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As described in section 9, LEXT has two `internal' data structures, the list and the image, although both may be further distinguished by their . Data can be read into, and written from, these internal ßs in a variety of ways.
The I/O is principally performed with the commands READ and WRITE. The same commands are used for both lists and images. When a new file is read, it is stored in the first available slot and its state is automatically set to `protected'. Further modification of the data will require it to be FREEd.
In IMAGE mode data access is performed via the Figaro DSA/DTA routines. Thus it is possible to read and write both FIGARO DST/HDS files or STARLINK NDF(SDF) files. The default is determined by the DCL logical name FIGARO_FORMATS which should be setup with DEFINE/JOB. For example, this should be set to ``NDF,DST'', if NDF's were the default, but specific DST's might need to be read.
It is important that images are read in in such a way as to leave the red end of the spectrum at the right, and with north up. All possible transformations can be accomodated by use of the qualifiers /1 up to /8. At the time of writing the WHT Tektronix chip required /3, which represents a rotation of .
In list mode the default external representation is as a Figaro DTA/HDS file. These files can be examined outside LEXT, with FIGARO EXAM or STARLINK TRACE. These files are unique to LEXT and are not appropriate for input into any other packages.
Any list may also be written or read as an ASCII file by specifying the /FORMATTED qualifier. The number of columns in the file must be specified in advance using SET NCOLUMNS. A special option to allow the list to be entered directly at the keyboard, use READ/FORMATTED HAND.
Extracted spectra may be written into a FIGARO `SPEC' file by specifying /SPEC. These can then be used as input into FIGARO routines which operate on 1D spectra.
The command FPRINT creates a formatted list of object information from the POS and MASK lists. Specifically the positions and dimensions of the slits on the mask, and the positions and magnitudes of the objects on the sky. The MASK list, which may be a subset of the POS list, is used as an index and each object is looked up in turn.
The /CAT qualifier, required to read and write the special object list files is described in more detail below. /MANUFACTURE is used to output the WHT style of MASK design instructions into an ASCII disk file.
A new list or image may be created and initialised with the command CREATE. If no qualifiers are present the new structure will contain only an array of constants, and will have TEMP.