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Section 4

Headers

The first 2 sections of this chapter define the structure and content of header keyword

records. This is followed in §4.3 with recommendations on how physical units should be

expressed. The final section defines the mandatory and reserved keywords for primary

arrays and conforming extensions.

4.1

Keyword Records

4.1.1

Syntax

Each 80-character header keyword record shall consist of a keyword name, a value indi-

cator (only required if a value is present), an optional value, and an optional comment.

Except where specifically stated otherwise in this standard, keywords may appear in

any order. It is recommended that the order of the keywords in FITS files be preserved

during data processing operations because the designers of the FITS file may have

used conventions that attach particular significance to the order of certain keywords

(e.g., by grouping sequences of COMMENT keywords at specific locations in the header, or

appending HISTORY keywords in chronological order of the data processing steps).

A formal syntax, giving a complete definition of the syntax of FITS keyword records,

is given in Appendix A. It is intended as an aid in interpreting the text defining the

standard.

4.1.2

Components

4.1.2.1

Keyword name (bytes 1 through 8)

The keyword name shall be a left justified, 8-character, space-filled, ASCII string with no

embedded spaces. All digits 0 through 9 (decimal ASCII codes 48 to 57, or hexadecimal

30 to 39) and upper case Latin alphabetic characters `A' through `Z' (decimal 65 to

90 or hexadecimal 41 to 5A) are permitted; lower case characters shall not be used.

FITS Standard