NAME:
	plch_fun

 PURPOSE:
	Two 2-d gaussian images, support routine for PLCHFIT.

 DESCRIPTION:
	Evaluate the sum of two 2-d guassian and a 2-d 2nd order polynomial
	and optionally return the value of it's partial derivatives.

 	Evaluate the equation y=f(i) where:

		X  = I MOD NX
		Y  = I / NX

		R1 = SQRT( (X-A0)^2 + (Y-A1)^2 )
		R2 = SQRT( (X-A0-A6)^2 + (Y-A1-A7)^2 )

		Z1 = R1/A3
		Z2 = R2/A3

 		F(I) = A2*EXP(-Z1^2/2) + A5*EXP(-Z2^2/2) + A4

	Function parameters
       --------------------------------------------
	A0  = X location of center of Pluto
	A1  = Y location of center of Pluto
	A2  = height of gaussian for Pluto
	A3  = the 1/e width of the guassian
	A4  = Constant term for the background
	A5  = height of gaussian for Charon
	A6  = X location of center of Charon relative to Pluto
	A7  = Y location of center of Charon relative to Pluto

 CATEGORY
       Function fitting
 CALLING SEQUENCE:
	FUNCT,I,A,F[,PDER]
 INPUTS:
	I = Independent variable, actually, just an index into 2-d array.
	A = PARAMETERS OF EQUATION DESCRIBED BELOW.

 OPTIONAL KEYWORD PARAMETERS:
	XSIZE = width of image

 OUTPUTS:
	F = value of function at each F(I).

 OPTIONAL OUTPUT PARAMETERS:
	PDER = (N_ELEMENTS(I),8 or 6) array containing the
		partial derivatives.  P(I,J) = Derivative
		at Ith point w/respect to Jth parameter.
 COMMON BLOCKS:
	r2gauss_com - contains the x-width of the array stored in A
 SIDE EFFECTS:
	NONE.
 RESTRICTIONS:
	NONE.
 MODIFICATION HISTORY:
	Written by Marc W. Buie, Lowell Observatory, 1993 January 13
	93/07/01, MWB, changed to remove linear and quadratic background
	   terms, added facility for forcing position of Charon.
  2000/11/08, MWB, removed use of obsolete ()