[daip] Re: OGEOM problems

[Lawrence Rudnick]

  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.
   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
>  
>