Telescope…a system of lenses, mirrors or both used to gather light from a distant object and form an image. Telescope…Has three major functions -Gather light -provide resolution -magnification
Galilean- It consists of a convergent lens as objective ( the lens that forms the image); and its eyepiece (or ocular), placed in front of the focus, is a divergent lens Keplerian - eyepiece, or ocular, is a convex (positive, or convergent) lens placed in back of the focus, the point at which the parallel light rays converge; and the instrument produces an inverted (real) image that can be projected or made visible.
Newtonian- The Newtonian reflecting telescope contained a small plane mirror that was angled at 45 degrees. This was inside the prime focus and brought the light to the side of the telescope. Cassegrain- Permits short tubes relative to the mirror diameter. Gregorian- The Gregorian version of the telescope contains a concave secondary mirror outside of the prime focus. This helps to reflect the light back through a hole into a primary mirror.
King, Henry C. The History of the Telescope http://en.wikipedia.org/wiki/Telescope http://en.wikipedia.org/wiki/X-ray_telescope http://www.astro.virginia.edu/class/oconnell/UIT/ http://www.refractortelescopes.co.uk/history.htm http://www.reflectortelescopes.co.uk/guides/a-short-history-of-reflector- telescopes/ http://www.britannica.com/EBchecked/topic/315254/Keplerian-telescope http://www.britannica.com/EBchecked/topic/224040/Galilean-telescope