Physics · Ch 6 — Optics
Reflecting Telescope
Reflecting Telescope
Modern telescopes use a concave (typically parabolic) mirror rather than a lens as the objective, because manufacturing very large lenses free of optical defects is difficult and expensive; telescopes with mirror objectives are called reflecting telescopes. Advantages of a mirror objective: only one surface needs to be polished and maintained (versus at least two curved surfaces on a comparable lens); the mirror can be structurally supported across its entire back surface rather than only around its rim; and mirrors weigh considerably less than an equivalent-aperture lens. The one practical drawback is that a simple concave-mirror objective focuses light back inside the telescope tube, where an eyepiece and observer would otherwise obstruct part of the incoming light; this is solv …
What this figure shows. Parallel light from a distant object enters the open end of the telescope tube and strikes a large parabolic primary mirror at the closed end, which reflects and focuses the light back up the tube; before reaching that focus, the converging beam is intercepted by a smaller, convex secondary mirror, which redirects it sideways out through an opening in the tube wall to an externally mounted eyepiece -- avoiding the practical problem of an eyepiece and observer's head blocking the incoming light if it were placed d …