Spectral Resolution
DiFX allows quite flexible selection of the desired number of "spectral points" spanning each individual data channel. Any number that can be factored as 2n · 5m can be specified, subject to these limitations:
- A maximum of 4096 points per channel, for routine DiFX processing.
- A total of 132,096, summed over all channels and polarization products, for compatibility with AIPS.
- A minimum spectral resolution of 2 Hz.
The number of spectral points must be the same for all data channels at any given time, although multiple passes are possible with different sets of channels. 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 points from any or all data channels. Only the selected spectral points are included in the output dataset. This capability is of value mainly in maser studies, where a recorded data channel 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 wideband data channels 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.
Spectral zooming does not work with mixed sideband observing, this can happen for HSA or global observations where some telescopes require upper sideband and others require lower sideband. In proposing observations that will use spectral zooming, the required number of spectral points before zooming should be specified in the Proposal Submission Tool. Currently, the location and width of the "zoom" bands must be communicated directly to VLBA operations before correlation.
