CORWTFN

CORWTFN specifies the weighting function to be used on the VLBA correlator. The weighting function is applied by weighting the data points before they are sent to the FFT. Valid options are:

• UNIFORM. No weights are applied. The data are used as is. The is the option that most users will want.

• ZEROPAD. With this option, the input data for the FFTs are padded with zeros by a factor of two. This reduces the resolution of each spectral point. With the FX correlator, the frequency response of each spectral channel is sharper than with an XF correlator and this may not be what is desired for some spectral line observations. If so, ZEROPAD is a way to reduce the effect. Actually reproducing the frequency response of an XF correlator is possible in theory, but not with any of the available windowing functions. Note that ZEROPAD should not be used with the 1:4 fan out because that causes there to be no overlapping of FFTs so half the data are not used.

• HANNING. With this option, a Hanning weighting is applied in before the FFT. This is done in the voltage (sample) domain and has the effect, after cross correlation, of applying a ``Hanning squared'' weighting on the final power spectra. This is probably not what the user wants.

• QANNING. With this option, an approximation of a ``root Hanning'' weighting is applied to get close to the expected Hanning response in the power domain after correlation. It is only an approximation because the windowing function cannot have the negative values that would be required to do this exactly.

Eventually more options may be available.

• Argument: Any text string of up to 16 characters
• Options: UNIFORM and HANNING are the useful options.
• Default: UNIFORM
• Usage: Only one value used -- the last.
• Example: CORWTFN = 'HANNING'