3.3.1 Required Keywords for an Extension Header

  The first keywords of a FITS extension header, from XTENSION to the last NAXISn, must appear consecutively in the order listed below.

1.

XTENSION (character) indicates the type of extension. This keyword must appear on the first card image in the header. With this keyword at the front, any software can immediately read the extension type and determine whether or not it is a type that the software can handle.

2.

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

3.

NAXIS (integer) gives the number of ``axes'' or analogous structures; a value of zero is allowed, which indicates there are no data records in the current extension. The maximum possible value is 999. Negative values are not allowed.

4.

NAXIS$n, n=1, \ldots ,$ NAXIS (integer) (NAXISn=0 for any n implies that the extension has no data) is the number of array elements along the nth axis or its analog.

In order to allow FITS readers that are unfamiliar with an extension to skip it, a header must specify the size of the extension in bits. To do so, two parameters, PCOUNT and GCOUNT, are defined in the header:

• PCOUNT (integer) - The value may be zero. Note that although this keyword originated in the Random Groups (section 3.2) rules, the interpretation for an extension may be different.
• GCOUNT (integer) - If simple image-like data (e.g., a table) are being written, its value will be 1.

  The PCOUNT and GCOUNT keywords may appear anywhere between the last NAXISn (or NAXIS=0)  keyword and the END keyword. The best place is immediately after the last NAXISn keyword; the existing standard extensions require that location, but otherwise software to read FITS files should not assume they will be there. They may be defined in any way that, along with the BITPIX, NAXIS, and NAXISn values, gives the correct data size using the following formula to derive NBITS, the number of bits excluding fill:
  

  NBITS = ABS(BITPIX) × GCOUNT times

   (PCOUNT + NAXIS1 × NAXIS2 × ... × NAXISm) (3.9)

  where m is the value of NAXIS. NBITS may not be negative.

  The number NRECORDS of data records following the header record is then given by

  NRECORDS = INT[(NBITS + 23039) / 23040)] (3.10)

  Rules for individual extensions may place further restrictions on the position of the PCOUNT and GCOUNT keywords.

  END is always the last keyword in a header. The remainder of the record following the END keyword is filled with ASCII blanks.

The data begin at the start of the first record following the last record of the header.