FITS header

Every header and data unit (HDU) in a FITS file starts with a header: plain ASCII keyword records, 80 characters each, ending with an END record and padded with spaces to a whole 2880-byte block. This page follows version 4.0 of the FITS Standard (IAU FITS Working Group, 2016, language-edited 2018). The data that follows the header is on the data page, the extension headers on the extensions page and the coordinate keywords on the world coordinates page.

Blocks and records

A FITS block is 2880 bytes, which holds 36 keyword records of 80 bytes.
Header records use only the ASCII characters 32 to 126 (space to tilde). There are no line ends between records.
The header ends with the END record. Any records left in the last block are filled with spaces (20).
The data of the HDU starts at the next 2880-byte boundary.

Keyword record

Bytes	Size	Content
1–8	8	Keyword name, left-justified and padded with spaces. Only `A`–`Z`, `0`–`9`, hyphen and underscore, no spaces inside. An index has no leading zeros (`NAXIS1`, never `NAXIS001`).
9–10	2	Value indicator `=` (equals sign, space) when the record has a value. Commentary keywords and `CONTINUE` do not use it.
11–80	70	The value, then an optional comment introduced by `/`. A space before the slash is strongly recommended.

Without the value indicator, bytes 9 to 80 are free text, as in COMMENT and HISTORY.

Value formats

Mandatory keywords must use the fixed format. Other keywords may use the free format, where the value can sit anywhere in bytes 11 to 80.

Type	Written as	Fixed format
Character string	Between single quotes; a quote inside is written twice (`'O''HARA'`). Leading spaces count, trailing spaces do not. Up to 68 characters in one record.	Opening quote in byte 11, closing quote in or before byte 80
Logical	`T` or `F`	Byte 30; in free format, the first non-space character in bytes 11 to 80
Integer	Optional `+` or `-`, then decimal digits	Right-justified in bytes 11 to 30
Real	Decimal number with an optional exponent, written with an upper-case `E` or `D`	Right-justified in bytes 11 to 30
Complex integer	Real and imaginary parts in brackets, separated by a comma: `(123, 45)`	No fixed format
Complex real	The same, with real numbers: `(123.23, -45.7)`	No fixed format
Undefined	Nothing (only spaces) in the value field after `=`	–

Long strings

A string longer than 68 characters is split with the CONTINUE convention, part of the standard since version 4.0:

Split the string into pieces shorter than 68 characters, never splitting a doubled quote.
End every piece except the last with an ampersand (&).
Write the first piece as the keyword’s value.
Write each later piece in a CONTINUE record: the name in bytes 1 to 8, spaces in bytes 9 and 10, then the quoted piece.

CONTINUE must not be used with mandatory or reserved keywords unless they are declared to take long strings.

Mandatory keywords of the primary header

Position	Keyword	Value
1	`SIMPLE`	`T`: the file conforms to the standard. Required here, and not allowed in extensions.
2	`BITPIX`	Bits per data value: 8, 16, 32, 64, -32 or -64 (see data).
3	`NAXIS`	Number of axes, 0 to 999. Zero means no data follows.
4	`NAXISn`	Length of axis n, for n = 1 to `NAXIS`, in order. Non-negative.
…	Other keywords	Any order.
Last	`END`	No value; bytes 9 to 80 are spaces.

No other keyword may come between SIMPLE and the last NAXISn. The mandatory keywords of an extension header are on the extensions page.

Reserved keywords for any HDU

Keyword	Value	Meaning
`DATE`	String	Date the HDU was created: `YYYY-MM-DD` or `YYYY-MM-DDThh:mm:ss[.sss…]`, in UTC for data made on Earth. Files written before 2000 may use `DD/MM/YY`.
`DATE-OBS`	String	Date of the observation, in the same format; the start of the observation unless the comment says otherwise. `DATExxxx` keywords follow the same rules.
`ORIGIN`	String	The organisation that created the file.
`AUTHOR`	String	Who compiled the data, for data taken from papers or several sources.
`REFERENC`	String	Bibliographic reference, ideally an ADS bibcode or a DOI.
`TELESCOP`	String	The telescope.
`INSTRUME`	String	The instrument.
`OBSERVER`	String	Who took the data.
`OBJECT`	String	The name of the object observed.
`COMMENT`	None	Commentary text in bytes 9 to 80.
`HISTORY`	None	Processing history, as text in bytes 9 to 80.
Blank name	None	Commentary text; bytes 1 to 8 are spaces.
`CONTINUE`	String	The next piece of a long string (above).
`CHECKSUM`	String	16-character checksum of the whole HDU (below).
`DATASUM`	String	Checksum of the data part, as a decimal number in quotes (below).
`EXTEND`	Logical	Primary header only. `T` says the file may contain extensions. Advisory: extensions are allowed without it.
`BLOCKED`	Logical	Primary header only, deprecated. Said the tape blocking was a multiple of 2880 bytes.
`EQUINOX`	Real	Equinox of the celestial coordinate system, in years (see world coordinates).
`EPOCH`	Real	Deprecated older name for `EQUINOX`.

Reserved keywords for arrays

These describe the values of a primary array or an IMAGE extension. How they are applied is on the data page.

Keyword	Value	Meaning
`BSCALE`	Real	Scale factor for the stored values. Default 1.0.
`BZERO`	Real	Offset added after scaling. Default 0.0.
`BUNIT`	String	Physical units of the scaled values.
`BLANK`	Integer	The stored value that marks an undefined pixel. Only for positive `BITPIX`.
`DATAMAX`	Real	Largest valid physical value, ignoring IEEE special values.
`DATAMIN`	Real	Smallest valid physical value, ignoring IEEE special values.

Reserved keywords for extensions

Keyword	Value	Meaning
`EXTNAME`	String	Name that tells apart extensions of the same type.
`EXTVER`	Integer	Version that tells apart extensions with the same type and name. Default 1.
`EXTLEVEL`	Integer	Level in a hierarchy of extensions. Default 1.
`INHERIT`	Logical	Extension headers only, directly after the mandatory keywords. `T` means the extension takes on the keywords of the primary header (below).

The INHERIT convention should be used only when the primary HDU has no data (NAXIS = 0). The mandatory primary keywords and the commentary keywords are never inherited, nor are keywords specific to tables. When a keyword is in both headers, the extension’s value wins. With INHERIT = F nothing is inherited.

Checksums

Both checksums are 32-bit ones’ complement sums, taken over the bytes as big-endian 32-bit integers (with the carry out of bit 31 added back into bit 0).

DATASUM is the sum over the data blocks of the HDU, written as an unsigned decimal number in quotes, such as '2503531142'. It must be updated before CHECKSUM.
CHECKSUM makes the sum over the whole HDU, header and data, come out as negative zero (all ones). To write it, set the value to sixteen 0 characters, sum the HDU, and take the complement of the sum.
The complement is encoded as 16 characters. Each of its four bytes is split into four parts that add up to the byte, 0x30 (ASCII 0) is added to each part, and the 16 values are interleaved as A1 B1 C1 D1 A2 B2 C2 D2 … Pairs are then adjusted so that every character is a letter or digit, never : to @ (3A–40) or [ to ` (5B–60).
Finally the 16 characters are rotated one place to the right (the last moves to the front), because the string starts in byte 12 of its record, one byte before a 4-byte boundary.

A complete primary header

This header describes a 100 by 100 image of 16-bit integers. Each line here is one 80-byte record; in the file the records follow each other with no line ends, and the block is padded to 2880 bytes with spaces. The T of SIMPLE is in byte 30, and the integers end in byte 30.

SIMPLE  =                    T / file conforms to the FITS standard
BITPIX  =                   16 / 16-bit two's complement integers
NAXIS   =                    2 / number of axes
NAXIS1  =                  100 / width in pixels
NAXIS2  =                  100 / height in pixels
EXTEND  =                    T / extensions may follow
OBJECT  = 'M31'                / name of the object
DATE    = '2016-07-22'         / date the file was written
END

Nine records, 720 bytes; the rest of the 2880-byte block is spaces

The same first record as bytes: 53 49 4D 50 4C 45 20 20 is SIMPLE with two spaces, 3D 20 is the value indicator, then 19 spaces and 54 (T).

Conventions outside the standard

Some widely used conventions are registered with the FITS Support Office but are not part of the standard, for example ESO’s HIERARCH convention for keyword names longer than eight characters. A reader that does not know a convention sees ordinary records and can skip them.

.FITS File Format – Header