Editing Parabolic reflector (section)

==Variations==

===Focus-balanced reflector===
[[File:focus-balanced_parabolic_reflector.svg|thumb|300px|An [[oblique projection]] of a focus-balanced parabolic reflector]]

It is sometimes useful if the [[centre of mass]] of a reflector dish coincides with its [[Focus (geometry)|focus]]. This allows it to be easily turned so it can be aimed at a moving source of light, such as the Sun in the sky, while its focus, where the target is located, is stationary. The dish is rotated around [[wikt:axis|axes]] that pass through the focus and around which it is balanced. If the dish is [[symmetrical]] and made of uniform material of constant thickness, and if ''F'' represents the focal length of the paraboloid, this "focus-balanced" condition occurs if the depth of the dish, measured along the axis of the paraboloid from the vertex to the plane of the [[wikt:rim|rim]] of the dish, is 1.8478 times ''F''. The radius of the rim is 2.7187&nbsp;''F''.{{efn|The closeness of this number to the value of "e", the base of natural logarithms, is just an accidental coincidence, but it does make a useful mnemonic.}} The angular radius of the rim as seen from the focal point is 72.68 degrees.

===Scheffler reflector===
The focus-balanced configuration (see above) requires the depth of the reflector dish to be greater than its focal length, so the focus is within the dish. This can lead to the focus being difficult to access. An alternative approach is exemplified by the '''Scheffler reflector''', named after its inventor, [[Wolfgang Scheffler (inventor)|Wolfgang Scheffler]]. This is a paraboloidal mirror which is rotated about axes that pass through its centre of mass, but this does not coincide with the focus, which is outside the dish. If the reflector were a rigid paraboloid, the focus would move as the dish turns. To avoid this, the reflector is flexible, and is bent as it rotates so as to keep the focus stationary. Ideally, the reflector would be exactly paraboloidal at all times. In practice, this cannot be achieved exactly, so the Scheffler reflector is not suitable for purposes that require high accuracy. It is used in applications such as [[Solar cooker#Paraboloidal reflectors|solar cooking]], where sunlight has to be focused well enough to strike a cooking pot, but not to an exact point.<ref>{{cite web| url=http://www.solare-bruecke.org/index.php?option=com_content&view=article&id=2&Itemid=2&lang=en|title=The Scheffler-Reflector | last=Administrator|website=www.solare-bruecke.org}}</ref>

===Off-axis reflectors===
[[File:off-axis_parabolic_reflector.svg|thumb|The vertex of the paraboloid is below the bottom edge of the dish. The curvature of the dish is greatest near the vertex. The axis, which is aimed at the satellite, passes through the vertex and the receiver module, which is at the focus.]]

A circular paraboloid is theoretically unlimited in size. Any practical reflector uses just a segment of it. Often, the segment includes the [[Vertex (curve)|vertex]] of the paraboloid, where its [[curvature]] is greatest, and where the [[axis of symmetry]] intersects the paraboloid. However, if the reflector is used to focus incoming energy onto a receiver, the shadow of the receiver falls onto the vertex of the paraboloid, which is part of the reflector, so part of the reflector is wasted. This can be avoided by making the reflector from a segment of the paraboloid which is offset from the vertex and the axis of symmetry. The whole reflector receives energy, which is then focused onto the receiver.  This is frequently done, for example, in satellite-TV receiving dishes, and also in some types of astronomical telescope (''e.g.'', the [[Green Bank Telescope]], the [[James Webb Space Telescope]]).

Accurate off-axis reflectors, for use in [[solar furnace]]s and other non-critical applications, can be made quite simply by using a [[rotating furnace]], in which the container of molten glass is offset from the axis of rotation. To make less accurate ones, suitable as satellite dishes, the shape is designed by a computer, then multiple dishes are stamped out of sheet metal.

Off-axis-reflectors heading from medium [[latitude]]s to a [[Geostationary satellite|geostationary TV satellite]] somewhere above the equator stand steeper than a coaxial reflector. The effect is, that the arm to hold the dish can be shorter and snow tends less to accumulate in (the lower part of) the dish.

[[File:ASTRA2Connect Dish.jpg|thumb|Off-axis satellite dish]]