3.11  Line-fitting

The best multiple-line-fitting IDL software is the CFIT package, created by Stein Vidar Haugan, and described in Software note #47.

Line profiles can have any general shape, and the `background' is normally fitted with a polynomial. Any number of parameters can be fitted; any number of lines can be fitted over a wavelength region.

The parameter's values can be allowed to vary within some fixed ranges, or can be kept constant. In many cases lines are closely spaced, and a proper de-blending can only be done if ranges for the positions and widths are given.

Each fitted line's parameters can be checked and modified interactively. The fit is performed with a non-linear weighted least squares fit, optimizing an initial user-supplied set of parameters in order to find the best agreement between the chosen model function and the observed data.

The data are given `weights' 1/s², where the s is the noise associated with the data, s = Ö(N) where N is the total number of photon-events/pixel.

The fitting is done passing a series of initial parameters that define each line to be fitted.

Only `real' lines that have a width at least equal to the instrument resolution should be fitted. This can be done by fixing the minimum line width equal to the instrument resolution.