\chapter{Introduction}

\class{} is a software package for reducing spectroscopic data obtained on
a single-dish telescope. It also has basic functionalities to reduce
continuum drifts like pointing or focus.

The originality of \class{} with respect to similar systems already in use
is in the way an observation may be identified. In addition to the
traditional scan number which can be used to uniquely refer to an
observation, the system also enables one to use {\em Selection Criteria} as
in a data base management system.  This faculty, added to a powerful
command monitor, \sic{}, allows easy manipulation of large volumes of data;
the list of observation numbers to be added to get the mean spectrum at one
position need no longer be typed in, but may be found by \class{} itself.

On a standard installation, \class{} is entered by just typing {\tt class}.
\class{} is divided in different parts, called ``Languages'', which have
somewhat independent functions:
\begin{itemize}
\item Language LAS contains all the general utility functions to handle the
  data structure, plot the spectra or drifts and calibrate them.
\item Language ANALYSE contains functions to analyse calibrated spectra in
  more detail.
\item Language FIT gathers the spectra fitting functionalities.
\end{itemize}
Those languages are described in this manual. In addition, \class{} imports
many functionalities defined and documented in other \gildas{} packages:
\begin{itemize}
\item The command line interpretor is imported through the SIC (basic), GUI
  (for widgets) and VECTOR (miscellaneous) languages.
\item The plotting possibilities through the GTVL (basic graphic actions),
  GREG1 (curve plotting), GREG2 (image plotting) and GREG3 (data cube
  plotting) languages.
\item And the ephemerids and atmospheric contributions through the ASTRO
  language.
\end{itemize}
In addition to this manual, the reader should thus consult the \sic{}
manual, and for further processing, the \greg{} manual.

This version of the \class{} documentation reflects the full rewritting of
\class{} in {\tt FORTRAN90}. In this process, many things have been
changed, hopefully improved. If you are an experienced \class{} user, you
may first want to consult the IRAM memo which describes only the changes in
this version of \class{}.

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