An optical system having a first positive optical element having an aspherical surface; and a second positive optical element having an aspherical surface. The first and second optical elements are arranged in a Keplerian configuration. The aspheric surface of the second optical element is related to...http://www.google.com/patents/US6295168?utm_source=gb-gplus-sharePatent US6295168 - Refractive optical system that converts a laser beam to a collimated flat-top beam
