[daip] how does uvsub really work?

[Eric Greisen]

Hanno Spreeuw writes:
 > Hello, I do not really understand how uvsub works (in default mode). I
 > understand that one can transform a model of clean components to the uv
 > plane.
 > That has to be subtracted from the input uv dataset. How can you subtract
 > visbilities from each other without getting negative visibility
 > amplitudes?
 > Or are they subtracted as complex numbers?

But of course they are complex numbers.  The low level routines
compute the Fourier transform of the CC model at the locations of the
input data and do something with it (add, subtract, divide).  The same
routines are used throughout AIPS by IMAGR, CALIB (w models), etc.

Eric Greisen