Editor's note: This story was originally posted 22 August 2013, but it's now graduated and become part of Astronomy 101! Please be aware that any references to events that seem current may not actually reflect events happening right now.
Ever look up in the sky and see the stars twinkling overhead? It may be one of the most beautiful sights in nature, but it's also one of the most annoying to astronomers. Astronomers call it seeing; it's caused by unsteadiness in the Earth's atmosphere and it blurs our details in astronomical images. This is one of the main reasons we've gone to space - without an atmosphere, images can be extraordinarily sharp. Even though it's relatively small and old by telescope standards, it is for this reason that the Hubble Space Telescope has been our clearest window to the Universe for more than twenty years.
That's about to change. Using a technology called adaptive optics, the Magellan telescope in Chile has begun producing images even sharper than those from Hubble. Adaptive optics isn't a new technology, but it's finally coming into its own.
How does it work? First let's consider the problem it's trying to fix. Light traveling from a star reaches the Earth moving in a straight line. If the Earth's atmosphere were unmoving, it would pierce right through and strike a single point on our telescope. Instead, variations in temperature cause the position and density of parcels of air to change constantly. Because of a physical law called Fermat's Principle, light will take the shortest available path. But, since the atmosphere is constantly changing, this shortest path is also changing. The result is that light from the same star will strike different places at different times, blurring out the image.
Adaptive optics works to counteract this by deforming the telescope's mirror to offset the change in strike location. This is easier said then done - the Earth's atmosphere changes so quickly that Magellan must make 1000 adjustments per second at each of 585 locations on the mirror. The results, however, are stunning - an image more than 30 times sharper than its uncorrected equivalent. Take a look at the left and middle panels of the above image. On the left we have the uncorrected image - it looks like a very blurry star. In the middle, with adaptive optics on, the star becomes far sharper. Zooming in, as the right panel does, reveals that this is in fact a double star - something we previously would not have known.
Why bother with all this work when Hubble has been producing similar images for decades? It comes down to cost. You could build ten or more ground-based telescopes for the cost of Hubble. That doesn't mean that efforts like those at Magellan come cheaply - a single night of observing is worth more than $17,000!