NAME:
  astterms
 PURPOSE:
  Evaulate the independent vectors to match an astrometric polynomial function.
 DESCRIPTION:

  This function is a support routine to ASTROM and handles part of the
  transformation between pixel (x,y) coordinates and the tangent plane
  coordinates (xi,eta).  The transformation from (ra,dec) to (xi,eta)
  is not handled in this routine.  The premise is that the transformation
  from the tangent plane to pixel coordinates can be done with a polynominal.
  I have implemented all of the common terms found in typical astrometric
  solutions.  In practice, the high order terms are probably not needed
  except for very large fields or for highly distorted fields caused by
  excessive optics.  Most CCD fields can be accurately modeled using just
  the linear terms.

  This function does NOT actually evaluate the transformation.  Instead,
  the indepedent values for the polynominal are computed.  The result is
  an array with (nterms,nvals) elements where nterms is the number of
  non-zero terms and nvals is the number of input x and y values (which must
  be of the same length.  The table below lists the contents of the i^th
  column in the output array.

 CATEGORY:
  Astrometry
 CALLING SEQUENCE:
  val=astterms(x,y,terms)
 INPUTS:
  x     - X - coordinate (scalar or vector)
  y     - Y - coordinate (scalar or vector)
  terms - Which terms are to be built
           This must be a 10 element vector, a 1 means use the term,
                   0 means don't use it.
              0 - const (always use this)
              1 - x     (always use this)
              2 - y     (always use this)
              3 - r
              4 - x^2
              5 - y^2
              6 - x^3
              7 - y^3
              8 - xy^2
              9 - yx^2

 OPTIONAL INPUT PARAMETERS:

 KEYWORD INPUT PARAMETERS:

 OUTPUTS:
  return value - Dependent value(s), if x,y was 1-d then this will be scalar.

 KEYWORD OUTPUT PARAMETERS:

 COMMON BLOCKS:

 SIDE EFFECTS:

 RESTRICTIONS:

 PROCEDURE:

 MODIFICATION HISTORY:
  97/06/17, Written by Marc W. Buie, Lowell Observatory