5.4 STScI Inheritance Convention

The rules of FITS do not specify whether or not keywords in the primary header apply to extension data in the same file. STScI has adopted the following conventions governing such inheritance of keywords (Zarate and Greenfield 1996).

1.

Primary header keywords can be inherited only if there is no primary data array.

2.

Primary header keywords apply to extensions only if the header contains the keyword INHERIT with the value set to T.

3.

Required FITS keywords are not inherited.

4.

If a keyword appears both in the primary header and an extension header, the value in the extension header will apply to the extension.

5.

The reserved commentary keywords, COMMENT, HISTORY, and , are never inherited.

6.

Changes to an inherited keyword appear only in the extension header, not the primary header, i.e., the inherited keyword will appear in the extension header with its new value and the primary header value remains unchanged.

As with any FITS convention, software may allow for overriding the inheritance convention for specific purposes. For example, the IRAF FITS kernel has a mechanism to override the rules given in items 1 and 2.