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The LEXT software package is an integral part of LDSS-2, in that it enables the complex operations required to observe with multislit masks to be carried out quickly and efficiently. The program performs three essential functions: it allows the observer to design the masks; it enables rapid acquisition of the target fields by calculating the telescope offset required to map the slits onto the targets; and it simplifies the data reduction. It also offers many general data processing tools. The present version has been developed for LDSS-2 but should work equally well for LDSS-1. The original LDSS-1 version was written by Richard Hook with modifications by staff at AAO.
LEXT is a Starlink-compatible program which runs on a workstation at the telescope but can also be used at Starlink nodes. Thus you can use it at your home Starlink node to design the masks before the observing run and afterwards to reduce your data. The complete package includes the executable image, source code, help files, LEXT procedures and DCL command procedures (for VMS) or make files (for UNIX) to maintain the software. The software is predominantly written in Fortran, and depends on several Starlink ADAM and FIGARO subroutine libraries. Refer to the LEXT Installation Guide for details of installation.
It should be noted that LEXT is resource-hungry and will probably need special quotas. Extended use for long periods (for example at the telescope) may eventually cause problems with virtual memory resources since the DSA library, which LEXT uses, appears to be a little careless on occasions about freeing virtual memory. If this happens, the solution is to exit from LEXT and restart. For this reason, and as a normal wise precaution, you should make sure that important intermediary data sets are saved to external disk files.
The following section gives a general introduction to the LEXT package. More specific information about using LEXT is given in: Section 2.2 for preparing masks; Section 4.8 for calculating image distortion corrections; Sections 5.3 and 5.4 for acquiring fields; and Section 6 for reducing data. These sections should act as a user guide, and hopefully contain the essential information required to run LEXT for most observing programs. A more comprehensive description of the functions available in LEXT is presented in the LEXT Reference Manual, (Appendix ii). Much of this information is also available in the large on-line HELP library which can be accessed from within the package.