[alma-config] Difference images for the simulations using the SIL

[John Conway]

Hi Steven (et al),

 I had a quick look this afternoon (the website was not accesible this
morning). It seems to me that the final approach of first subsampling the
model  image -then  creating uv data from that seems the best approach. We
know this approach should work, because it worked for A-Array.

In the above approach the same map is used to generate uv data 
is also used as the final comparison map. Otherwise if one
creates the uv data with the full resolution model, maps it for C array 
with cellsize 4 times bigger than the original model, then 
the  subsampled comparison  map has to be generated by another program
from the full resolution model - using SUBIM which introduces 
registration problems. I think the  from a philosphical point of 
view one whould do a minimum of different preocessing to the 
image and uv data streams before forming the difference.
Unfortunately this new approach means re-gerating the uv data 
and remapping the B,C,D array simulations, but that should go
fairly automatically. 

On the issue of metrics rather than waiting for us all to agree 
on a final metric I suggest you should go ahead and do the
simlpest things first(!)  Pure difference
images side by side of the two arrays will 
give us a first order idea of if there is any difference in 
imaging quality at all and what the nature of any difference 
is. Fractional errors on the bright regions are also easy to 
compute - you could start with simply dividing the difference 
between the difference and the true image, setting the 
'magic-blanking' cutoff at say 1% of the peak brightness.
This would be just to get a quantitative idea of what the sizes of 
the  'on-source' fractional errors are, these 
can then be compared to other sources of errors (pointing, phase
etc etc). For a final comparison we may need more metrics 
(e.g 'off-source' dynamic range) or fractional errors at 
different model cutoff levels  but I think if we can look at the 
first order results we will be much better placed to discuss this
(at the next telecon perhaps).

  John.

P.S I've been busy with teaching for the last 2.5months, 
but I intend to work on ALMA configuration stuff in the next couple
of weeks, write up my strawman as a memo etc.

P.P.S Perhaps another telecon in the next few weeks 
would be useful to discuss Steven SIMs and get the
configuration  ball rolling again and agree a new timetable
for decining the configurtion issue etc tec.


 

 

On Wed, 8 Nov 2000, Steven wrote:

> Hello. Of late I have been wondering what the requirements, and indeed
> definitions, are for fractional difference maps using the CLEAN images and
> convolved models. More immediately my concern has been how to even create
> simple subtractive difference maps for array resolutions other than A, where
> the model and final CLEANed image are different sizes and have different
> pixel scales. Registration of these maps has proved problematic due to
> downsampling, windowing etc., and to show where I am at, I have put together
> a page on my website which demonstrates the current state of the art
> graphically. This may be found by following a link in the 8th November news
> item at the top of my usual index page at
> http://www.stevenheddle.co.uk/ALMA/ALMA_IND.HTM
> 
> I would be grateful if you could have a look at it.
> 
> Cheers,
>     Steven
>