10

SECTION 2. DEFINITIONS, ACRONYMS, AND SYMBOLS

IEEE special values Floating-point number byte patterns that have a special, re-

served meaning, such as -0, ±∞, ±underflow, ±overflow, ±denormalized, ± NaN.

(See Appendix E).

Indexed keyword A keyword name that is of the form of a fixed root with an ap-

pended positive integer index number.

Keyword name The first eight bytes of a keyword record which contain the ASCII

name of a metadata quantity (unless it is blank).

Keyword record An 80-character record in a header block consisting of a keyword

name in the first 8 characters followed by an optional value indicator, value and

comment string. The keyword record shall be composed only of the restricted set

of ASCII text characters ranging from decimal 32 to 126 (hexadecimal 20 to 7E).

Mandatory keyword A keyword that must be used in all FITS files or a keyword

required in conjunction with particular FITS structures.

Mantissa Also known as significand. The component of an IEEE floating-point number

consisting of an explicit or implicit leading bit to the left of its implied binary point

and a fraction field to the right.

MEF Multi-Extension FITS , i.e., a FITS file containing a primary HDU followed by

one or more extension HDUs.

NOST NASA/Science Office of Standards and Technology.

Physical value The value in physical units represented by an element of an array and

possibly derived from the array value using the associated, but optional, linear

transformation.

Pixel Short for `Picture element'; a single location within an array.

Primary data array The data array contained in the primary HDU.

Primary HDU The first HDU in a FITS file.

Primary header The first header in a FITS file, containing information on the overall

contents of the file (as well as on the primary data array, if present).

Random Group A FITS structure consisting of a collection of `groups', where a group

consists of a subarray of data and a set of associated parameter values. Random

groups are deprecated for any use other than for radio interferometry data.

Record A sequence of bits treated as a single logical entity.

FITS Standard