%N pbder %F $MIR/src/subs/pb.for
%D Get the value of the derivative of a primary beam.
%P rjs
%: image-data
%B
      real function pbder(pbObj,x,y)

      integer pbObj
      real x,y
  ---------------------------------------------------------------------
  Determine the derivative (w.r.t. frequency) of the primary beam
  at a particular location.

  Input:
    pbObj      The primary beam object.
    x,y        Grid location of the position of interest.
  Output:
    pbDer      The primary beam derivative wrt frequency.
