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1.4.1 Current Header Data Unit (CHDU)

A basic concept used throughout the FITSIO interface is that of the Current Header - Data Unit (CHDU). When a FITS file is first opened, the CHDU is defined to be the primary header and data array (i.e., the first HDU in the FITS file). Most of the FITSIO subroutines only operate within the CHDU and will only read or write header keywords or data values in the CHDU. Other FITSIO routines are provided to move to other HDUs within the FITS file or to append a new HDU to the end of the FITS file.