[daip] IMAGR and UVRANGE - no residuals?

[Eric Greisen]

Adam Ginsburg writes:
 > The image I get from NITER=0 (also, DOTV 1 before it fails) is a
 > reasonable naturally weighted, uncleaned image.
 > 
 > On 1/3/07, Eric Greisen <egreisen at nrao.edu> wrote:
 > > Try telling IMAGR NITER=0 (no cleaning) and take a look at the image.
 > > I am betting that you have an image that is nearly constant.

I'm not sure what you mean by reasonable.  Your beam is 15x12 pixels
in size.  The dirty beam has a peak value of 1.4 Jy (in the corner I
presume) which suggests that some things are not going well in the
imaging.  The corners can get bad with very large numbers of uv
samples being gridded into modestly small images - round-off error
from so many computations, corrected by a large factor in the corners
does make for trouble.  This then messes up the histogram of things.

The messages from IMAGR say that it is trying to load only pixels >
2.75 mJy which is nearly the peak in your image.  It chooses this
level since the next step down in the histogram has more than 20000
pixels which is the limit allowed for cleaning.  Try setting
MAXPIXEL=100000 to see if that helps.

It is possible that your beam histogram is so abnormal that it is
distorting the selection algorithm.  Then, even if the image isn't
completely flat, it may decide that it should only clean a few points
and may them misjudge even that.  Try making your image larger, IMSIZE
of 1024 or 2048, to see if the beam becomes better behaved.

Eric Greisen