Twinkle, twinkle, little star, how I wonder what you are. A beautiful poem and song, but the fact that stars twinkle is terribly frustrating to astronomers. It is impossible to study an astronomical object in detail if its image is constantly twinkling, moving and changing.
The atmosphere is constantly changing due to wind and temperature, so as light from a star passes through the atmosphere it gets bent in a very unpredictable way many times – hence the twinkling. This means that all the useful information about the star is all muddled by the time it gets to the telescope and scientific instrument. In other words, the image is blurry and no amount of focussing is going to be able to correct it.
How do astronomers tackle this problem? One way, of course, is to put the telescope in space so that there is no atmosphere in between object and telescope to muddle the image. We have done this already – The Hubble Space Telescope, with its 2.4m mirror, was the best thing that happened to astronomy since the telescope was invented. Unfortunately, it cost a colossal amount of money and it had to be repaired in space at extra cost because the mirror was the wrong shape.
Since Hubble, scientists have managed to build ground-based telescopes which have much bigger mirrors (and so much more light-gathering power) and so return often better images than Hubble. But scientists want to look deeper and further into the universe. Objects further away are smaller and fainter and so the atmospheric disturbance has a big effect on the image. It would be wonderful if we could put up a telescope with a really big mirror into space, but this is expensive and a massive technological and engineering challenge. So scientists have come up with an ingenious solution to this problem which does not involve putting the telescope into space.
It is called Adaptive Optics and it essentially involves changing the shape and/or orientation of the mirror (perhaps with magnets) hundreds of times per second to exactly undo the muddling effect of the earth’s atmosphere. So we can have Hubble’s advantage of avoiding the atmosphere and avoid Hubble’s disadvantage by having fewer restrictions on the size of the mirror. We can have our cake and eat it.
The idea was first conceived by Horace Babcock in 1953, but it wasn’t until the 1990s that the technology became available and good enough to be developed and put to use. Let’s say we want to look at a galaxy, but it’s blurry so we need to know what shape to put the mirror in to undo the muddling effect. We can do this by looking at a nearby star and altering the shape of the mirror so that the star no longer twinkles in the telescope’s view. Since the star is close to the galaxy, the atmospheric disturbance will be very similar for the galaxy and so the altered mirror will undo the muddling effect for the galaxy and we will be able to see it in much more detail. What if there’s no star nearby? Astronomers make one! By shining a laser close to the galaxy, astronomers can make an artificial star and we apply the same technique just as for the real one.
Now-a-days, there is no ground-based telescope being built which does not have Adaptive Optics. However, this does not make space telescopes obsolete. They are able to stare at a patch of sky for much longer than ground-based telescopes – this was how the Hubble Deep fields were created. Also, the atmosphere takes away certain wavelengths which carry a lot of information which we aren’t able to restore (e.g. Ultra-Violet is taken away by the ozone layer). For the wavelengths which can get through the atmosphere though, space telescopes are no match for the big-mirrored ground-based telescopes.