4.4. KEYWORDS

29

Table 4.6.

Interpretation of valid BITPIX value.

Value

Data Represented

8

Character or unsigned binary integer

16

16-bit two's complement binary integer

32

32-bit two's complement binary integer

64

64-bit two's complement binary integer

-32

IEEE single precision floating-point

-64

IEEE double precision floating-point

SIMPLE Keyword  The value field shall contain a logical constant with the value T if

the file conforms to this standard. This keyword is mandatory for the primary header

and must not appear in extension headers.1 A value of F signifies that the file does not

conform to this standard.

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 in the associated data array. The only valid values of BITPIX are

given in Table 4.6. Writers of FITS arrays 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. A value of zero

signifies that no data follow the header in the HDU.

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 in-

dexed keyword shall contain a non-negative integer representing the number of elements

along axis n of a data array. A value of zero for any of the NAXISn signifies that no data

follow the header in the HDU (however, the random groups structure described in §6

has NAXIS1 = 0, but will have data following the header if the other NAXISn keywords

are non-zero). If NAXIS is equal to 0, there shall not be any NAXISn keywords.

END Keyword  This keyword has no associated value. Bytes 9 through 80 shall be

filled with ASCII spaces (decimal 32 or hexadecimal 20). The END keyword marks the

1

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

FITS Standard