30

SECTION 4. HEADERS

Table 4.7.

Example of a primary array header.

Keyword Records

SIMPLE

=

T

/

file does

conform to FITS standard

BITPIX

=

16

/

number of

bits per data pixel

NAXIS

=

2

/

number of

data axes

NAXIS1

=

250

/

length of

data axis 1

NAXIS2

=

300

/

length of

data axis 2

OBJECT

= 'Cygnus X-1'

DATE

= '2006-10-22'

END

logical end of the header and must occur in the last 2880-byte FITS block of the header.

The total number of bits in the primary data array, exclusive of fill that is needed

after the data to complete the last 2880-byte data block (§3.3.2), is given by the following

expression:

Nbits  = |BITPIX| × (NAXIS1 × NAXIS2 × · · · × NAXISm),

(4.1)

where Nbits must be non-negative and is the number of bits excluding fill, m is the

value of NAXIS, and BITPIX and the NAXISn represent the values associated with those

keywords. Note that the random groups convention in the primary array has a more

complicated structure whose size is given by Eq. 6.1. The header of the first FITS

extension in the file, if present, shall start with the first FITS block following the data

block that contains the last bit of the primary data array.

An example of a primary array header is shown in Table 4.7. In addition to the

required keywords, it includes a few of the reserved keywords that are discussed in

§4.4.2.

4.4.1.2

Conforming Extensions

All conforming extensions, whether or not further specified in this standard, must use the

keywords defined in Table 4.8 in the order specified. Other keywords must not intervene

between the XTENSION keyword and the GCOUNT keyword. The BITPIX, NAXIS, NAXISn,

and END keywords are defined in §4.4.1.1.

FITS Standard