The optimisation in LONGSLIT allows for a flat base. Although this is generally satisfactory, for some data the base varies significantly. The user then has various options: the base can be subtracted from the using CSUB (which subtracts a polynomial fit to the base from the data); the data can be divided by the base; or the base can be left as it is, but a polynomial fit can be subtracted during the actual fitting. The latter option has the advantage that any plots of line profile etc. will still show the base as in the original data. This option is available by the use of the routine FITCONT, or totally within LONGSLIT. In general it is better to use the version in LONGSLIT, but this will not always cope with ``difficult'' data sets, so FITCONT may be required. The main difference here which allows FITCONT to deal with these cases is that it allows the weighting to be set using a cursor (in LONGSLIT it is done simply by excluding the areas within line boundaries, which is less versatile). This is similar to CSUB in fitting a Chebyshev polynomial to the base. However the data itself is not altered. When LONGSLIT fits a line it can subtract this base. Alternatively the base may be interpolated over the line using cubic splines (FITCONT is not needed for this).
FIGARO A general data reduction system