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25th of November 2021
The Parkes variable baseline interferometer
by Ron Ekers
Of all the observations and experiments conducted at the Parkes Observatory, arguably the most innovative was the variable baseline interferometer, which was used in the late 1960s and early 1970s. An interferometer combines the signals from two (or more) separate telescopes, in this case with the 18m (60-foot) Kennedy antenna, given the Wiradjuri name Giyalung Guluman, used with the 64m (210-foot) Dish, Murriyang.

Initially interferometer visibilities between Parkes and the 18m antenna were measured continuously while the 18m moved along the baseline at a few km/hr. The 18m telescope was powered by a hydraulic system pulling on a fixed cable like a punt. The cable carriage in the picture above laid the communication and LO cable on the ground as the 18m moved along the track. The cable carriage was pulled along at the correct speed by a cable and pulley connected to the 18m telescope. The one-dimensional structure of sources could be determined in about 15 minutes using this system. With other telescopes at Cambridge or Caltech it took months to collect enough spacings.

The LO cable was supposed to be phase stable (it was a temperature compensated coaxial cable) but the mechanical process of bending it to lay it down caused phase errors which we could not correct. In the later years of its life the 18m was moved to fixed locations and phase calibrated separately for each baseline.

This may have been the only operational variable baseline interferometer ever built, although the French had also proposed such a system to be called hyper-synthesis. They wanted to do this at mm wavelengths so the baseline could be changed faster than the atmospheric variations. Space VLBI was in effect also a variable baseline interferometer, with the satellite-borne telescope moving rapidly compared to the ground-based telescopes. (Ron gives more details of the variable baseline interferometer in a video available from the Parkes 60th webpage.)

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