[daip] Re: Multi-Resolution Clean in AIPS

[Greg Ball]

Hi Eric,

thanks very much for your detailed answer.  I have a couple more 
specific questions that I hope you can answer.


>         Their algorithm is rather different and should not be much of
> a guide.

I would like to clarify exactly what the IMAGR MR clean is doing and how 
it is supposed to work.  My understanding is the following:

For each field, images are made using a series of beams, beam (i) being 
the "point-source" beam convolved with a Gaussian of FWHM W(i).  For W=0 
the image is cleaned as usual using a delta function and the point-source 
beam; for W>0 the clean components are Gaussians, FWHM W(i).  Each clean 
component is subtracted from ALL the images.

Cleaning an extended source with delta functions produces well-known 
"bed-of-nails" type artifacts, but cleaning an unresolved source with a 
Gaussian produces a "bullseye" artifact.  So at each step we have to clean 
the image which corresponds to the real size scale of the brightest 
emission.  The effect of the Gaussian convolved beams is that a feature of 
some finite size will be increasingly bright, in units of J/beam, for 
larger beams while the beam is smaller than the source, but will then stay 
about the same brightness in Jy/beam as the beam gets still larger.  So we 
want to clean the image where the peak brightness reaches roughly its 
limiting value.

In practice, what we do is weight the peak brightness for each image by 
some power of the beam area, controlled by IMAGRPRM(11).  This ensures 
that the peak-brightness-versus-W(i) curve turns over, and hopefully, that 
the turn over point is the size scale we are after.  We must have 0 < 
IMAGRPRM(11) < 1 so that a) the weights decrease as W increases, and b) 
the initial increasing part of the curve, which should be about linear at 
first, is not "squashed".  (I won't go into interpretation of the other 
IMAGRPARM values since you recommend leaving them zero.)

So much for my understanding of the algorithm.

> The most important thing is to be experimental with it - watch it on
> the TV with DOTV true throughout.

I'm trying this, but it isn't clear to me what sort of intervention is 
appropriate.  Should try to put clean boxes around different 
features at different resolutions as cleaning proceeds, to direct the 
process (rather then just ensuring my original box contains all the 
emission)?  Forcing clean to pick a particular field to clean next?
Changing IMAGRPRM(11) with TELL to get different behavior?

Thanks for any suggestions.

Greg Ball


--
Gregory H. Ball             Harvard-Smithsonian Center for Astrophysics
Graduate Student            60 Garden St MS #10, Cambridge MA 02138, USA
Astronomy Department        Email: gball at cfa.harvard.edu
Harvard University          Office: +1 617 496-4946