[daip] Re: Multi-Resolution Clean in AIPS

[Eric Greisen]

Greg Ball writes:
 > Hi Eric.
 > 
 > >    Actually it gets it wrong all the time - no extended source is
 > > round - so we depend on Clean being iterative and working to undo its
 > > mistakes.  What we try for is to keep the early mistakes small since
 > > they are hard to undo in a reasonable time.
 > 
 > Given this way of thinking about it, I am wondering:
 > 
 > Do we expect all the resolutions to produce mostly positive components,
 > or only one?

    With a modest loop gain (0.1) most will be positive most of the
time but the negative components are needed.

 > 
 > When using the clean components as a model, e.g. in CALIB or VPLOT, how 
 > should we choose NCOMP?  It seems that choosing NCOMP by inspecting the 
 > list of components on each field might lead to an overall inconsistent set 
 > of components;  To ensure consistency, is it safe to use a flux cutoff in 
 > this case, or is taking *all* the components necessary?

     The problem with FLUX in this case is that it means different
things in the different fields.  And one certainly does NOT want to
cut off at the first negative.  I usually try to control the earlier
Cleans so that the self-cal can use all components and on later
iterations just Clean more deeply.

 > 
 > > The extended source parts mostly cut down on the bed of nails errors
 > > and somewhat fill in the negative bowls that surround objects when
 > > there are too few short spacings.
 > 
 > Is it your experience that these corrections to the image have a big 
 > impact on self-calibration?

I honestly do not know.  I would suspect that the bed-of-nails does
affect the self-cal and the negative bowl will bother the short
spacings.  But the bed-of-nails and the bowl are both measures of the
lack of (missing) data allowing structures we "know" to be wrong - so
the impact should not be great although iot can impact the science...

Eric Greisen