4.4. KEYWORDS

31

Table 4.8.

Mandatory keywords in conforming extensions.

#

Keyword

1

XTENSION

2

BITPIX

3

NAXIS

4

NAXISn, n = 1, . . . , NAXIS

5

PCOUNT

6

GCOUNT

.

.

.

(other keywords)

.

.

.

last

END

XTENSION Keyword  The value field shall contain a character string giving the name

of the extension type. This keyword is mandatory for an extension header and must

not appear in the primary header.1 To preclude conflict, extension type names must

be registered with the IAUFWG. The current list of registered extensions is given in

Appendix F. An up to date list is also maintained on the FITS Support Office web site.

PCOUNT Keyword  The value field shall contain an integer that shall be used in any

way appropriate to define the data structure, consistent with Eq. 4.2. In IMAGE (§7.1)

and TABLE (§7.2) extensions this keyword must have the value 0; in BINTABLE extensions

(§7.3) it is used to specify the number of bytes that follow the main data table in the

supplemental data area called the heap. This keyword is also used in the random groups

structure (§6) to specify the number of parameters preceding each array in a group.

GCOUNT Keyword  The value field shall contain an integer that shall be used in any

way appropriate to define the data structure, consistent with Eq. 4.2. This keyword

must have the value 1 in the IMAGE, TABLE and BINTABLE standard extensions defined

in §7. This keyword is also used in the random groups structure (§6) to specify the

number of random groups present.

The total number of bits in the extension data array (exclusive of fill that is needed

after the data to complete the last 2880-byte data block) is given by the following

1

This requirement differs from the wording in the original FITS papers. See Appendix H.

FITS Standard