Telescope earliest existing record of a telescope was a 1608 patent submitted to the government in the Netherlands by Middelburg spectacle maker Hans Lippershey for a refracting telescope. The actual inventor is unknown but word of it spread through Europe. [[Galileo Galilei|Galileo]] heard about it and, in 1609, built his own version, and made his telescopic observations of celestial objects. The idea that the [[Objective (optics)|objective]], or light-gathering element, could be a mirror instead of a lens was being investigated soon after the invention of the refracting telescope. The potential advantages of using [[parabolic reflector|parabolic mirrors]]—reduction of [[spherical aberration]] and no
Janssen (lunar crater) lies entirely within the outer wall, in the northeast quadrant of the floor. A number of other lesser, but still notable craters mark the crater floor. Connected to the northeast rim is Metius, and to the north is the heavily worn Brenner. Southeast of Janssen are the co-joined craters Steinheil and Watt. Astride the southwest wall is the smaller Lockyer. Further to the east, although appearing nearby due to elongation, is the huge Vallis Rheita. In the southern two-thirds of Janssen can be discerned the remains of a large, concentric crater, the wall of which is overlaid by Fabricus. The