Editing Mirror (section)

===Wedge===
Wedge errors are caused by the deviation of the surfaces from perfect parallelism. An [[optical wedge]] is the angle formed between two plane-surfaces (or between the principle planes of curved surfaces) due to manufacturing errors or limitations, causing one edge of the mirror to be slightly thicker than the other. Nearly all mirrors and optics with parallel faces have some slight degree of wedge, which is usually measured in [[second of arc|seconds]] or [[minutes of arc]]. For first-surface mirrors, wedges can introduce alignment deviations in mounting hardware. For second-surface or transmissive mirrors, wedges can have a prismatic effect on the light, deviating its trajectory or, to a very slight degree, its color, causing [[chromatic aberration|chromatic]] and other forms of [[Optical aberration|aberration]]. In some instances, a slight wedge is desirable, such as in certain laser systems where stray reflections from the uncoated surface are better dispersed than reflected back through the medium.<ref name=walk1998/><ref name=klei1989/>