The technique used is called "adaptive optics" because one telescope mirror has a flexible surface that compensates for distortions produced by the earth's atmosphere.  By removing the blurring effect of the earth's atmosphere, the sharpness of an astronomical image is improved by a factor of ten.  The future of astronomy will rely heavily on adaptive optics techniques.
 
This project of two Max-Planck Institutes is called ALFA (Adaptive optics with a Laser For Astronomy) and uses the 3.5 m telescope at the Spanish-German Observatory in Calar Alto, Spain.  ALFA is at the forefront of adaptive optics technology because it is one of the first instruments that can produce an artificial star in the sky using a yellow laser  beam projected from the ground. The laser beam causes sodium atoms to fluoresce 90 km above the ground, generating light that appears star-like.  The apparent star seen by the telescope is always required for adaptive optics to function anywhere that the telescope is pointed.

For the first time on December 7, 1997, German astronomers were able to use the artificial laser star to remove the blurring effects of the earth's atmosphere and sharpen their astronomical images.  An unresolved pair of stars in the constellation Perseus which are thought to harbor a young, protoplanetary system appeared as separate stars when the laser star was used in the operation of ALFA The image sharpness is comparable to that achieved by the Hubble Space Telescope, which at 600 km altitude does not have to contend with imaging through the earth's
atmosphere.
 
ALFA: Comparison with HST