Refracting Telescopes

Figure 2.7. A refracting telescope with a long focal length objective lens and a short focal length eyepiece.

Answer: (a) Objective lens: to gather light and form an image. (b) Eyepiece: to magnify the image formed by the objective. (c) Focal length of objective lens.

A reflecting telescope has a highly polished curved-glass mirror, the primary mirror, mounted at the bottom of an open tube. When starlight shines on this mirror, it is reflected back up the tube to form an image at the prime focus.

You can record the image at the prime focus, or you can use additional mirrors to reflect the light to another spot. The Newtonian telescope, originated by British scientist Sir Isaac Newton in 1668, uses a small, flat mirror to reflect the light through the side of the tube to an eyepiece (Figure 2.8).

The Cassegrain telescope uses a small convex mirror, a secondary mirror, to reflect the light back through a hole cut in the primary mirror at the bottom end of the tube (Figure 2.9). It is more compact than a refractor or Newtonian reflector of the same aperture. The Schmidt-Cassegrain telescope combines an extremely short-focus spherical primary mirror at the back end of a sealed tube with a thin lens at the front.

Reflecting telescopes range in size from a beginner's 76-mm (3-inch) Newtonian reflector to the world's largest, the 10.4-m (34-foot) Gran Telescopio Canarias atop a peak on the Canary Island La Palma, Spain.

Figure 2.8. A Newtonian reflecting telescope with a primary mirror, a small diagonal secondary mirror, and an eyepiece.

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