
2006.10.27

  I have been working to fix the PSF modeling in psphot.  The PSF
  model consists of a flux model for an individual object using an
  analytical model with a number of parameters.  For a collection of
  PSF objects, the variation of the parameters as a function of
  position are then themselves fitted with a model.  The PSF model for
  a single object consists of a radial profile with a functional form
  f(z) where a given value of z defines an elliptical coutour of the
  form z = \frac{x^2}{2\sigma_x^2} + \frac{y^2}{2\sigma_y^2} +
  \sigma_{xy}xy.



  The term \sigma_{xy} is difficult to model as a simple function of x
  and y (eg, a low-order polynomial).  

  A better model can be constructed for
  \frac{\sigma_{xy}}{(\sigma_x^{-2} + \sigma_y^{-2})^2}, which varies
  like a^2 \sin 2\theta 

