Bigger is better

In astronomy, size is important. As you make the diameter of your telescope larger, you benefit in two ways. Firstly, just like a large bucket collects more rain than a small one, you can collect more light from the objects you are trying to study. The second benefit is that the resolution of the image you take increases.

Over the years, the telescopes got larger (in diameter) and larger until astronomers noticed a problem. Over a certain size, the resolution of the images was no longer increasing. This is because there is a wobbling atmosphere is the way which distorts the image and ultimately limits the resolution of your images. From the ground, the best resolution you can get is about half an arcsecond - one arcsecond is equivalent to angular size of a 10 pence piece or US dime at a distance of 2 km - and this has been one of the limits to building telescopes larger than a few meters.

One way to get around the problem is to go into space. Since it was launched (and fixed), the Hubble Space Telescope has produced thousands of amazing images better than any from the ground even though at 2.4m in diameter, it is smaller than many professional telescopes. However, putting a telescope in space brings its own problems. Apart from putting observing trips out of the question, it's harder to fix, it costs a lot of money and your observatory may be at risk if the guy in charge of the funding decides to go to Mars instead.

Now the HST has been great, but there is only ever going to be a limited number of people that can use it. Wouldn't it be good if we could somehow remove the effect of the atmosphere from the images that are taken by bigger (and much cheaper) telescopes on the ground? In recent years this has become possible and goes by the name of adaptive optics. Basically, the telescope's mirror gets distorted many times a second in order to take out the distortion caused by the atmosphere.


European Southern Observatory concept image of the 100m Over-Whelmingly Large (OWL) telescope
These advances have allowed us to think about creating truly gigantic telescopes with diameters of 50 to 100m. There have been several proposals for these massive telescopes and these fall under the label ELT or extremely large telescope. At NAM on Friday, Dr Isobel Hook (who talked about the big rip last year) discussed the scientific case for these ELTs. Just think, a 100 m telescope would have a resolution 40 times better than the HST. It would be able to observe Earth-like planets around other stars as well as the earliest galaxies and supernova explosions. But is it even possible? Apparently, initial studies are positive but it doesn't come cheaply; the price tag is about one billion Euros. If it does get financed, it will be a gigantic engineering challenge.

As a final note, I should point out that it isn't just optical astronomy that is going big. Radio astronomers have joined up their telescopes as interferometers for years, giving a telescope of diameter equivalent to the size of the planet Earth. Although this very long baseline interferometry (VLBI) has got the resolution, it doesn't have the collecting area. One plan, imaginitively called the Square Kilometer Array (SKA), is to build a telescope with an area of one square kilometer. Both of these projects - ELT and SKA - would hope to become operational in the next 10 to 20 years. If they do, it should be an exciting time in big science.

Posted in astro blog by Stuart on Sunday 10th Apr 2005 (17:23 UTC) | Permalink
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