XAS files

XAS deals with two basic kind of files : Auxiliary and ancillary files are plain ASCII files, intended for sequential access. For portability reasons they are implemented as stream_lf files on all systems (they can be NFS-mounted across all systems, including VMS-to-Unix).

Data files are binary,intended for Fortran DIRECT access and include the following categories :

A VOS routine layer allows generic file access using Fortran in a portable manner.

XAS reduced files are binary files in native machine representation, which share a common layout design (all files are direct access, with a natural record length corresponding e.g. to the image or table width, and a number of records corresponding to the number of image or table rows; all files share the same kind of header). They include the following classes :

XAS files are logically divided in a data part (organized as above) and a header. The header is logically made of a number of numeric- or ASCII-valued keywords. Keyword naming is closely consistent with FITS usages. For practical reasons the header is located at the end of the file, in order to be extensible (it is common practice to add HISTORY and comments after file manipulation), so that each file is made of : A layer of portable routines above the VOS ones allows basic access to XAS files, and handling of header keywords. An higher layer allows handling of specific data structures (classes).

In addition to the above, a class of "XAS ASCII tables" is unofficially defined as a tabular ASCII file preceded by a specially formatted one-line "magic" header.

XAS files can be used as they are from IDL or SAOimage, or exported to MIDAS, IRAF, XSPEC after conversion to FITS. An utility allows also to convert a XAS file among different operating system internal representations.