The main reason for using this routine is to ensure that data have a defined transformation from WCS coordinates to pixel coordinates. It can also be used to reduce the size of data files by excluding dead detectors (see parameter DETPURGE). This command should be run before attempting to merge multi-subsystem data.
There are two main modes, selected by parameter MERGE. If MERGE is FALSE, then the time slices in each input NDF are sorted independently of the other NDFs, and each output NDF contains data only from the corresponding input NDF. If MERGE is TRUE, then the input NDFs are sorted into groups that contain NDFs from the same observation and sub-system (that is, all NDFs in a group have the same value for the OBSIDSS FITS keyword). For each group, the time slices in all NDFs in the group are sorted into a single list. This list is then divided up into chunks (in a manner selected by parameter SIZELIMIT), and the time slices are written out sequentially to a number of output NDFs. If any time slice is present in more than one input NDF, then the data values for the two or more input time slices are merged into a single time slice.
MERGE = TRUE should be used to sort the time slices contained in a set of sub-scans from a sub-system.
SMURF -- the Sub-Millimetre User Reduction Facility