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In general, a FITS file can contain multiple Header Data Units, also called extensions. CFITSIO only operates within one HDU at any given time, and the currently selected HDU is called the Current Header Data Unit (CHDU). When a FITS file is first created or opened the CHDU is automatically defined to be the first HDU (i.e., the primary array). CFITSIO routines are provided to move to and open any other existing HDU within the FITS file or to append or insert a new HDU in the FITS file which then becomes the CHDU.