Description

The intent of cvel is to transform channel labels and the visibilities to a spectral reference frame which is appropriate for the science analysis, e.g. from TOPO to LSRK to correct for Doppler shifts throughout the time of the observation. Naturally, this will change the shape of the spectral feature to some extent. According to the Nyquist theorem you should oversample a spectrum with twice the numbers of channels to retain the shape. Based on some tests, however, we recommend to observe with at least 3-4 times the number of channels for each significant spectral feature (like 3-4 times the linewidth). This will minimize regridding artifacts in cvel.

If cvel has already established the grid that is desired for theimaging, clean should be run with exactly the same frequency/velocity parameters as used in cvel in order to avoid additional regridding in clean.

Hanning smoothing is optionally offered in cvel, but tests have shown that already the regridding process itself, if it involved a transformation from TOPO to a non-terrestrial reference frame, implies some smoothing (due to channel interpolation) such that Hanning smoothing may not be necessary.