Ken Tapping, October 20 2010

Our observatory Annual Open House, held in September, was a combination of three events. Firstly, there was our usual annual show and tell; secondly, we were celebrating the observatory's 50th Birthday, and finally, there was a special ceremony recognizing the role of the observatory and its scientists in the Canadian development of Very Long Baseline Interferometry, or VLBI.

The human eye has an angular resolution of about one arc-minute, which is a sixtieth of a degree. The Moon has an angular size of about 30 arc-minutes. Angular resolution, or the ability to see fine detail, is dictated by how large the imaging lens is compared with the lengths of the waves being detected, in this case light waves. Telescopes and binoculars have lenses or mirrors larger than the lenses in our eyes, so with their aid we can see even more detail.

Radio waves are much longer than light waves. Therefore, to see a similar level of detail in the objects observed, radio telescope antennas have to be correspondingly larger. For example, at a wavelength of 21 cm, which is one of the more important wavelengths for radio astronomy, the 250 ft (75 m) dish at the Jodrell Bank radio observatory has an angular resolution of about 10 arc-minutes, roughly ten times worse than the human eye. Things were improved a bit when we found we can make big radio telescopes out of lots of small antennas connected together. For example, the row of seven 9-m dishes at our observatory, which are distributed along a 600-m long line, equal the human eye in their ability to discern detail.

The VLBI image on the right shows much finer structure than the traditional array of radio telescopes image on the left.

The angular resolution problem became even more important in the early 1960's, when it turned out that one of the radio sources catalogued at Cambridge University in the UK in 1959 was odd This object, called 3C273, was very bright, very far away, and varied in intensity over weeks. It therefore had to be very small by cosmic standards, and radiating enormous amounts of energy. Then it turned out that 3C273 was only one example of a class of objects, which became known as quasars. It's not surprising that radio astronomers wanted to image a quasar or two to get insights into what they are and how they work. However no single radio telescope in the world had fine enough resolution to see more than a blob that was more to do with the radio telescope than the quasar. The largest arrays of connected radio telescopes were little better.

Canada was the first country to succeed in making arrays of radio telescopes which are not connected together. By recording the radio waves on video tape at each antenna, together with very precise timing signals, it was possible to take the tapes to a processing station afterwards and combine the signals to emulate a radio telescope with a size equal to the distance between the antennas. This is now called VLBI. Moreover, with this technique there is in theory no limit to how far apart the antennas can be. One recent experiment used a radio telescope on a satellite as part of the array. Thanks partly to VLBI observations, we now know that quasars were more common in the young universe and that they are almost certainly powered by black holes, and Canada did it first.