[daip] clean component filtering

[Eric Greisen]

Andy Biggs wrote:

> By the way, my attempts to experiment with the CC filtering in IMAGR 
> were somewhat thwarted, as I said, by some other unknown problem. I was 
> wondering though, do IMAGRPRM(8,9) have the same meanings as CUTOFF and 
> CPARM(1,2) in CCEDT i.e. does IMAGR sum all clean components within 
> radius IMAGRPRM(9) and reject them if this sum is less than IMAGRPRM(8)? 
> It's not terribly clear from the help which may have a typo:
> 
> (8) If non-zero, select only those Clean components having
>                 > ABS(IMAGRPRM(8)) Jy within a radius of IMAGRPRM(9)
>                 cells of the component.  If IMAGRPRM(8) < 0, the abs
>                 value of the flux near the component is used.
> 
> Is ABS(IMAGRPRM(8)) correct? The next sentence seems to say that ABS is 
> only used when this parameter is -ve.

I think Amy explained how this works - it is looking for weak and 
isolated components.  It sums around a component and drops that 
component only if the sum is too low.  The ABS is correct and required 
in the above sentence because the sign of IMAGRPRM(8) conveys a special 
meaning.  If IMAGRPRM(8) = 0.001 then all components with less than 
0.001 Jy in the IMAGRP(9) area around them will be deleted including 
those with, for example, -100 Jy around them.  If IMAGRP(8)= -0.001
then all components surrounded by -0.001 to 0.001 Jy are dropped but 
seriously negative as well as positive regions are retained.

Some folks use this to deal a bit with the bowls around sources although 
the source itself is in the bowl and so reduced and this algorithm will 
not fix that part of the problem.  Restarting the clean after a 
filtering may encourage it to find more positive/real stuff however.

Eric Greisen