pf - parabola fit

pf -3,15 see:		Fit a parabola to stack 1 between -3 and 15 Km/s
			(assuming horizontal plot units are velocity).
			Put the parabola in stack1 and overlay the plot
			of the present stack 1 with the parabola.

pf -3,14 rm:		Fit and subtract a parabola between -3 and 15.


Pf fits a positive concave down parabola to the data in stack 1 over the
horizontal range given.  The archetypical use of pf is in fitting the spectrum
of IRC+10216.  The parameters of the fit are stored in the header variables
(see below).

Unless the dp: flag is given, the parameters of the fit will be printed
on the terminal in horizontal plot units.  A single header will be
printed for each command line.

See: first plots stack 1 if there is not a current plot, overwrites
stack 1 with the best fit parabola, and then overlays the parabola on
the plot.

Rm: subtracts the fitted parabola from the stack, puts the residuals in 
stack 1 and plots them.

Values returned by pf are which are stored in the stack header are
  ftmax - peak temperature of fitted parabola 
  fterr - error in ftmax
  fpeak - horizontal location of ftmax in plot units
  fperr - error in fpeak
  fwhm - fitted full width at half maximum in plot units
  fwerr - error in fwhm
  area - area of parabola in (K * plot units).
  chisq - reduced chi-squared of the fit
  nfree - number of free parameters (usually the number of channels - 2)
As usual, these are accessible via the "dot mechanism", e.g. 'p .fwhm'.

If the fit did not converge all the values are set to .DR.
See Bevington for a discussion of the fitting technique.

This document was last updated on Tue Oct 17 16:24:05 EDT 2000 by Marc W. Pound.