ACSIS data comes in two forms: time-series cubes and spatial/spectral cubes. Time-series cubes have frequency on the first axis, detector on the second, and time on the third. As data comes in off the telescope, the time slices are written to disk. Because data acquisition is asynchronous, time slices are not necessarily written in sequential order. Further, there is a 500-megabyte size limit for raw data files, more than one file can be written per observation. These files are called subscans.
ACSIS can be configured to take data at two (or more) frequencies or bandwidth modes at the same time in different subsystems. Subsystems are treated as individual and separate observations by Orac-dr except for hybrid mode observations, where two (or four, for RxA, RxB, and RxW) subsystems are set up to overlap in frequency space in such a way that overlapping channels observe the same frequency.
Astronomers would rather deal with spatial/spectral cubes (heretofor referred to as cubes – when time-series cubes are discussed they will be called time-series cubes instead of time-series cubes. Time-series cubes are regridded onto a spatial grid, creating a cube with right ascension and declination on the first two axes and frequency on the third.