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A parabolic (or paraboloid or paraboloidal) reflector (or dish or mirror) is a reflective surface used to collect or project energy such as light, sound, or radio waves. Its shape is part of a circular paraboloid , that is, the surface generated by a parabola revolving around its axis.
Each optical element (surface, interface, mirror, or beam travel) is described by a 2 × 2 ray transfer matrix which operates on a vector describing an incoming light ray to calculate the outgoing ray. Multiplication of the successive matrices thus yields a concise ray transfer matrix describing the entire optical system.
Newtonian telescope design. A Newtonian telescope is composed of a primary mirror or objective, usually parabolic in shape, and a smaller flat secondary mirror.The primary mirror makes it possible to collect light from the pointed region of the sky, while the secondary mirror redirects the light out of the optical axis at a right angle so it can be viewed with an eyepiece.
From top: Parabolic mirror showing Foucault shadow patterns made by knife edge inside radius of curvature R (red X), at R and outside R. Foucault testing is commonly used by amateur telescope makers for figuring primary mirrors in reflecting telescopes. [5] [6] The mirror to be tested is placed vertically in a stand. The Foucault tester is set ...
For mirrors with parabolic surfaces, parallel rays incident on the mirror produce reflected rays that converge at a common focus. Other curved surfaces may also focus light, but with aberrations due to the diverging shape causing the focus to be smeared out in space. In particular, spherical mirrors exhibit spherical aberration. Curved mirrors ...
A convex secondary mirror is placed just to the side of the light entering the telescope, and positioned afocally so as to send parallel light on to the tertiary. The concave tertiary mirror is positioned exactly twice as far to the side of the entering beam as was the convex secondary, and its own radius of curvature distant from the secondary.
The Gregorian telescope consists of two concave mirrors: the primary mirror (a concave paraboloid) collects the light and brings it to a focus before the secondary mirror (a concave ellipsoid), where it is reflected back through a hole in the centre of the primary, and thence out the bottom end of the instrument, where it can be viewed with the aid of the eyepiece.
The focal ratio (f-number, the ratio of the focal length to the dish diameter) of typical parabolic antennas is 0.25–0.8, compared to 3–8 for parabolic mirrors used in optical systems such as telescopes. In a front-fed antenna, a "flatter" parabolic dish with a long focal length would require an impractically elaborate support structure to ...