Raw data take the form of multiple NDFs within an HDS container file for UKIRT data, or individual FITS files for AAT, INGRID, ISAAC, NACO, NIRI, and Classic Cam data. For UFTI, UIST, and IRCAM they comprise one NDF for the data array and dynamic headers, such as the start time of the exposure, and another for static headers. Each container file is converted to a single NDF in $ORAC_DATA_OUT with a merged set of headers.
The Michelle HDS container file 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 and NIRI, ORAC-DR converts a multi-extension FITS file into a multi-NDF HDS container file following UKIRT conventions.
ORAC-DR -- imaging data reduction