Principles
For detailed information on specific designs of reflecting, refracting, and catadioptric telescopes: see the main articles on Reflecting telescopes, Refracting telescopes, and Catadioptrics.
The basic scheme is that the primary light-gathering element the objective (1) (the convex lens or concave mirror used to gather the incoming light), focuses that light from the distant object (4) to a focal plane where it forms a real image (5). This image may be recorded or viewed through an eyepiece (2) which acts like a magnifying glass. The eye (3) then sees an inverted magnified virtual image (6) of the object.
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—David Hume (1711–1776)