FITS incorporates a philosophy along with the data format. The underlying goal is to provide a standardized, simple, and extensible means to transport data between computers or image processing systems. Any FITS reader should be able to cope with any FITS formatted file, skipping over portions (extensions) and ignoring keywords that the reader does not and need not understand.
Simplicity requires that reading and writing FITS should be implemented in a fairly straightforward way on any computer used for astronomical reduction and analysis. Simplicity also implies that the structure of the file should be self-defining and, to a large degree, self-documenting.
The first word in FITS is ``flexible''. The format needs to be flexible to facilitate extensibility for different applications. Hence, the number of strict rules is not large. Because the files are self-defining, FITS can fulfill a large range of data transport needs. FITS can be used not only for unambiguous transportation of n-dimensional, regularly-spaced data arrays, but also for additional information associated with such a matrix. FITS can also transport arbitrary amounts of text within standard data files. The ``history'' of manipulations of the data can thus easily be recorded in self-documenting data files. FITS is sufficiently general for a wide variety of applications. The introduction of new keywords permits addition of new information as needed, and the use of extensions allows almost unlimited flexibility in the type of information to be stored. Thus, FITS can grow with the needs of the astronomical community.
The great flexibility of FITS is a potential weakness as well as a strength. While there is a great temptation to proliferate keywords and new extension types, caution should be exercised in this process. Because FITS is a worldwide medium of data exchange, extension formats need to be coordinated under the International Astronomical Union FITS Working Group (IAUFWG) to prevent duplication and inconsistencies in usage, and agreements should be reached governing keyword conventions in particular fields. The structure under the IAUFWG provides an overall authority for the FITS standard, but additions to FITS are not created by the IAUFWG but are designed by FITS users and then acted upon by the international structure. Although the number of strict rules is not large, there is an extensive set of recommended practices. Creators of FITS files should adhere to these recommendations if at all possible; in particular, the rules of FITS should not be exploited to create files that try to mimic the local format, and, although in technical compliance with the rules, depart from the recommendations to such an extent that they don't look like FITS files. General adherence to recommended practice will simplify the task of the FITS software developers; if a FITS file contains too many unconventional but permitted constructs, many FITS readers may not be able to handle it. Not everything that is permitted is wise.
Users who develop extensive libraries of FITS files need assurance that they will not have to periodically revise these files because of changes in the standard. This requirement gives rise to one of the fundamental principles of FITS: no change in the rules should render old FITS files unreadable or out of conformance--the principle of ``once FITS, always FITS.'' This philosophy is reflected in data reduction and analysis packages in which all obsolete implementations are trapped and processed in the most accurate manner possible. While adherence to this principle has perpetuated some constructs that have proven with time to be awkward, it is better than the alternative of requiring revision of existing FITS files.
Changes in the FITS rules may add new structures that old software cannot handle. Revised software will be required for new standard extensions, but revising a software package is a far smaller effort than updating a full data library would be. As far as is possible, however, FITS should be expanded in such a way that the old software will still be able to process those parts of the file which it is capable of handling. In such a case, software should not fail or give incorrect results when confronted with the new extension or conventions; it should simply ignore them and continue to process those parts of the file that it can understand.
FITS is defined as a logical structure, not tied to the properties of any particular medium, thus allowing its continued use as the technology changes. Conventions for its adaptation to any medium are independent of the logical structure of FITS. Because its original development was for 1/2-inch magnetic tape, its structure is well adapted to that medium and the conventions are long established. More recently, more generalized conventions have been adopted for the expression of FITS on magnetic tape and on magnetic and optical disk. Conventions can be defined for new media as they develop.