XAS files
XAS deals with two basic kind of files :
- binary data files
- auxiliary and ancillary 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 :
- SAX-specific "telemetry" or "FOT" files
- reduced data files in instrument-independent form (XAS files)
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 :
- image format data
- images and pseudo-images (any 2-d equispaced data)
- response matrices (a couple of files with instrument response and reference energy grid)
- tabular format data
- spectra
- time profiles
- photon lists
- any other generic table
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 :
- one or more records (for a total of no less than 28 bytes) of basic information(a magic
number, the record length and the number of data and header records)
- the data part
- the header part
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.
sax.iasf-milano.inaf.it/Sax/Xas/files.html
:: original creation 1996 Oct 17 12:13:51 CEST ::
last edit 1996 Oct 17 12:13:51 CEST