images with 4 or 5 pixels 
per beam at each array tested and 100 - 200 beams across - as we
agreed in Tuscon - for there to be any point to doing the C-array 
simulations (which are most important becausue they are  a proxy in fact 
for 3 different arrays B,C and D). After all from a physical point of
view as one
goes to lower reolution if anything we expect to have more filled 
beam areas because of imcreased brightness temperature sensitivity.
There is I agree a practical limit right now because we cannot easily do 
simulations of  mosiacied images, but we should chose the 
most complex images consistent with being in the regime which 
we can easily simulate. 


 Cheers
   John


P.S. I have put a corrected version of the A-array pad 
positions on my web site.







  


 

On Fri, 28 Jul 2000, Min Yun wrote:

> I have conducted a small set of imaging tests on Cygnus A model in
> order to see if we have everything we need for imaging simulations
> and all in correct format.  You can find my initial findings at:
> www.aoc.nrao.edu/~myun/mma/imaging/sim.html.
> 
> This is a demonstration of the feasibility and is not meant to be 
> a rigorous test of the imaging capability of the strawperson
> arrays.  The same input model image was used for all different 
> simulated observations. The comparison images are
> constructed by convolving the input image with the synthesized 
> Gaussian beams of the simulated observations -- as I discussed
> in my earlier e-mail. All images are shown with logarithmic 
> stretch in order to bring out the low level extended features.
> 
> I find little evidence for gridding/sampling effects in the simulated 
> observation images. One practical issue I have come
> across is that all model images needed to be scaled up by some 
> factor (numbers of pixels per beam as well as the ratio in area
> between the model and the reconstructed beam) because the unit of 
> the input image is in "Jy/beam" rather than "Jy/pixel"
> -- the simulated and reconstructed images are expected to be 
> in "Jy/beam" unit as well. We may need to worry about any
> error arising from computing the beam area in numbers of pixels 
> during the comparison between the model and simulated image.
> 
> Please comment on what I have done here.  Any suggestions to improve
> the process are welcome.
> 
> 
> 
> 					-- Min
>