Other initial prompts

`arc' will also prompt for the arc line half width in pixels (the parameter called `sigma'), which it uses as a guide when finding the centres of the arc lines, and for the initial order of polynomial to use for the fit. `arc' performs running fits, and unless you are using a previous line list (see below), the first fit will have to be made to a low order, simply because there will not be enough lines for a higher order. Once enough lines have been identified, `arc' will start to use the order you specified initially. You will be able to change the values for both `sigma' and `order' interactively during the fitting process.

`arc' will also ask if lines from the previous fit are to be used. During a fit, `arc' keeps writing out to disk the positions and wavelengths of the lines identified so far. (`arc' always writes to the file `arlines.lis', thus any such file should be deleted or renamed before you run `arc'.) If you reply `yes' to this prompt, `arc' will read in the file giving the previous list of identified lines. This allows you to start again where you left off the previous time-either because `arc' or the computer crashed (perish the thought!) during the previous fit, or simply because, on reflection, you are unhappy with the fit you obtained and want to experiment with other orders, a different selection, etc. The default for this previous list file is always `arlines.lis', since that is the name of the file that `arc' writes. However, you may specify a different file. You may, for example, have one file that you want to use as the basis for fits to a number of arcs, which you have renamed from `arlines.lis' (the name `arc' will have given it originally when it was produced) to, say, `basefit.lis'.

You can also use a previous fit if the arc you are identifying is very similar to the previous arc, but is shifted slightly. This is often the case with arcs taken at intervals throughout a night. `arc' will notice if the file that was used to create the previous fit is different to the file you are fitting. If this is the case, you are prompted for the `xcorr' keyword, which allows you to request that `arc' locate the previous spectrum used and attempt to determine a linear shift by cross-correlating the two arcs and applying the determined shift to the arc line list. (If you are going to use this option, specifying a slightly larger `sigma' than that used for the previous arc will give the line finding algorithm a little more flexibility when it looks for the lines at their shifted positions.) This is a simple operation, but the shift is determined over the whole of the arc. There is an alternative, messier, way to indicate a shift to `arc' on the basis of a single selected line, but this is described later.