NAME:
	star_fun

 PURPOSE:
	Single 2-d gaussian image, support routine for STARFIT.

 DESCRIPTION:
	Evaluate the sum of one 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

		R  = SQRT( (X-A0)^2 + (Y-A1)^2 )

		Z  = R/A3

 		F(I) = A2*EXP(-Z1^2/2) + 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,

 CATEGORY
       Function fitting

 CALLING SEQUENCE:
	STARFIT_FUN,I,A,F[,PDER]

 INPUTS:
	I = Independent variable, actually, just an index into 2-d array.
	A = Parameters of equation described above.

 OPTIONAL KEYWORD PARAMETERS:
	XSIZE = width of image

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

 OPTIONAL OUTPUT PARAMETERS:
	PDER = (N_ELEMENTS(I),9) array containing the
		partial derivatives.  P(I,J) = Derivative
		at Ith point w/respect to Jth parameter.
 COMMON BLOCKS:
	starfit_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