----- file created by makedoc on: Tue Apr  1 15:29:52 2008
----- Documentation for /scisoft/xop2.3/extensions/shadowvui/src/lib/curfit.pro -----

 NAME:
	CURFIT
 PURPOSE:
	Non-linear least squares fit to a function of an
	arbitrary number of parameters.
	Function may be any non-linear function where
	the partial derivatives are known or can be approximated.
 CATEGORY:
	E2 - Curve and Surface Fitting
 CALLING SEQUENCE:
	yfit = curfit(x,y,w,a,sigmaa)
 INPUTS:
	X = Row vector of independent variables.
	Y = Row vector of dependent variable, same length as x.
	W = Row vector of weights, same length as x and y.
		For no weighting
		w(i) = 1., instrumental weighting w(i) =
		1./y(i), etc.
	A = Vector of nterms length containing the initial estimate
		for each parameter.  If A is double precision, calculations
		are performed in double precision, otherwise in single prec.

 KEYWORD PARAMTERS:
	funct = string containing the name of the fitting function.
	    Default = 'funct'
	quiet = 1 to suppress printing interation information.
	max_its = Maximum number of iterations the function will
		do before giving up. Default=1000.
	accept = Acceptance criterion for acceptable fit. 
		This number is equal to the relative change in the
		ChiSquare statistic from one iteration to the next. 
		Default=.00001

 OUTPUTS:
	A = Vector of parameters containing fit.
	Function result = YFIT = Vector of calculated
		values.
 OPTIONAL OUTPUT PARAMETERS:
	Sigmaa = Vector of standard deviations for parameters
		A.
	
 COMMON BLOCKS:
	NONE.
 SIDE EFFECTS:
	The function to be fit must be defined and called FUNCT.
	For an example see FUNCT in the IDL User's Libaray.
	Call to FUNCT is:
	FUNCT,X,A,F,PDER
 where:
	X = Vector of NPOINT independent variables, input.
	A = Vector of NTERMS function parameters, input.
	F = Vector of NPOINT values of function, y(i) = funct(x(i)), output.
	PDER = Array, (NPOINT, NTERMS), of partial derivatives of funct.
		PDER(I,J) = Derivative of function at ith point with
		respect to jth parameter.  Optional output parameter.
		PDER should not be calculated if parameter is not
		supplied in call (unless you want to waste some time).
 RESTRICTIONS:
	NONE.
 PROCEDURE:
	Copied from "CURFIT", least squares fit to a non-linear
	function, pages 237-239, Bevington, Data Reduction and Error
	Analysis for the Physical Sciences.

	"This method is the Gradient-expansion algorithm which
	compines the best features of the gradient search with
	the method of linearizing the fitting function."

	Iterations are perform until the chi square changes by
	only 0.001% or until 1000 iterations have been performed.

	The initial guess of the parameter values should be
	as close to the actual values as possible or the solution
	may not converge.

 MODIFICATION HISTORY:
	Written, DMS, RSI, September, 1982.
	Modified by D.L. Windt, AT&T Bell Labs, March, 1989.

