The output PSF image psfimage is normally a 2D image containing
the image header parameters, "XPSF", "YPSF", "PSFMAG" and "PSFRAD"
which define the centroid, magnitude and size of the PSF model, the
parameters "FUNCTION", "PSFHEIGH", "NPARS", and "PAR#" which define
the analytic component of the PSF, and a single look-up table of
residuals from the analytic fit subsampled by a factor of 2 with
respect to the parent image.
If the DAOPARS parameter varorder = -1, the PSF is fit by the
analytic function and psfimage has no pixel file.
If the DAOPARS parameter varorder = 1 or 2, then two or five
additional lookup tables are computed and psfimage is a 3D image
with 3 or 6 planes respectively. The first two additional look-up
tables contain the first derivatives of the PSF wrt the x and y
positions in the image (varorder = 1), and the next three contains
the second derivatives with respect to x ** 2, xy, and y ** 2
(varorder = 2).
The positions and magnitudes of each of the stars contributing to
the PSF model are also stored in the PSF image header.