ELF general procedures

Although the command descriptions given later describe functionality in detail, it is probably worth just summarising how to do a simple Gaussian fit, for illustration. The steps would typically be:

• POP the data of interest into the current arrays. Use (e.g.) RXR to restrict the number of data points to the minimum consistent with adequately defining the line(s) of interest and a small amount of continuum.

• Although you can represent the continuum by a polynomial with free parameters, convergence is enormously improved if you first subtract a continuum (using PF, or CDRAW, for example, together with ASUB). If you really must incorporate a background polynomial in the fit, keep the degree as low as possible (e.g. zero) to avoid indeterminacy.

• Type ELFINP to invoke ELFs special command language, and input your first guesses at the values of the parameters to be optimised. Make life easier for yourself by keeping the number of non-linear parameters (line centre positions and widths) as small as possible, and try to make your guesses good ones. Type QELF to leave the command language processor.

• Type ELFOPT to start the ELF Fit Coefficient OPTimisation. If you have many free parameters, and the machine is being heavily used, go for a cup of tea, after you've checked the first iteration of the optimisation to make sure everything is as you expect.

• Later...you can push a copy of the `best fit' model onto the DIPSO stack using ELFPUSH. You can also save the fit coefficients on the separate fit coefficient stack, for later use or reference, with ELFPUSHC.

A serious program crash while optimising, such as divide by zero or overflow, may occur occasionally. Such problems are usually caused by overspecified fits, or starting values that are grossly in error.