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LEXT is a monolithic program which is essentially a tool-kit to support multiobject instrumentation. Although experienced users can gain the most from its powerful features, inexperienced users can carry out sophisticated target acquisition operations with ease by making use of several special procedures written especially for LDSS-2. There are procedures to carry out all the essential functions of LEXT, and for most observers they will provide the easiest way to use LEXT. A list of the most useful procedures is given in Table 5.
Several logical names and symbols must be set up to run LEXT and there must also be some start-up files in your LEXT directory. The details of what is required are given in the LEXT manual. At the telescope, you will access LEXT from the account LEXTOBS, most conveniently run from the LPVS3 VAXstation. This account automatically sets up the relevant logical names and symbols, and LEXT can be invoked from DCL by simply typing:
$ LEXTAfter some startup messages, you should arrive at a command line prompt:
LEXT -- IMAGE 1 >From here you can invoke any of the LEXT functions; a good one to start with being:
LEXT -- IMAGE 1 > HELPThe on-line help facility uses the Starlink portable HELP facility which is different in some respects from the VMS HELP facility. In particular, if you request help on a command and its qualifier on the command line, you should leave a space between them. For example: HELP MAKEFF /PLOT will work but HELP MAKEFF/PLOT will not. Alternatively you can type HELP MAKEFF and then type /PLOT in response to the HELP facility prompt for the MAKEFF topic.
LEXT operates in two modes: IMAGE mode for handling image data such as acquisition images and the 2-d spectra; and LIST mode for handling tables of data, such as a list of object positions. There are several special types of LIST used by different LEXT functions. For example a MASK list contains details of the mask configuration and a BOXES list holds information on the predicted extent of the spectra for a particular observing configuration.
The user can toggle between working in LIST and IMAGE mode simply by typing LIST or IMAGE. The current mode is displayed within the command line prompt. LISTs and IMAGEs can be read in, using the READ command, to internal slots. There is a stack of slots for IMAGEs and one for LISTs. Subsequent processing takes place on these slots, and when they require saving they can be written back to disk files with the WRITE command.
There are a number of LEXT commands which simply deal with the bureaucracy of the stack system. For example DIRECTORY shows the currently used slots in the stack, NEXT, BACK and USE allow the user to move the current slot pointer. The relevance of this is that many other commands work, by default, on the current slot. However, many commands will search the stack for LISTs of the right type and warn you if there is more than one list of the correct type present.
Many of the simple commands (see the on-line HELP for full details) can be used on both LISTs and IMAGEs but with different results. For example, in LIST mode, typing:
LEXT -- LIST 3 > READ ALISTwill first read in a list called ALIST and will then prompt the user for a constant to add to a given column of the LIST (which is also prompted for) in the current slot. Note that, in this example, we start by pointing at LIST 3. The slot pointer is changed to 4 as ALIST is read into the next available slot which happens to be 4. The old data values will be over-written. In IMAGE mode, typing:
LEXT -- LIST 4 > ADDC
LEXT -- IMAGE 2 > ADDC 6.3will add 6.3 to all the data values in the current IMAGE held in slot 2. Notice that here we have specified the parameter on the command line, which is an alternative to allowing the program to prompt.
There are also a selection of more sophisticated commands to perform LDSS-2 specific tasks. Many of these require standard information, such as the pixel scale, the dispersion of the grism etc. This information is stored in a large set of internal parameters, which are then used as required by each routine. A list of some of the most important parameters is shown in Appendix D. To see what the current settings are type:
LEXT -- LIST 2 > SHOW/ALLThis works in both modes, as do all the remaining examples in this section.
Internal parameters may be changed individually using the command
LEXT -- LIST 2 > SET parameter valueThe currently SET parameters may be output to an external ASCII format file using the command SAVESET. By default the saved file is called LEXT.SET, but any name can be specified on the command line. Files with can be read back by simply typing their name, e.g. to set the parameters in MYFILE.SET type:
LEXT -- LIST 2 > MYFILE.SETThese files are examples of LEXT procedures, which are basically lists of LEXT commands to be executed one after another (see the example file in Appendix D). In the case of LEXT.SET all these commands are simply to SET internal parameters.
There are also several other procedures which have been written to facilitate the tasks of mask design, field acquisition etc. These reside in a directory with logical name LDSS_PROC and are invoked by typing the procedure name. LEXT searches LDSS_PROC as well as looking in the current directory for the specified procedure file. Thus both the procedures RUMBA.PROC in LDSS_PROC and RUMBA.PROC in the current directory could be invoked by typing:
LEXT -- IMAGE 4 > RUMBAIf both procedures were present, LEXT would preferentially use the one in the current directory.
Several procedures are discussed in the appropriate sections of this manual, including PREDICT_SPECTRA, EDIT_MASK, OPTIMIZE_MASK and DRAW_MASK to help in designing masks, and FIND_TWEAK to help acquiring fields with the telescope. A list of procedures is shown in Table 5.