NAME:
	starfit

 PURPOSE:
	Single gaussian fit to a stellar image.

 DESCRIPTION:
 	Fit 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(-Z^2/2) +
			A4 + A5*X + A6*Y + A7*X^2 + A8*Y^2

	Function parameters				Initial guess
       --------------------------------------------    --------------------
	A0  = X location of center of Pluto,		Maximum in image
	A1  = Y location of center of Pluto,		Maximum in image
	A2  = height of gaussian for Pluto,		Max - Min from image
	A3  = the 1/e width of the guassian,		GUESS
	A4  = Constant term for the background,		0
	A5  = Linear term in X for the background,	0
	A6  = Linear term in Y for the background,	0
	A7  = Quadratic term in X for the background,	0
	A8  = Quadratic term in Y for the background,	0

 CATEGORY:
	Function fitting

 CALLING SEQUENCE:
  starfit, image, guess, model, a, sigmaa

 INPUTS:
	image  - Input image to be fitted, must be 2-d.
	guess  - Input information for starting guess.
		  guess = estimate of the 1/e full width of the seeing.

 OUTPUTS:
	model  - The fitted model image is returned.
	a      - The final coefficient array.
	sigmaa - The uncertainties on the coefficients.

 OPTIONAL OUTPUT PARAMETERS:

 COMMON BLOCKS:
	starfit_com - contains the xwidth of the image.

 SIDE EFFECTS:
	None.

 RESTRICTIONS:

 PROCEDURE:

 MODIFICATION HISTORY:
	Written by Marc W. Buie, Lowell Obs., 1993 January 13.