Afocal lens system

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The Kepler telescope is an afocal system

As afocal refers to a lens system that acts dispelling neither collecting nor. This means that bundles of rays arriving in parallel are refracted convergently or divergently within the system , but additional lenses or mirrors in the beam path cause the light to exit again in a parallel direction. In other words: a beam arriving parallel to the optical axis also leaves the system parallel to the axis. Strictly speaking, this only applies in paraxial optics , since otherwise the spherical aberration can cause a deviation.

An afocal system, such as a double-sided telecentric lens , has no focal length, or it is infinite. It also has no main and nodal points .

Beams arriving at an angle to the optical axis emerge at a different angle to the optical axis than they originally arrived. This causes a change in the imaging scale when the afocal system is used as an attachment in front of another imaging system. This can be, for example, the eye to which a telescope is placed.

Some zoom lenses contain an afocal system as an assembly.

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