[daip] synthesized beam area

[Eric Greisen]

Gary A. Welch writes:

 > Task BLSUM outputs mean flux per beam in the blotch region (I'm using a dirty 
 > cube).  I want to convert to total flux in the blotch region.  So I need a good 
 > value for the "beam area" used by BLSUM.  
 > 
 > The only info which seems potentially useful are the values of "Conv size" in 
 > the image header.  Is the "beam area" just the area of the ellipse defined by 
 > those two values?  Is there a better way?
 > 

If you sum the image over the pixels, then you need to divide by the
beam area which is 1.1331 * Major * Minor where Major and Minor are in
cells.

This is a sumewhat dangerous thing to do on a dirty image.  The beam
area is in fact 0 if you did not put in a zero spacing and the weight
of that zero spacing more or less if you did.  In a Clean image we
have done a model and installed the model components with a known,
non-zero beam given by the above formula.

The reality is that - if you observed a source s.t. you saw all its
flux and it covers a very limited area then you can get that flux this
way.

Negative bowls around (and covering) sources due to missing short
spacing can easily contain the same but negative flux as the positive
bits.

Eric Greisen