2.2. DEFINED TERMS

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FITS Flexible Image Transport System.

FITS block A sequence of 2880 8-bit bytes aligned on 2880 byte boundaries in the

FITS file, most commonly either a header block or a data block. Special records

are another infrequently used type of FITS block. This block length was chosen

because it is evenly divisible by the byte and word lengths of all known computer

systems at the time FITS was developed in 1979.

FITS file A file with a format that conforms to the specifications in this document.

FITS structure One of the components of a FITS file: the primary HDU, the random

groups records, an extension, or, collectively, the special records following the last

extension.

FITS Support Office The FITS information web site that is maintained by the

IAUFWG and is currently hosted at http://fits.gsfc.nasa.gov.

Floating-point A computer representation of a real number.

Fraction The field of the mantissa (or significand) of a floating-point number that lies

to the right of its implied binary point.

Group parameter value The value of one of the parameters preceding a group in

the random groups structure, without the application of the associated linear

transformation.

HDU Header and Data Unit. A data structure consisting of a header and the data

the header describes. Note that an HDU may consist entirely of a header with no

data blocks.

Header A series of keyword records organized within one or more header blocks that

describes structures and/or data which follow it in the FITS file.

Header block A 2880-byte FITS block containing a sequence of thirty-six 80-character

keyword records.

Heap The supplemental data area following the main data table in a binary table

standard extension.

IAU International Astronomical Union.

IAUFWG International Astronomical Union FITS Working Group.

IEEE Institute of Electrical and Electronic Engineers.

IEEE NaN IEEE Not-a-Number value; used to represent undefined floating-point val-

ues in FITS arrays and binary tables.

FITS Standard