Total Number of Channels

The total number of available channels for a selected bandwidth and correlator mode, with and without Hanning smoothing, is listed in Appendix A. In the case of on-line Hanning smoothing the channel separation is equal to the frequency resolution. Without Hanning smoothing an unresolved spectral line will closely resemble a sinc function and, therefore, the frequency resolution (FWHM) is equal to 1.2 times the channel separation. The number of channels (2$^{n}$) shown in the tables is the maximum for each particular combination of total bandwidth and observing mode. Channels are numbered 0 through (2$^{n}$-1). The central channel is 2$^{n-1}$. Channel 0 does not contain any astronomically useful information and is always replaced by the average of the central 75% of the available band. Although rarely used, one can select a subset of channels to be recorded, thus reducing the data rate. In that case, the number of channels chosen must be a power of 2 and not less than 4. Ordinarily, one would want to specify a start channel, $k$. If 2$^{m}$ channels are chosen, the new channels run from 1 to 2$^{m-1}$ and correspond to the old channels $k+1$ through $k -1 + 2^{m}$ and the original center of the band is now at $2^{n-1} - k$. Channel zero retains its original value.

Note that there is a hard limit of about 10,000 to the product of the number of baselines and the number of frequency channels, divided by the integration time in seconds. This limit is set by the speed at which the on-line computer system can process and record the data.