Raw data take the form of multiple NDFs within an HDS container file for UKIRT data, or individual FITS files for AAT, INGRID and ISAAC data. For UISTthey comprise of one NDF for the data array and dynamic headers, such as the start time of the exposure, and another for static headers. For the HDS containers, initial reduction steps operate on each of the NDFs individually, only merging them to a simple NDF once the interleaving step is complete.
The Michelle HDS container also has NDFs for the individual chop beams. However, these cannot be merged until the data variance is calculated from the individual beams. Michelle reduced chopped data become simple NDFs once the recipe takes the difference of the two beams.
ORAC-DR automatically converts AAT FITS files into single NDFs in $ORAC_DATA_OUT which retain the original FITS headers. For INGRID, ORAC-DR converts a multi-extension FITS file into a multi-NDF HDS container file following UKIRT conventions.
ORAC-DR -- spectroscopy data reduction