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Proposal 5: A convention for pre-allocating space in FITS headers for future use

Definition

Any completely blank FITS keyword records (filled with 80 ASCII space characters) that immediately precede the END keyword should be considered as unused non-significant space that may be reused when appending new keywords to the header. The first non-blank keyword that preceeds the END keyword marks the 'effective' end of the header. When a new keyword is appended to the header, it should be written immediately after this effective end keyword.

By default, this unused header space, if any, should be preserved when copying the FITS header to a new file.

Discussion

This convention allows the creator of a FITS file to reserve space in each FITS header for additional keywords that may be written at a later date. This is especially useful when dealing with very large FITS files, where there is significant overhead associated with inserting a new header block to make room for more keywords (which requires rewriting all the subsequent data).