3.5.1 Header

The image extension follows the rules for required keywords in extensions stated in section 3.3.2. The first keywords of a FITS image extension header, through GCOUNT, must appear consecutively in the order listed below.

1.

XTENSION (character) has the value 'IMAGE' for image extensions.

2.

BITPIX (integer) provides the number of bits per ``pixel'' value. The values allowed are the same as those for the primary data array.

3.

NAXIS (integer) is, for an image extension, the number of axes in the extension data array. A value of zero is acceptable and indicates that no data are associated with the current header. The maximum possible value is 999. Negative values are not allowed.

4.

NAXISn, $n=1, \ldots ,$NAXIS (NAXIS=0 -> NAXIS1 not present) (integer) is the number of elements along axis n of the array. The NAXISn keywords in an image extension are governed by the same rules as those in a primary header.

5.

PCOUNT (integer) has the value 0 for image extensions.

6.

GCOUNT (integer) has the value 1 for image extensions.

(...the other keywords follow until...)

END (no value) The last keyword must be END. This card image has no ``='' in column 9 or value field but is filled with ASCII blanks.

The remainder of the last header record should be filled with ASCII blanks.

Note that while the rules for generalized extensions do not specify where the PCOUNT and GCOUNT keywords are located between the last NAXISn and the END keywords, the image extension rules require that they appear immediately after the last NAXISn card image.