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Next: Some Hints on Keyword Up: Primary Header Previous: Required Keywords Reserved Keywords

  Many of the following reserved keywords were originally suggested by Wells, Greisen, and Harten (1981). If a reserved keyword is used, the meaning and structure must be as described here. Keywords other than the reserved keywords should not be used in their place to express the same concepts. Reserved keywords may appear in any order between the required keywords and the END keyword.

Some of these keywords describe the data array.

The reserved keywords permit complete specification of a linear coordinate system for any axis. Other coordinate systems can be identified through the name given by CTYPEn, comments, and user-specified keywords.

Default values have not been defined for any of these keywords.

These reserved keywords, from FITS Paper I, allow the definition of simple rectangular coordinate systems, but they do not prescribe the relation between the plane rectangular coordinate system of the FITS array and the spherical coordinate region of the sky that it represents. This question, along with the current comprehensive proposal under consideration by the FITS community, is discussed in section 4. Part of the proposal replaces the simple (CROTAn, CDELTn) method with a more comprehensive method of defining coordinate transformations in three dimensions.

Note that DATAMAX and DATAMIN apply to the physical values represented, not to the numbers in the FITS file. In determining the values for DATAMAX and DATAMIN, special values such as the IEEE special values and values derived from integer array members set to the value of the BLANK keyword are not considered.

Some keywords provide information on the observations represented or the production of the data set.

Other keywords signal a card image with comments or other text.

These keywords are the only ones without values that can have ``='' in column 9. Users may define other keywords to contain comments as well. For these keywords, column 9 may not contain ``=''. The reason this restriction does not apply to the COMMENT, HISTORY and keywords is principally historical; the original FITS paper defined columns 9-80 as being a comment and did not restrict the content. For the COMMENT, HISTORY, and blank field keywords, the reader will be able to tell by the keyword name that there is no value. However, for user-defined keywords, the reader has no other way of telling a priori whether the field has a value or not. To minimize confusion, it is best to avoid ``='' in column 9 even after COMMENT, HISTORY, and blank keyword fields as well.

next up previous contents
Next: Some Hints on Keyword Up: Primary Header Previous: Required Keywords