This I/O system expects (but does not require) a FITS file to end with the extension ".fit" or ".fits", and thus it is most strongly recommended that users refrain from creating structures called "FIT" or "FITS" at the top level of an HDS file (Asterix will first assume that img.fits is a FITS file, and only if this fails will it check for the data structure "FITS" in the file img.sdf).
This recognition scheme can be enforced or overridden by use of the percent mechanism. File names can be specified with the following strings appended with no intervening white space, "%hds" and "%fits". Thus "img.fits%hds" will only attempt to access the "FITS" object in the file "img.sdf" and will not attempt to access the file "img.fits" should that opperation fail.