At bandwidths of 32MHz or less you may have trouble calibrating the delays due to insufficient S/N in the narrow channels. VIS will display wildly varying delays in this case. (Note: At narrow bandwidth the delay calibration is much less critical, so you may be able to skip it completely.) As a rule of thumb: You want the delays to be less than 32/bandwidth[MHz] nanoseconds, however if you don't form a continuum channel but process the channels individually in the reduction you can use the channel bandwidth instead of the observing bandwidth in this equation. This means, if VIS displays wildly varying delays, but the amplitudes are roughly ok, you can probably skip the delay calibration (use Interactive mode in CACAL). If after all this you still think you need to improve your delay calibration (most people believe that they have to) you have the following options:
293 at 3cm.
The gains will just be equalized on these sources,
i.e., no absolute calibration is performed.
If you have `randomized' your delays by calibrating on a source that wasn't strong enough, you can restore the global delay values using the CACAL Reset option (choose Interactive and then Reset, then D for delays). These values are often good enough for spectral line observing.