Editing Collimated beam (section)

==Collimation and decollimation==
"Collimation" refers to all the optical elements in an instrument being on their designed [[optical axis]]. It also refers to the process of adjusting an optical instrument so that all its elements are on that designed axis (in line and parallel). The [[Binoculars#Alignment|unconditional aligning]] of binoculars is a 3-axis collimation, meaning both optical axis that provide stereoscopic vision are aligned parallel with the axis of the hinge used to select various [[interpupillary distance]] settings. With regards to a telescope, the term refers to the fact that the optical axis of each optical component should be centered and parallel, so that collimated light emerges from the eyepiece. Most amateur reflector telescopes need to be re-collimated every few years to maintain optimum performance. This can be done by simple visual methods such as looking down the optical assembly with no eyepiece to make sure the components are lined up, by using a [[Cheshire eyepiece]], or with the assistance of a simple laser collimator or [[autocollimator]]. Collimation can also be tested using a [[shearing interferometer]], which is often used to test laser collimation.

"Decollimation" is any mechanism or process which causes a beam with the minimum possible [[ray (optics)|ray]] divergence to diverge or converge from parallelism. Decollimation may be deliberate for systems reasons, or may be caused by many factors, such as [[refractive index]] inhomogeneities, occlusions, [[scattering]], [[Deflection (physics)|deflection]], [[diffraction]], [[Reflection (physics)|reflection]], and [[refraction]]. Decollimation must be accounted for to fully treat many systems such as [[radio]], [[radar]], [[sonar]], and [[optical communication]]s.