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: http://xmm.vilspa.esa.es/sas/5.4.1/doc/slconv/node6.html
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It may be desirable to apply (in addition to the source selection described above) some definite upper limit to the number of sources to be plotted - say, to plot only the first 50 brightest sources. Such a limit may be difficult or impossible to achieve solely by use of the expression parameter. For this reason, withncut and ncut parameters have been provided. However, this parameter alone cannot provide full versatility. It is not sufficient just to specify the number of sources to keep: it is also necessary to determine which sources to keep. This means in practice that the sources must be sorted into a series before the series can be truncated after the ncutth member. The parameter ncutsortexpression has been provided to allow the sources to be sorted. This works as follows: the task looks in parameter ncutsortexpression for an expression involving column names; this expression is evaluated and the sources are then ordered in decreasing order of the resulting value.
For example,
slconv srclisttab=fred.ds:SRCLIST withncut=yes ncutsortexpression=RATE ncut=10
produces a ds9 region file containing data for the first 10 brightest sources from the SRCLIST table in file `fred.ds' (that is, those 10 sources in the table which have the highest values of RATE).