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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:

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


next up previous contents
Next: Reserved Keywords for Extension Up: Extensions Previous: Extensions