7.1 Spectral Resolution

DiFX currently supports powers-of-2 numbers of spectral points spanning each individual sub-band, up to 4096 for routine DiFX processing, and up to 32,768 if required and adequately justified. (The latter limit is the maximum resolution currently supported by AIPS.) Oversampled data (essential for extremely high spectral resolution with the original VLBA baseband subsystem, but no longer necessary) can be decimated appropriately. Currently, both the number of spectral points, and the oversampling factor, must be the same for all sub-bands at any given time, although multiple passes are possible with different sets of sub-bands. The actual spectral resolution obtained, and statistical independence of the spectral points, depends on subsequent smoothing and other processing.

DiFX also supports ``spectral zooming'', selection of a subset of correlated spectral channels from any or all sub-bands. Only the selected channels are included in the output dataset. This capability will be of value mainly in maser studies, where the recorded band may be much wider than the maser emission in two main categories of observations: (1) Maser astrometry with in-beam continuum calibrators. Wideband observing is required for maximum sensitivity on the calibrators, while zooming allows high spectral resolution at the frequencies where maser emission appears. (2) Multiple maser transitions. When wide bands are used to cover a large number of widely separated maser transitions, spectral zooming allows the empty portions of high-resolution spectrum to be discarded.

In proposing observations that will use spectral zooming, the required number of channels before zooming should be specified in the Proposal Submission Tool. Currently, the location and width of the ``zoom'' sub-bands must be communicated directly to VLBA operations before correlation.