Index: trunk/psphot/doc/psfmodel.txt
===================================================================
--- trunk/psphot/doc/psfmodel.txt	(revision 15000)
+++ trunk/psphot/doc/psfmodel.txt	(revision 15017)
@@ -1,2 +1,13 @@
+
+2007.09.25
+
+  The PSF model has parameters which vary across the image.  This 2D
+  is can be represented as either a polynomial (ordinary or chebychev)
+  or as an image map.  The user requests the maximum X & Y scale
+  (PSF.TREND.NX & PSF.TREND.NY) for the variations, and
+  psphotChoosePSF / pmPSFtry attempt to find the best choice for the
+  scale.  For the polynomial representations, NX and NY are the order
+  of the variation.  For the image map, NX and NY are the size of the
+  image map.
 
 2007.09.21