Seeing by radio

Ken Tapping's weekly astronomy column from Dominion Research Observatory


Even in these days of getting our TV from cable distribution systems or over the internet, there must be some of us out there who have been on the roof with an antenna, pointing it in various directions while family members relayed whether the picture was getting better or worse. The antenna was directional; it was designed to concentrate its sensitivity in one direction. The trick was to get the antenna pointed in the direction of the TV transmitter. We do the same thing with satellite TV dishes. They are highly directional and need to be pointed quite accurately at the satellite.

The early radio telescopes were much the same. We pointed the antennas in the direction of the cosmic radio source we wanted to measure. We got the strongest signal when the source was in the middle of the patch of sky the antenna could “see”. If the patch of sky seen by the antenna was bigger than the source we were looking at we could only measure the properties of the signal. There was no way we see the structure of the source.

There is a solution – make the antenna bigger, because as the antenna gets bigger, the patch of sky seen by the antenna gets smaller. For example, the largest radio telescope antenna in Canada is located in Algonquin Provincial Park in Ontario, and has a diameter of 46 m.  At a wavelength of 2.8 cm, the antenna can see a patch of sky about one per cent the size of the Sun in the sky. By scanning the antenna left and right and up and down, we can measure the brightness of each point on the solar disc and then combine the measurements to make a radio image of the Sun.

This is not an efficient way to see. If our eyes worked that way we would have to walk very slowly. Instead of having just one sensor on the antenna, we have a single antenna, the lens, and then have a large number of sensors covering the image projected by the lens.  At our observatory we are developing a radio telescope that works just like that. It uses a new Canadian technology called the Phased Array Feed Demonstrator, or PHAD.

However there is another approach. Rather than making devices like our eyes, we can emulate the eyes of insects. Instead of one lens and lots of sensors, they have thousands of lenses each of which has only one sensor. Then the information from all these sensors are combined in the insect’s brain to make an image. Mother Nature chose to use lots of sensors to keep the brain’s processing simple. In radio astronomy the problem is different, the sensors – radio telescope antennas – are complex and expensive. Computer power is cheaper. In addition, most things we see in the sky are not changing very quickly, so we reduce the number of sensors and compensate using more advanced signal processing.  That’s what we do at our observatory to image the radio sky, and what is done with other radio astronomical imagers around the world. The idea of seeing by radio might seem strange, but radio waves, like light, are electromagnetic waves, so there is no reason we cannot “see” using them.

Jupiter rises around 7 p.m., Mars comes up around 2 a.m. The Moon will reach last quarter on the 19th.

Ken Tapping is an astronomer with the National Research Council’s Herzberg Institute of Astrophysics, and is based at the Dominion Radio Astrophysical Observatory, Penticton.