NAME:
  astchi1
 PURPOSE:   (one line only)
  Astrometric goodness-of-fit for one image based on rotation and offset
 DESCRIPTION:
 CATEGORY:
  Astrometry
 CALLING SEQUENCE:
  chisq = astchi1(x,y,xpred,ypred,xoffset,yoffset,rotang,alpha)
 INPUTS:
  x       - Measured X position of an object, this is in raw chip coordinates
  y       - Measured Y position of an object, this is in raw chip coordinates
  xpred   - Predicted X location of the object
  ypred   - Predicted Y location of the object
  xoffset - Offset to apply to X positions
  yoffset - Offset to apply to Y positions
  rotang  - Rotation to apply to X coordinates (applied after rotation)
  alpha   - 2-element vector that gives the relative weighting between
              the chisq values for the X and Y axes.  [1.,1.] would apply
              equal weights.  [1.,0.] would return just the X chisq, and
              so on.
 OPTIONAL INPUT PARAMETERS:
 KEYWORD INPUT PARAMETERS:
 OUTPUTS:
  return value is a scalar number giving the goodness-of-fit statistic
 KEYWORD OUTPUT PARAMETERS:
 COMMON BLOCKS:
 SIDE EFFECTS:
 RESTRICTIONS:
 PROCEDURE:
 MODIFICATION HISTORY:
  Written by Marc W. Buie, Southwest Research Institute, 2010/01/13