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SECTION 4. HEADERS

The underscore (` ', decimal 95 or hexadecimal 5F) and hyphen (`-', decimal 45 or

hexadecimal 2D) are also permitted. No other characters are permitted.1 For indexed

keyword names that have a single positive integer index counter appended to the root

name, the counter shall not have leading zeroes (e.g.,NAXIS1, not NAXIS001).

4.1.2.2

Value Indicator (bytes 9 and 10)

If the 2 ASCII characters `=' (decimal 61 followed by decimal 32) are present in bytes 9

and 10 of the keyword record this indicates that the keyword has a value field associated

with it, unless it is one of the commentary keywords defined in §4.4.2.4 (i.e., a HISTORY,

COMMENT, or completely blank keyword name) which by definition have no value.

4.1.2.3

Value/Comment (bytes 11 through 80)

In keyword records that contain the value indicator in bytes 9 and 10, the remaining

bytes 11 through 80 of the record shall contain the value, if any, of the keyword, followed

by optional comments. In keyword records without a value indicator, bytes 9 through 80

should be interpreted as commentary text, however, this does not preclude conventions

that interpret the content of these bytes in other ways.

The value field, when present, shall contain the ASCII text representation of a literal

string constant, a logical constant, or a numerical constant, in the format specified in

§4.2. The value field may be a null field; i.e., it may consist entirely of spaces, in which

case the value associated with the keyword is undefined.

The mandatory FITS keywords defined in this standard must not appear more than

once within a header. All other keywords that have a value should not appear more

than once. If a keyword does appear multiple times with different values, then the value

is indeterminate.

If a comment follows the value field, it must be preceded by a slash (`/', decimal 47

or hexadecimal 2F).1 A space between the value and the slash is strongly recommended.

The comment may contain any of the restricted set of ASCII text characters, decimal 32

through 126 (hexadecimal 41 through 7E). The ASCII control characters with decimal

values less than 32 (including the null, tab, carriage return, and line feed characters),

and the delete character (decimal 127 or hexadecimal 7F) must not appear anywhere

within a keyword record.

4.2

Value

The structure of the value field depends on the data type of the value. The value field

represents a single value and not an array of values.1 The value field must be in one

1

This requirement differs from the wording in the original FITS papers. See Appendix H.

FITS Standard