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Introduction

LEXT is a software package designed to provide facilities to aid the use of the Low Dispersion Survey Spectrograph, LDSS-2 instrument at the William Herschel Telescope. It was written for use with the LDSS multi-slit spectrograph, at the Anglo Australian Telescope. It is intended primarily as an aid to observations in the areas of mask preparation, target acquisition and fast data reduction. Nonetheless, it provides powerful tools for the extraction of the spectra, and is designed to interface to the Figaro system which offers extensive further facilities. It is implemented to run under VAX/VMS and is partly incorporated into the STARLINK ADAM environment.

Thus, its functions can be considered to be in three categories:

• simple, general purpose, data processing routines. eg. adding two images
• more sophisticated tasks which deal with the extraction and calibration of the multiple spectra from 2D images.
• tools which are rather specific to the LDSS system, such as multislit mask design, and target acquisition.

The detailed working of these commands is the subject of this document. For a more general discussion of the common operations and procedures, and examples of their use, the reader is recommended to refer to the LDSS-2 Observer Manual.

Unlike many other software packages, such as FIGARO, LEXT is a monolithic program. External images are read into the package and stored in a stack, where they can be operated on in situ. There is also a stack for tabulated information, called lists, which are used to store and communicate data such as slit positions etc. (see Section 9). A large number of internal parameters control aspects of the interface, such as graphics pen colour, and store standard instrumental data, such as the physical size of the detector pixels. Many of the more sophisticated commands rely on the presence of list information defining, for example, the approximate position of the spectra within the image. It is intended largely for interactive use, and provides no special facilities for batch processing.