As astronomers attempt to understand the limits of the physical universe, they must look deep into the night sky with a sharp eye. Unfortunately, looking into the night sky is like looking up from the bottom of a swimming pool. Turbulence in the upper atmosphere causes spatial and temporal anomalies in atmosphere's refractive index and any planar wavefront of light passing through this turbulence will experience phase distortions by the time it reaches a ground-based telescope. These phase distortions blur the images obtained by the telescope and result in resolution an order of magnitude worse than the theoretical capabilities of the telescope. The power of ground-based telescopes to observe and resolve distant faint astronomical objects is limited by the effects of the atmosphere on the light coming from these objects. The desire to avoid the image degradation due to the atmosphere was one of the main motivations behind the Hubble Space Telescope.
In recent years, astronomers have developed the technique of `adaptive optics' to actively sense and correct wavefront distortions at the telescope during observations. A telescope with adaptive optics measures the wavefront distortions with a wavefront sensor and then applies phase corrections with a deformable mirror on a time scale comparable to the temporal variations of the atmosphere's index of refraction. Adaptive optics dramatically improves image resolution and increase the image's coherence. One application of adaptive optics uses the improved coherence of the images to couple large optical telescopes into an interferometric array. Optical interferometers, before only feasible for small, specially designed telescope, will soon become a much more widely used astronomical tool as ESO's Very Large Telescope Interferometer and the Keck Imaging Interferometer come on line.