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Current Header Data Unit (CHDU)

A basic concept used throughout FITSIO is that of the current Header Data Unit (CHDU) within the FITS file. The FITSIO subroutines which read or write information only operate on the 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). FITSIO subroutines 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.