NAME:
  psfstack
 PURPOSE:
  Generate an average numerical psf by stacking observed images.
 DESCRIPTION:

 CATEGORY:
  CCD data processing

 CALLING SEQUENCE:
  psfstack,image,x,y,psf

 INPUTS:
  image - Input image from which psf is collected.
  x     - Scalar or vector list of positions of stars (x coordinate).
  y     - Scalar or vector list of positions of stars (y coordinate).

 OPTIONAL INPUT PARAMETERS:

 KEYWORD INPUT PARAMETERS:
  DW    - half-width of box centered around PSF, final size is 2*DW+1,
             default size is DW=9 pixels.
  SILENT- Flag, if set suppresses all printed output.
  SNRTHRESH - Signal-to-noise ratio threshold.  Any source pointed to by x,y
                 that has a lower SNR than this will be ignored.  The default
                 is 25.

 OUTPUTS:
  psf   - Final average psf, normalized to unit _volume_

 KEYWORD OUTPUT PARAMETERS:

 COMMON BLOCKS:

 SIDE EFFECTS:

 RESTRICTIONS:

 PROCEDURE:

 MODIFICATION HISTORY:
  97/10/22, Written by Marc W. Buie, Lowell Observatory