Introduction

This chapter describes the long-slit  reduction package in a general way. One-dimensional spectra are considered to be a particular case of long-slit spectra and are also handled by the package. More instrument-specific operating instructions may be given in appendices to this MIDAS manual or in the relevant ESO Instrument Handbooks.

The package provides about 50 basic commands to perform the calibration and to display the results, defined in the context LONG. A tutorial  procedure, TUTORIAL/LONG, illustrates how to operate the package. The context SPEC is a low-level context including general utility commands required by the different spectroscopy packages ECHELLE, IRSPEC and LONG. These commands are refered to and summarized in this chapter. The graphical user interface   XLong is a MOTIF-based interface providing an easy access to the commands of the package. This interface is described in the Appendix . Standard spectral analysis  can be performed with the graphical user interface XAlice (command CREATE/GUI ALICE). The use of general MIDAS commands for spectral analysis is also discussed in this chapter.

The main characteristics of the LONG context are derived from its modularity, that allows to perform, in a first step, the image cosmetics and photometric corrections -- which are detector dependent -- and then, to correct for the geometric distortions. The method gives particular emphasis to an accurate wavelength calibration  and correction of all distortions along the whole slit, so that the spatial structure of extended objects can be examined in detail.

Although the method is applicable to different instrumental configurations, we assume, in the following description, that the generic instrument consists of a spectrograph coupled to a CCD detector. In the current version it is assumed that the CCD frame is oriented with the rows in the dispersion direction and the columns along the slit.

Note chapter only provides a synopsis of the commands needed for the reduction of long-slit. It is important to realise that it can neither substitute the HELP information available for each command nor be exhaustive, especially not with regard to the usage of general utilities such as the MIDAS Table System, the AGL Graphics package, etc. The Appendix provides a practical approach to the reduction of long-slit spectra.