IRAF help page for package utilities, program curfit

• input
• function = legendre
• order = 4
• weighting = uniform
• uniform
• user
• statistical
• instrumental
• interactive = yes
• axis = 1
• listdata = no
• verbose = no
• power = no
• calctype = double
• device = stdgraph
• cursor = stdgcur

curfit input

input

function = legendre

order = 4

weighting = uniform

user

statistical

instrumental

interactive = yes

axis = 1

listdata = no

verbose = no

power = no

calctype = double

device = stdgraph

cursor = stdgcur

A curve is fit to data read from either an image section or a list. The type of curve is set by the function parameter as either a legendre polynomial, chebyshev polynomial, linear spline or cubic spline, with the order of the fit (or number of spline pieces) set by order. If data is read from an image, the axis parameter is used to reduce the dimensionality of the image; it specifies the axis along which the image is projected. For example, when axis = 1, the image is compressed to a column. Axis = 2 would project the image along a line; axis = 3 indicates projection in the z direction, etc.

The input data must be ordered in x because of a restriction in the interactive plotting package. If the input is from a list, the data are sorted prior to fitting; image input data are assumed to be ordered in x and are not explicitly sorted by curfit.

If the input is from a list the user may specify a set of weights, weighting = user or a set of errors, weighting = instrumental. An additional weighting option weighting = statistical can be used for both list and image data. The default is weighting = uniform.

When interactive = yes, the curve is plotted and cursor commands allow for interactive examining and adjustment of the fit. The full range of interactive cursor commands is available including those for changing the function type, order, and rejection criteria, and examining the residuals.

The final fit parameters are written to STDOUT with the format controlled by parameters verbose and listdata. By default, the function type, order, and resulting chi-square are printed as well as the coefficients and their standard deviations. If verbose is set to yes, a list of X, Y_calculated, Y_input, and W_input is also printed. If listdata is set to yes, the only printed output will be a listing of X, Yc, Y and W. This provides a list suitable as input to graph or any other list oriented utility. Setting listdata to yes overrides the verbose option.

When power = yes, the coefficients are converted to power series coefficients of the form a0 + a1*X + a2*X**2 +a3*X**3 .... Only legendre and chebyshev coefficients are converted; a conversion of spline coefficients is meaningless. Also, errors in the coefficients are not converted.

The user has a choice of single or double precision calculations. Generally double precisions is used since the calculation time is only slightly longer. The single precision calculation is used in many other tasks which do many fits. This task provides a test tool to compare the results between the two levels of precision.

1. The x,y pairs in file test.data are interactively fit with a fourth order legendre polynomial. The printed output is shown.

cl> curfit test.data

	NOAO/IRAF V2.0 Hammond@lyra Fri 11:45:41 13-Dec-85
	file = test.data
	function = legendre
	grow = 0.
	naverage = 1
	order = 4
	low_reject = 0., high_reject = 0.
	niterate = 1
	sample = *
	total points = 8
	sample points = 8
	nrejected = 0
	deleted = 0
	square root of reduced chi square = 3.008706E-6
		coefficent	  error
 	1	   2.633E1	  1.098E-6
 	2	   3.150E1	  1.820E-6
 	3	   8.167E0	  1.896E-6
 	4	 -1.621E-6	  2.117E-6

cl> curfit m74[501:512,1:512] axis=2 func=spline3 order=5

3. Use curfit as a filter to overplot a smoothed curve to an existing plot of the data points. The command line for graph is shown as well as the curfit command. Note the interactive flag for curfit is turned off.

cl> graph points.list point+ mark=box wx1=.13 xlab="X VALUES" >>> ylab="Y VALUES" title="Legendre fit to points.list"

cl> type points.list | curfit list+ inter- | graph append+