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The distortion along the slit 
can be
modelled using an image containing one or several spectra of (ideally:
point-) sources with well defined continuum. The command
SEARCH/LONG can be used to generate a table with the positions of the
spectra at several wavelengths. (Contrary to the general conventions
used throughout this chapter, for this particular application the
dispersion direction must be parallel to the `Y'-axis!) The
distortion is then modelled by a two-dimensional polynomial fitted to
these positions (using, e.g., REGRESSION/TABLE). If the
resulting coefficients are stored in the keyword COEFY*,
(*=I,R,D) and combined with suitable (if necessary: dummy)
coefficients for the dispersion resolution in the keyword
KEYLONG*, (*=I,R,D), the command RECTIFY/LONG can be applied.
Note that these steps are not implemented as a convenient-to-use high-level command procedure.