Keywords: class

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# 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 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 `class`. CLASS is divided in different parts, called ``Languages'', which have somewhat independent functions:

- Language LAS contains all the general utility functions to handle the data structure, plot the spectra or drifts and calibrate them.
- Language ANALYSE contains functions to analyse calibrated spectra in more detail.
- Language FIT gathers the spectra fitting functionalities.

Those languages are described in this manual. In addition, CLASS imports many functionalities defined and documented in other GILDAS packages:

- The command line interpretor is imported through the SIC (basic), GUI (for widgets) and VECTOR (miscellaneous) languages.
- The plotting possibilities through the GTVL (basic graphic actions), GREG1 (curve plotting), GREG2 (image plotting) and GREG3 (data cube plotting) languages.
- And the ephemerids and atmospheric contributions through the ASTRO language.

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 `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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