The technical specification for the FITS format is described in several files, each of these files is a plain ASCII document:
AAREADME.TXT provides general overview information about the FITS format.
FITS_BAS.TXT is descriptive information about the FITS format that is updated by NASA on a monthly basis and posted to USENET.
FITS_SRC.TXT lists where sources of FITS information, including a FAQ, can be found. This material is updated periodically by the FITS Support Office at NASA GSFC.
FORMAT.TXT describes the FITS format.
XTENSION.TXT describes extensions to the basic FITS format.
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