28

SECTION 4. HEADERS

Table 4.5.

Mandatory keywords for primary header.

#

Keyword

1

SIMPLE = T

2

BITPIX

3

NAXIS

4

NAXISn, n = 1, . . . , NAXIS

.

.

.

(other keywords)

.

.

.

last

END

field delimiter by a single space character. An example, using a non-standard keyword,

is

EXPTIME = 1200.  / [s] exposure time in seconds

This widespread, but optional, practice suggests that square brackets should be used

in comment fields only for this purpose. Nonetheless, software should not depend on

units being expressed in this fashion within a keyword comment, and software should

not depend on any string within square brackets in a comment field containing a proper

units string.

4.4

Keywords

4.4.1

Mandatory Keywords

Mandatory keywords are required in every HDU as described in the remainder of this

subsection. They must be used only as described in this standard. Values of the

mandatory keywords must be written in fixed-format.

4.4.1.1

Primary Header

The SIMPLE keyword is required to be the first keyword in the primary header of all

FITS files. The primary header must contain the other mandatory keywords shown in

Table 4.5 in the order given. Other keywords must not intervene between the SIMPLE

keyword and the last NAXISn keyword.

FITS Standard