Wideband correlators for radio astronomy

A description of modern wideband digital correlators being developed at the National Radio Astronomy Observatory (NRAO) for radio astronomy is presented. The NRAO is developing several modern wideband instruments for spectroscopic analysis in radio astronomy. One instrument now in the system test phase is a spectrometer intended for use on the 100 meter Green Bank Telescope (GBT) being constructed by the NRAO. This spectrometer has a maximum bandwidth of 6.4 GHz comprised of 8 separate 800-MHz-wide segments and can develop 16,384 points of spectral resolution in its wideband mode. Factor of 2 trade-offs between bandwidth and resolution can be made. In an alternate narrow bandwidth mode, the GBT spectrometer can develop spectra with 262,144 point resolution over a maximum bandwidth of 1600 MHz comprised of 32 separate 50-MHz-wide segments, again with bandwidth/resolution trade-offs possible. Additional spectrometers being built or planned using hardware developed for the GBT system include a spectrometer for the NRAO 12 Meter Telescope in Tucson and a test correlator for the proposed Millimeter Array (MMA). A design for the final MMA correlator is also being studied at the NRAO. The MMA is projected to be an aperture synthesis array consisting of about 40 radio telescopes (the exact array size is currently uncertain). The correlator proposed for the MMA will be able to analyze the auto- and cross-spectra of the array output with a bandwidth of up to 16 GHz per antenna.