The Galileo Project – The telescope[permanent dødt link], With this instrument he observed the Moon, discovered four satellites of Jupiter... The next discovery, that of sunspots, was made by several observers, including Galileo, independently
The Galileo Project – The telescope[permanent dødt link], It had a plano-convex objective (the lens toward the object) with a focal length of about 30-40 inches., and a plano-concave ocular with a focal length of about 2 inches... which meant that only a quarter of the full Moon could be accommodated in the field.
The Galileo Project – The telescope[permanent dødt link], As early as 1611, in his Dioptrice, Johannes Kepler had shown that a telescope could also be made by combining a convex objective and a convex ocular... the addition of yet a third convex lens would make the image erect again.
The Galileo Project – The telescope[permanent dødt link], In his study of sunspots, Scheiner had experimented with telescopes with convex oculars in order to make the image of the Sun projected through the telescope erect. ...although the image was inverted, it was much brighter and the field of view much larger than in a Galilean telescope.
The Galileo Project – The telescope[permanent dødt link], Astronomers eschewed the third convex lens (the erector lens) necessary for re-inverting the image because the more lenses the more optical defects multiplied
The Galileo Project – The telescope[permanent dødt link], By the early 1670s, Johannes Hevelius had built a 140-foot telescope... The objective was mounted on a building or pole by means of a ball-joint and aimed by means of a string; the image was found by trial and error.
The Galileo Project – The telescope[permanent dødt link], Others were unable to grind mirrors of regular curvature, and to add to the problem, the mirror tarnished and had to be repolished every few months, with the attending danger of damage to the curvature... The reflecting telescope became a reality in the hands of first James Hadley and then others... In the second half of the eighteenth century, in the hands of James Short and then William Herschel, the reflecting telescope with parabolically ground mirrors came into its own.
http://telescopes.stardate.org/ – History of the Telescope – Isaac NewtonArkiveret 26. februar 2011 hos Wayback Machine, The primary mirror, at the bottom of the telescope tube, curved inward slightly, in a spherical shape... Light from an astronomical object struck this mirror and reflected back up the telescope tube, where it hit a flat secondary mirror. This mirror, which was tilted at a 45-degree angle, in turn reflected the light to an eyepiece at the side of the tube.
http://telescopes.stardate.org/ – History of the Telescope – Isaac NewtonArkiveret 26. februar 2011 hos Wayback Machine, The primary mirror, at the bottom of the telescope tube, curved inward slightly, in a spherical shape... Light from an astronomical object struck this mirror and reflected back up the telescope tube, where it hit a flat secondary mirror. This mirror, which was tilted at a 45-degree angle, in turn reflected the light to an eyepiece at the side of the tube.
