Correlator Configurations

by Stephan W. Witz last modified Apr 04, 2012

The EVLA correlator is very flexible, and will be able to provide data in many ways. For the Open Shared Risk Observing (OSRO) program available to the community during the period Sep 2011 through Jan 2013 are offering two independently tunable basebands, where each baseband has up to eight sub-bands. Possible sub-band widths are 128 MHz, 64 MHz, 32 MHz, all the way down in factors of 2 to 0.03125 MHz. All sub-bands must have the same bandwidth and channelization in both basebands, and be contiguous in frequency within each baseband. We will offer three different OSRO modes: full polarization, dual polarization, and single polarization, with 64, 128, and 256 channels per sub-band, respectively. There is always the possibility during offline processing to smooth in frequency to reduce dataset sizes or to improve spectral response.

Starting with the D-configuration in September 2011, we have been providing options for configuring WIDAR for OSRO in the following three ways:

1. "OSRO Full Polarization": Four polarization products. This configuration offers 4 polarization products for each sub-band, each of which has 128 MHz bandwidth with 64 channels. It is possible to decrease the subband bandwidth by powers of two, keeping the same number of channels, to provide the capabilities in the following table:
2. "OSRO Dual Polarization": Two polarization products. This configuration offers 2 polarization products for each sub-band, each of which has 128 MHz bandwidth with 128 channels. It is possible to decrease the sub-band bandwidth by powers of two, keeping the same number of channels, to provide the capabilities in the following table.
3: "OSRO Single Polarization": One polarization product (new for OSRO observing). It offers 1 polarization product for each sub-band, each of which has 128 MHz bandwidth with 256 channels. It is possible to decrease the sub-band bandwidth by powers of two, keeping the same number of channels, to provide the capabilities in the following table.

These capabilities are being provided with integration times no shorter than 1 second in A configuration (3 seconds in B/C/D configurations), and Doppler setting will be available with these correlator configurations.

If it is likely that the data will need to be resampled spectrally in order to Doppler track to a line rest frequency, care should be taken to make sure the spectrum is oversampled to avoid the subsequent introduction of Gibbs ringing or the need to reduce the spectral resolution by Hanning smoothing.

All observations with the EVLA correlator should be treated as traditional VLA spectral line observations, in that they will require observation of a bandpass calibrator. They may also require observation of a delay calibrator. Users should contact NRAO staff for advice on setting up observations with the EVLA correlator.