3.1.1.1 Required Keywords

 The following keywords are required for all Basic FITS headers for all time, and must appear in the order given below. All except SIMPLE must appear in other headers as well, in the same order. The value field must appear in the fixed format described in section 3.1.1 .

1.

SIMPLE (logical) - A value of ``T'' signifies that the file conforms to FITS standards. A value of ``F'' is used for files that resemble FITS files but depart from the standards in some significant way. One example would be files where the numbers are in the DEC VAX internal storage format rather than the standard FITS most significant byte first. The ``FITS'' files produced by some hardware that contain non-standard data formats such as two-byte unsigned integers or give the month number before the day number in date formats are another example. Such files might be convenient for internal use by a particular organization or for exchange between users with the same hardware for whom convenience is more important than standardization, when they wish the files to have an overall FITS-like structure. No installation should use them as the standard format for communication with outside users. Files with SIMPLE  = F should not be described as FITS files.

2.

BITPIX (integer) describes how an array value is represented:

No other values for BITPIX are valid.

The name comes from the original FITS design, in which the values of the array were regarded as pixels in a digital image (BITs per PIXel). With the use of negative values of BITPIX to signify floating point array values, the number of bits per data array member is the absolute value of BITPIX.

3.

NAXIS (integer) is, for the primary header, the number of axes in the data following the associated primary data array. A value of zero is acceptable and indicates that no data are associated with the current header. The most common reason for a primary HDU with no data is that all the data in the file are extensions. The maximum possible value is 999. Negative values are not allowed.

4.

NAXIS$n, n=1, \ldots ,$NAXIS (NAXIS=0 -> NAXIS1 not present) (integer) is the number of elements along axis n of the array; NAXIS1 describes the most rapidly varying index of the array, NAXIS2 the second most rapidly varying, etc. This convention is the same as the one used in FORTRAN. A value of zero for any of the NAXISn signifies that no data array is associated with the header. None of the NAXISn may be negative. The rules of FITS do not strictly forbid use of NAXISn keywords for values of n>NAXIS. While some groups use such keywords for special purposes, the practice is not recommended as a general rule.

...the other keywords follow until...

5.

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.

Other keywords may appear only between the last NAXISn and END keywords. The remainder of the last header record should be filled with ASCII blanks.

These keywords prescribe the size of the primary data array in bits, NBITS, through equation 3.1,

NBITS=	ABS(BITPIX) x
	(NAXIS1 x NAXIS2 x ... NAXISm),	(3.1)

where m is the value of NAXIS and the keyword names represent the values of those keywords.

Examples 1 and 2 in Appendix A illustrate primary headers that precede data arrays.