Secondary mirror
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A secondary mirror (or secondary) is a second light gathering and focusing surface in a reflector telescope. Light gathered by the primary mirror is directed towards a focal point typically past the location of the secondary. The secondary directs the light either out a side opening of the tube (Newtonian reflector) or back towards a focal point behind and through the primary (Cassegrain reflector).
The secondary is typically supended by X-shaped struts (sometimes called a "spider") in the path of light between the souce and the primary, however it is important to realize that this does not cause large distortions in the image. Every point on the primary individually forms an image, with additional area increasing the brightness of the final image, not changing the shape of the image. That said, secondaries and their supports can cause minor distortion to the point spread function, resulting in the cross-shaped "diffraction spikes" radiation from bright sources in many popular images.
See also: telescope