The line selection process

`arc' will read in the specified spectrum and display a portion of it on the graphics device. Initially, the portion will be 200 channels long; you can change this should you want to. You will be invited to use the cursor to indicate an arc line.

Normally, you move the cursor until it is close to a line whose wavelength you know (you will often find it useful to have a hard plot of the whole arc in front of you as you perform the fit) and select it by hitting the space bar. (Strictly, you can use any key that does not have some specific function, but there are rather a lot that do and the space bar is a safe one to use.) `arc' will then try to find a line close to the point you have indicated, and if it finds one will show you its centre on the display with an arrow. The algorithm used to find a line centre is one described as `convolution with the derivative of a Gaussian', and it incorporates some requirements as to just what constitutes a line, which can lead on rare occasions to its being unable to find a line centred near the position indicated. You can think of the algorithm as a fit to a Gaussian of fixed sigma.

You will be told the channel number of the line, and asked to enter a wavelength. Strictly, you enter a wavelength followed by a type, e.g.

   3850 argon

The type should be one of the names specified in response to the `What type of Arc' prompt. `arc' then looks in the table for that type and selects the line whose wavelength is nearest to the one you specified. If you do not specify a type (which is by far the most usual case), `arc' uses the first of the types. Since in most cases only one type was specified, this means that `arc' will normally just look in the one table that it read in. Which is what you would expect. So the response is usually just a number, e.g.

   3850

`arc' will then tell you what line it assumes you mean:

   Wavelength is 3350.924 OK? /YES/ >

If you reply `y', `yes', or just hit the return key, `arc' will use that wavelength. If you reply `no', or `n', it will ask you for the wavelength again. If you just hit return in response to the wavelength prompt, the line will be deleted and you will be back with the cursor looking for another line.