next up previous contents Back To Software Index
Next: Routine Names Up: Basic CFITSIO Conventions Previous: Piped FITS Files

Current Header Data Unit (CHDU)

A basic concept used throughout CFITSIO is that of the current Header Data Unit (CHDU) within the FITS file. The CFITSIO routines 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). 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.