46

SECTION 7. STANDARD EXTENSIONS

Table 7.1.

Mandatory keywords in image extensions.

#

Keyword

1

XTENSION= 'IMAGE

'

2

BITPIX

3

NAXIS

4

NAXISn, n = 1, . . . , NAXIS

5

PCOUNT = 0

6

GCOUNT = 1

.

.

.

(other keywords . . . )

.

.

.

last

END

BITPIX Keyword  The value field shall contain an integer. The absolute value is

used in computing the sizes of data structures. It shall specify the number of bits that

represent a data value. The only valid values of BITPIX are given in Table 4.6. Writers

of IMAGE extensions should select a BITPIX data type appropriate to the form, range of

values, and accuracy of the data in the array.

NAXIS Keyword  The value field shall contain a non-negative integer no greater than

999, representing the number of axes in the associated data array. If the value is zero

then the image extension shall not have any data blocks following the header.

NAXISn Keywords  The NAXISn keywords must be present for all values n = 1, ...,

NAXIS, in increasing order of n, and for no other values of n. The value field of this

indexed keyword shall contain a non-negative integer, representing the number of ele-

ments along axis n of a data array. If the value of any of the NAXISn keywords is zero,

then the image extension shall not have any data blocks following the header. If NAXIS

is equal to 0, there should not be any NAXISn keywords.

PCOUNT Keyword

The value field shall contain the integer 0.

GCOUNT Keyword  The value field shall contain the integer 1; each image extension

contains a single array.

FITS Standard