[daip] Re: OGEOM problems

[Eric Greisen]

Lawrence Rudnick writes:
 >   Eric - Thanks for your note.
 > Here are the results of what I assume you'd agree is a fair
 > test, viz a beam of width 45" (gaussian, immod) with a sampling
 > of 15".  Shift -.5,-.5 pixels followed by +.5, +.5 shift.
 > As you'll see, OGEOM makes MAJOR errors in this,
 > 8.5% problems at the peak, a bias in shifting the image by almost
 > half a pixel, etc.
 >     I urge you most strongly to take remedial actions of warnings,
 > changes of default for reweight, and longer term, rewrite of
 > interpolation. 
 >    Over the years, as you know, my students and I have made
 > multiple tests for systematic problems.  It never entered our
 > mind that NRAO would have allowed such a simple utility
 > program to generate such large errors.  Obviously, our
 > mistake.

    Perhaps I am just tired but I am rather insulted by this remark.
There has never been any deliberate attempt on the part of any member
of the AIPS group to defraude the astronomical community, yet say NRAO
as a whole.

I may look at this - and then again I may adopt the attitude that I do
not look at things for angry people...

Eric Greisen

 >    lr
 > 
 > 
 >    Difference image  ORIGINAL - SHIFTED TO&FRO
 > Original peak  1000 on this scale, at pixel  31, 31 (see below)
 >            25             30             35
 >   39    0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0
 >   38    0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0
 >   37    0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0
 >   36    0  0  0  0  0  0  0 -1 -1 -1  0  0  0  0  0  0  0
 >   35    0  0  0  0  0  0 -2 -5 -5 -4 -2 -1  0  0  0  0  0
 >   34    0  0  0  0  1  0 -9-17-14 -8 -5 -2 -1  0  0  0  0
 >   33    0  0  0  1  5  3-16-19  9 14 -2 -5 -2  0  0  0  0
 >   32    0  0  0  3 12 11-17  4 85 79 14 -8 -4 -1  0  0  0
 >   31    0  0  0  4 17 14-35-21 85 85  9-14 -5 -1  0  0  0
 >   30    0  0  0  3 13  6-55-84-21  4-19-17 -5 -1  0  0  0
 >   29    0  0  0  1  5  1-31-55-35-17-16 -9 -2  0  0  0  0
 >   28    0  0  0  0  1  1  1  6 14 11  3  0  0  0  0  0  0
 >   27    0  0  0  0  0  1  5 13 17 12  5  1  0  0  0  0  0
 >   26    0  0  0  0  0  0  1  3  4  3  1  0  0  0  0  0  0
 >   25    0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0
 >   24    0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0
 >   23    0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0
 > 
 > Original image, same amplitude scale
 >             25             30             35
 >   39    0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0
 >   38    0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0
 >   37    0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0
 >   36    0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0
 >   35    0  0  0  0  0  0  2  5  7  5  2  0  0  0  0  0  0
 >   34    0  0  0  0  0  4 18 46 62 46 18  4  0  0  0  0  0
 >   33    0  0  0  0  2 18 85214292214 85 18  2  0  0  0  0
 >   32    0  0  0  0  5 46214540735540214 46  5  0  0  0  0
 >   31    0  0  0  0  7 62292735+++735292 62  7  0  0  0  0
 >   30    0  0  0  0  5 46214540735540214 46  5  0  0  0  0
 >   29    0  0  0  0  2 18 85214292214 85 18  2  0  0  0  0
 >   28    0  0  0  0  0  4 18 46 62 46 18  4  0  0  0  0  0
 >   27    0  0  0  0  0  0  2  5  7  5  2  0  0  0  0  0  0
 >   26    0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0
 >   25    0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0
 >   24    0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0
 >   23    0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0
 > 
 > >Your mistake is to interpolate images by simple interplation means
 > >when they are udersampled by an order of magnitude (min is 2.4 pixels
 > >per cell in X and in Y or about 6 in area) and they can be
 > >interplaoted correctly only by FFT means).  OGEOM, LGEOM, et al are
 > >simple interpolators and if you put all the flux in one pixel and then
 > >shift by a half a correct result would be 0.0.
 > >
 > >I agree that some warning about undersampled images is needed.  But I
 > >would argue that if this is important to your science the onus was on
 > >you to test things.
 > >
 > >Eric Greisen
 > >  
 > >