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CORCHAN(1) specifies the number of spectral channels per baseband channel (one polarization component) that the correlator will write. CORCHAN(2) specifies the number of channels to use in the internal FFT in the correlator.

The VLBA correlator (both old and new) will, by default, make 128 channel spectra which are then channel averaged on-line for continuum observations. The typical number of output channels for continuum observations is the greater of 16 or twice the baseband channel bandwidth in MHz. Over averaging can cause loss of amplitude due to ``delay smearing'' -- the effect of large phase slopes across the channels going into an average. Delay offsets can either come from a large field of view or from errors in the a priori clocks used in processing. The twice bandwidth number is determined by the clocks and should represent a minimum.

Corrections for this effect are made in AIPS, but the corrections are probably not perfect. Besides delay smearing, over averaging 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).

For spectral line observations, the DiFX correlator can do a vary large number of channels - "it's just software". But excessive numbers of channels lead to excessive output data rates and data set sizes. Up to 4096 channels per baseband channel are supported normally and up to 32768 channels can be supported if needed and justified. There are no special restrictions in full polarization mode. The 32768 limit is set by the AIPS postprocessing path. 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 10 Mbyte/sec. That is per second of observe time. The data rate in bytes/sec is given APPROXIMATELY as

4 * Ns * (Ns + 1) * Nc * Nsp * P / 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 Tavg = time average in seconds

The second argument can be used to specify the size of the FFT used in correlators such as DiFX. Normally that argument can be ignored and the FFT size will be set to the larger of 128 or the first argument. The ability to set that argument is provided in support of the muliple phase center capability added to DiFX in 2010. When using that option, the spectral resolution of the transforms done before the different phase centers are split must be high enough that the differential delays, which show up in the data as a phase slope in frequency, do not cause smearing. See the discussion of multiple phase center processing for more details advice.

Note the correlation parameters CORAVG, CORCHAN, CORPOL, CORTAPE, and CORSHIPn must be specified when the project will be correlated in Socorro and data are being recorded.

next up previous contents
Next: CORDFMT Up: Details of SCHED Parameters Previous: CORAVG2   Contents
Craig Walker 2014-06-17