Question: How big are the largest telescopes?

Helen Johnson answered on 19 Nov 2014:

Hi Will, the largest telescope in the world is the ‘Gran Telescopio Canarias’ on the Canary Islands. Telescopes are usually measured by the size of their primary mirror, since this is what is important for collecting as much light as possible, and getting a good resolution – seeing a really fine amount of detail. The largest is 10.4m across. However, this isn’t always the best measure of a ‘good’ telescope.

Some other factors come into what makes a telescope ‘good’, such as the location – you need really clear skies, a stable atmosphere and for the air to be very dry too. This is why some of the best telescopes in the world are in Hawaii or the Chilean desert (e.g. the VLT which I’ve used! There are 4 main telescopes with mirros 8m across – see this awesome video http://www.youtube.com/watch?v=uVCFc0ZiRt8). The effect of the atmosphere is very important. If you’re in Space like the Hubble Space Telescope, then you can get spectacular images because you’re above the atmosphere of the Earth, which sort of ‘wobbles’ and distorts the light in between the stars and our telescopes. Nowadays we have special equipment on ground-based telescopes (called Adaptive Optics) which tries to correct for the effects of the atmosphere. We’re getting very good at it!

Hope this wasn’t too long and rambly!