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#### Background Models

The default model is a flat background, which is quite sufficient for many objects such as many emission nebulae where there is not a strongly varying background. For some objects, however, this will not be sufficient, and allowance must be made for a varying background (e.g. a stellar continuum). LONGSLIT provides three solutions to this problem

1. Fit a Chebyshev polynomial to the base immediately before fitting the line profile. The polynomial it fitted to the whole of the spectrum, except for those parts within the boundaries of the identified lines. The order for the polynomial is stored in the results block, and this is used to re-create the fit as required (e.g. for profile plots). Although this cuts down on storage requirements, it does mean that if you identify any further lines, these will cause the program to make incorrect assumptions. For this the blocking is, of course the same as for the profile fitting. The order can be varied as required for different lines. If this option is used, it is essential that the spectrum is not trimmed (subsetted) too close to the line.

2. Use a cubic spline to interpolate the values in the range of fitting. As with the Chebyshev polynomial the blocking is as for the fit, and the areas within the line boundaries are not used to constrain the splines.

3. Use Chebyshev polynomials, but perform their fitting using FITCONT. LONGSLIT can then pick up the coefficients (from a special coefficient structure), and use them to obtain the base. Since the blocking to be used by LONGSLIT is not known at this stage (by FITCONT at least), the fits are to single cross-sections, and the results are added when required. The areas to be excluded from the fitting are marked with a cursor (this is more versatile than when the fitting is performed by LONGSLIT).

To select a model other than a flat base, use the DEFINE option in the main menu, in which case you will be asked for the background model. Note that once set up, the model remains in force for fitting until the user leaves the program.

If you select option (i) above at the MANUAL menu, LONGSLIT gets you to decide upon the order to fit at that time--various plots are available to help. If you select this from DEFINE, then you be asked to decide upon the order before the first fit, unless you have already given it the order (in WINDOW for example). For this reason, this option cannot yet be used in batch mode.

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FIGARO A general data reduction system