NAME:
  fn_hg1

 PURPOSE:
  Evaluate the 1-parameter Henyey-Greenstein phase function.

 DESCRIPTION:

 CATEGORY:
  Mathematical

 CALLING SEQUENCE:
  fn_hg1, g, a, F, /radians

 INPUTS:
  g:  Phase angle(s) in degrees.
  a:  Asymmetry parameter (negative is back, positive is forward).

 OPTIONAL INPUT PARAMETERS:

 KEYWORD INPUT PARAMETERS:
  radians:  Flag, if true, phase angle is in radians instead of default degrees.

 OUTPUTS:
  F:  The value of the function at each g.

 KEYWORD OUTPUT PARAMETERS:

 OPTIONAL OUTPUT PARAMETERS:

 COMMON BLOCKS:
  None.

 SIDE EFFECTS:
  None.

 RESTRICTIONS:
  None.

 PROCEDURE:
 MODIFICATION HISTORY:
  1999-Nov-13 Created by Will Grundy
  2001-Oct-29 WMG added radians keyword