The XTENSION keyword is required to be the first keyword of
all image extensions.
The card images in the header of an image
extension must use the keywords defined in Table 8.3
in the order specified. No other keywords may intervene between the
XTENSION and GCOUNT keywords.
Table 8.3:
Mandatory keywords in image extensions.
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 5.2.
The value field shall contain a non-negative integer no greater than
999, representing the number of axes in the associated data
array. A value of zero signifies that no data follow the
header in the image extension.
The value field of this indexed keyword shall contain a non-negative
integer, representing the number of elements along axis n of
a data array. The NAXISn must be present
for all values n = 1, ..., NAXIS, and for no other values of
n. A value of zero for any of
the NAXISn signifies that no data follow the
header in the image extension.
If NAXIS is equal to 0,
there should not be any NAXISn keywords.
