Over the past few decades, astronomers around the world have been building large telescopes with "pupils" or apertures of 8 to 10 meters in diameter (Gemini Observatory; European Southern Observatory; W.M. Keck Observatory; Subaru Observatory and the Large Binocular Telescope). Large apertures collect more light from astronomical sources; permitting astronomers to observe fainter objects and enabling measurements to be made of very distant sources (i.e. galaxies and quasars) whose light was emitted when the universe was a fraction of its present age.
Another advantage of large apertures is that it also permits more detail to be seen than with smaller apertures. For example, compare what you can see through a pair of binoculars with what you can see with your naked eye. The binocular lenses are significantly larger than the pupil of your eye so that you can see not only fainter objects but also more detail such as the craters on the surface of the moon. The amount of detail seen is technically known as the angular resolution of the telescope, and depends upon both the size of the aperture and the wavelength of the light being measured.
However, the Earth's atmosphere has a detrimental, two-fold affect on ground-based telescopic images. First, certain wavelengths of light are absorbed by the gases in the atmosphere and second, turbulence in the atmosphere causes images to blur. This means that large ground-based telescopes can see fainter objects that smaller telescopes can't, but the resolution is generally no better than a 0.1-meter aperture telescope. This atmospheric blurring can be easily observed with the naked eye as the twinkling of starlight.
From the lunar surface or from orbit above the Earth's surface, with no blurring atmosphere (for example, the Hubble Space Telescope), stars do not twinkle. Astronomers use a technique known as adaptive optics (AO) to try and achieve full resolution of large ground-based telescopes, despite the pesky muddling of the atmosphere.
The distortion of light by the atmosphere changes on very rapid timescales. In order to correct for this, the AO system needs to measure the distortion of the incoming starlight using an instrument called a wavefront sensor (WFS). The measurements from the WFS are then used to control an optical element called a deformable mirror (DM). The DM surface changes on very rapid timescales, canceling out the atmospheric distortion to a large degree.
However, a lot of interesting astronomical sources are either too faint or too extended to be used for sensing the shape of the wavefront. So a nearby bright star must be used. These stars are known as natural guide stars (NGS). The NGS needs to be very close to the astronomical target, a situation that rarely occurs.
Instead, astronomers create their own artificial laser guide star (LGS), which can be "placed" nearby the target of interest, a technique pioneered by the US Air Force Research Laboratory. There are two ways to generate LGSs. The first is to propagate a laser so that it focuses at a range of 10-15 kilometers from the telescope so that Raleigh scattering in the atmosphere forms an artificial point source. The second approach is to tune a laser to 589 nm and focus it at an altitude of 90-100 kilometers, where a thin layer of sodium atoms deposited by meteors exists.
The energy of the laser light excites electrons of the sodium atoms causing them to jump to a higher energy level. This is a relatively unstable energy level for the electrons and they soon decay back to the ground state and emit photons with the same energy. This resonance scattering process creates a sodium LGS.
Further information about adaptive optics can be found at a number of web sites such as the Center for Adaptive Optics (http://cfao.ucolick.org). The Gemini Observatory has an animation illustrating Sodium LGS adaptive optics at http://www.gemini.edu/videos/pio/geminiLGS_v9_animation.mov.
Special thanks to Dr. Julian Christou, Gemini Observatory.