• Question: how big is your telescopic device?

    Asked by noah to Evan on 13 Nov 2017.
    • Photo: Evan Keane

      Evan Keane answered on 10 Nov 2017:

      Hi Noah,

      Thanks for your question. The telescope, shown in that picture which I took from my office desk, is the Lovell Telescope at Jodrell Bank. It’s 76 metres in diameter so it’s pretty massive! I’ve worked with bigger ones but there aren’t really too many that are bigger. There is the Effelsberg telescope in Germany and the Green Bank telescope in the USA – they are both 100 metres. Bigger than those there are only 2, and they are so big that they actually can’t move as they’re too heavy. They are basically big light buckets that just sit there and watch the sky as it goes past. Those are the Arecibo and FAST telescopes which are 300 and 500 metres in diameter respectively (although only about 200 and 300 metres of those are actually usable at one time).

      There is an issue in astronomy that you want to see as far as possible which means you need a very big dish to collect more light. At some point you hit a snag that the dishes get too heavy and can’t move without collapsing on themselves, so you can’t look anywhere in the sky only straight up which is not ideal. But also as well as looking to great distances, you want to see as much sky at once as you can, and to have a large field of view you need a small dish! Large dishes see far but only have a tiny field of view. So you end up needing a big telescope and a small telescope at the same time which is an issue. There is a way to ‘have it all’ however which is to get an array of many small telescopes. You can get the signals from all of them and combine them with a computer (actually a supercomputer … it is a bit of a ‘big data’ issue) so that you can make a big telescope by combining the power of the smaller ones. Then you get the wide field of view of a small telescope, but you get the sensitivity (ability to see to large distances) of a big telescope (all the small telescopes added together add up to a big telescope).

      It is the area of the telescopes that matter. So if you wanted to have a bunch of 15-metre telescopes you would need to have about 178 of them to have the same area as one 200-metre telescope. That is the best way to have a 200-metre telescope as an actual 200-metre telescope isn’t really steerable. Another advantage is you can keep adding 15-metre dishes to make your telescope ever bigger as they are much easier to build. I work on a project called the “Square Kilometre Array”. The idea is to build a telescope with an area that is 1 square kilometre, or 1 million square metres. That is the same as one 1130-metre diameter telescope which would be impossible to build. But we can certainly build thousands of 15-metre telescopes and to combine them all we need are computers which (thankfully) get better each and every day. It’s not easy, but it is do-able.