CORCHAN specifies the number of spectral channels per baseband channel (one polarization component) that the correlator will write. The VLBA correlator will typically make 256 channel spectra which are then channel averaged on-line for continuum observations. The typical number of output channels for continuum data is 16 per baseband. Over averaging can cause loss of amplitude due to ``delay smearing'' -- the effect of large phase slopes across the channels going into an average. Corrections for this effect are made in AIPS, but the corrections are probably not perfect. Over averaging also can lead to SNR loss when bandpass calibration is done. This is because edge channels are discarded as part of bandpass calibration because of the frequency shifts required to adjust for the effects of the fringe rotators (doppler effect, essentially). It is probably wise to keep a minimum of 16 channels and keep the channel bandwidth less than 0.5 MHz. The latter constraint, due to possible delay errors, suggests that observations with 16 MHz BBC bands should use 32 output channels.
For spectral line observations, up to 1024 spectral channels per baseband channel can be provided in parallel-hand-only correlation mode and up to 256 channels in full polarization mode. Requesting too many channels can cause the data sets to be so large as to be difficult to impossible to manage. Also, the combination of the average time, the number of channels, and the number of baselines must be such that the output data rate is less than 0.5 Mbyte/sec. The data rate in bytes/sec is given APPROXIMATELY as
4 * Ns * (Ns + 1) * Nc * Nsp * P * Su / Tavg
where Ns = number of stations Nc = number of (BaseBand) channels (1, 2, ... 16) Nsp = spectral resolution (8, 16, 32 ... 512) P = 2 for polarization, 1 for none Su = speed up factor (1, 2, 4) Tavg = time average in seconds