# Chapter 1 Introduction

This document describes the Fortran-callable subroutine interface that is provided as part of the CFITSIO library (which is written in ANSI C). This is a companion document to the CFITSIO User’s Guide which should be consulted for further information about the underlying CFITSIO library. In the remainder of this document, the terms FITSIO and CFITSIO are interchangeable and refer to the same library.

FITSIO/CFITSIO is a machine-independent library of routines for reading and writing data files in the FITS (Flexible Image Transport System) data format. It can also read IRAF format image files and raw binary data arrays by converting them on the fly into a virtual FITS format file. This library was written to provide a powerful yet simple interface for accessing FITS files which will run on most commonly used computers and workstations. FITSIO supports all the features described in the official definition of the FITS format and can read and write all the currently defined types of extensions, including ASCII tables (TABLE), Binary tables (BINTABLE) and IMAGE extensions. The FITSIO subroutines insulate the programmer from having to deal with the complicated formatting details in the FITS file, however, it is assumed that users have a general knowledge about the structure and usage of FITS files.

The CFITSIO package was initially developed by the HEASARC (High Energy Astrophysics Science Archive Research Center) at the NASA Goddard Space Flight Center to convert various existing and newly acquired astronomical data sets into FITS format and to further analyze data already in FITS format. New features continue to be added to CFITSIO in large part due to contributions of ideas or actual code from users of the package. The Integral Science Data Center in Switzerland, and the XMM/ESTEC project in The Netherlands made especially significant contributions that resulted in many of the new features that appeared in v2.0 of CFITSIO.

The latest version of the CFITSIO source code, documentation, and example programs are available on the World-Wide Web from:

            https://heasarc.gsfc.nasa.gov/docs/software/fitsio/

Any questions, bug reports, or suggested enhancements related to the CFITSIO package should be sent to the FTOOLS Help Desk at the HEASARC:

            https://heasarc.gsfc.nasa.gov/cgi-bin/ftoolshelp

This User’s Guide assumes that readers already have a general understanding of the definition and structure of FITS format files. Further information about FITS formats is available from the FITS Support Office at https://fits.gsfc.nasa.gov/. In particular, the ’FITS Standard’ gives the authoritative definition of the FITS data format. Other documents available at that Web site provide additional historical background and practical advice on using FITS files.

The HEASARC also provides a very sophisticated FITS file analysis program called ‘Fv’ which can be used to display and edit the contents of any FITS file as well as construct new FITS files from scratch. Fv is freely available for most Unix platforms, Mac PCs, and Windows PCs. CFITSIO users may also be interested in the FTOOLS package of programs that can be used to manipulate and analyze FITS format files. Fv and FTOOLS are available from their respective Web sites at:

            https://fv.gsfc.nasa.gov
            https://heasarc.gsfc.nasa.gov/docs/software/ftools/