Editing Parabolic reflector (section)

===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.