Editing Parabolic reflector (section)

===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]]