Index: /trunk/psphot/doc/2Dmodels.txt
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+
+I am testing a new concept for the 2D representation of various values
+measured by psphot.  I have been using 2D polynomials to fit the
+variations of, eg, the psf parameters as a function of position in the
+images.  this has been somewhat successful, but it has also been
+fraught with a number of problems.  the worst issue has been the fact
+that the polynomial form is not physically motivated.  in some cases,
+eg the variation of the psf shape parameters, the true variations for
+real distributions are not well represented by a polynomial.  I have
+done some ad hoc tricks to get the functional form to look roughly
+like a polynomial, but this is really quite a hack.  the other problem
+is that it is difficult to decide if the polynomial does an acceptable
+job representing the variations.
+