3 Definitions, acronyms, and symbols

          Used to designate an ASCII blank.

ANSI

American National Standards  Institute.

Array

A sequence of data values.  This sequence corresponds to the elements in a rectilinear, n-dimension matrix ( $0 \leq n \leq 999$).

Array value

The  value of an element of an array in a FITS file, without the application of the associated linear transformation to derive the physical  value.

ASCII

American National Standard Code for Information Interchange.

ASCII blank

The ASCII character for blank  which is represented by hexadecimal 20 (decimal 32).

ASCII character

Any  member of the 7-bit ASCII character set.

ASCII NULL

Hexadecimal  00.

ASCII text

ASCII characters  hexadecimal 20-7E.

Basic FITS

The FITS structure  consisting of  the primary header  followed by a single primary  data array.

Bit

A single binary digit.

Byte

An ordered sequence of eight consecutive bits treated as a single entity.

Card image

A sequence of 80 bytes containing  ASCII text , treated as a logical entity.

Conforming extension

An extension whose keywords and organization  adhere to the requirements for conforming extensions defined in Sect. 4.4.1 of this standard. 

DAT

4 mm Digital Audio Tape.

Deprecated

This term  is used to refer to obsolete structures that should not be used for new applications but remain valid.

Entry

A single value in a table.

Extension

A  FITS HDU appearing  after the primary HDU in a FITS file.

Extension name

The identifier used to distinguish a particular extension HDU from others of the same type, appearing as the value of the  EXTNAME keyword. 

Extension type

An extension  format.

Field

A set of zero or more table entries collectively described by a single format.

File

A sequence of one or more records terminated by an end-of-file indicator appropriate to the medium.

FITS

Flexible Image Transport System.

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.

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.

GSFC

Goddard Space Flight Center.

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 records.

Header

A series of card images organized within one or more FITS logical records that describes structures and/or data which follow it in the FITS file.

Heap

A supplemental data area, currently defined to follow the table in a  binary table extension.

IAU

International Astronomical  Union.

IAUFWG

  International Astronomical Union FITS Working Group.

IUE

International Ultraviolet Explorer. 

IEEE

Institute of Electrical and Electronic  Engineers.

IEEE NaN

IEEE Not-a-Number  value.

IEEE special values

Floating point number byte patterns that have a special, reserved meaning, such as -0, $\pm \infty$, $\pm$underflow, $\pm$overflow, $\pm$denormalized, $\pm$NaN  (see Appendix .15).

Indexed keyword

A  keyword that is of the form of a fixed root with an appended positive integer count.

Keyword

The first eight bytes of a header card image.

Logical record

A record comprising 2880 8-bit bytes.

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.

Matrix

A data array of two or more dimensions.

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.

Picture element

A single location within an array.

Pixel

Picture element.

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.

Record

A sequence of bits treated as a single logical entity.

Reference point

The point along a given  coordinate axis , given in units  of pixel number, at which a value and increment are defined.

Repeat count

The  number of values represented in a binary table  field.

Reserved keyword

An  optional keyword that may be used only in the manner defined in this standard.

Special records

A series of 23040-bit (2880 8-bit byte) records, following the primary  HDU,  whose internal structure does not otherwise conform to that for the primary HDU or to that specified for a conforming extension  in  this standard.

Standard extension

A  conforming  extension  whose  header and data content are specified explicitly in this standard.

Type name

The value of the XTENSION keyword , used to identify the type  of the extension  in the data following.

Valid value

A member of a data array or table corresponding to an actual physical quantity.