
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.

