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Since version 3.1 of A User's Guide for the Flexible Image Transport System (FITS) was published, there have been several significant developments in the FITS community: This new version of the User's Guide has been written to reflect those changes. The discussion of the image extension and the rules for the binary table extension have been moved from section 5 (Advanced FITS) to section 3 (FITS Fundamentals) . The papers describing the image and binary table extensions have now been published in the Astronomy and Astrophysics Supplement Series and are now considered to be among the fundamental FITS papers. Section 4 on World Coordinates has been updated to include the refinements incorporated in the current proposals, and it also discusses in greater detail how widely the different proposed conventions are used in the general community. The discussion of the three proposed binary table conventions, which are not part of the formal structure endorsed by the IAUFWG, remains in section 5. The discussion of applications of binary tables has been significantly expanded, and descriptions of a number of binary table and other conventions have been added. Section 6 has been rewritten to emphasize network sources of FITS information, and it discusses a number of sites on the World Wide Web that contain FITS documents, software, and sample files. Many of these sites did not exist when version 3.1 of the User's Guide was written, and others have been greatly expanded since. Three additional sample FITS headers, one using an ASCII table and two using binary tables, have been added to Appendix A. Appendix B lists the IEEE floating point number type corresponding to every bit pattern. Thanks for comments on an earlier draft of version 4 go to M.Calabretta, D.Jennings, W.Pence, and R.Thompson. Also, thanks to D.Leisawitz for providing the DIRBE FITS header (example 6) and W.Pence for providing the ASCA FITS header (example 7). To be of most use to readers, a guide such as this one must go beyond the formal rules to discuss common practices that are not specified by those rules. In addition, users who are designing and developing FITS files need to know how FITS is likely to develop. Developing a formal standard for FITS has clarified a number of points that had been unclear or ambiguous in the original FITS papers. Some of the issues were regarded as not appropriate for a formal standard but deserving of further detailed discussion; at the recommendation of the Technical Panel developing the NASA/Science Office of Standards and Technology (NOST) standard, they are included in this Guide. Queries to the FITS Support Office and discussion on the FITS network newsgroup sci.astro.fits and the associated fitsbitselectronic mail exploder have identified other points in need of explanation. This Guide also describes a number of conventions that are widely used but have not been formally adopted by the FITS governing structure under the IAUFWG. Description in this Guide of such conventions is intended neither as a NASA endorsement nor as a requirement for use of these conventions by NASA projects. Where an issue is controversial, this Guide attempts to provide the arguments on all sides. FITS is continually expanding: new conventions are proposed, existing proposals are modified, new issues are raised; and the FITS committees act. Some of this progress will occur during the period this Guide is being proofed and undergoing physical composition. Thus, some of these developments may, unfortunately, not make it into the current User's Guide. To keep up with current events, use the resources described in Section 6. Similarly, Web sites may be reorganized and the URLs corresponding to an individual page may change. In that case, the new location can generally be found by going to the main page for the site and following appropriate links. As always, comments about the Guide, in particular about areas that need clarification or expansion, are encouraged. Send questions or comments to
FITS Support Office  
Astrophysics Data Facility  
Code 631  
Goddard Space Flight Center  
Greenbelt MD 20771  
Telephone: +1-301-286-2899  
Electronic Mail: fits@nssdca.gsfc.nasa.gov