The Keck II 36-segment 10-meter primary. The long tube to the lower left (with the blue lens) is the laser guide star "launch tube" which projects an artificial star in the upper atmosphere that is used by the adaptive optics to correct for "wavefront error induced by atmospheric instability", in other words, imperfect seeing. Photo by Alan Midkiff on April 18, 2006 while on behind-the-scenes tour of Keck Observatories.