A Cheshire eyepiece or Cheshire collimator is a simple tool that helps aligning the optical axes of the mirrors or lenses of a

telescope A telescope is a device used to observe distant objects by their emission, Absorption (electromagnetic radiation), absorption, or Reflection (physics), reflection of electromagnetic radiation. Originally, it was an optical instrument using len ...

, a process called

collimation A collimated beam of light or other electromagnetic radiation has parallel rays, and therefore will spread minimally as it propagates. A laser beam is an archetypical example. A perfectly collimated light beam, with no divergence, would not disp ...

. It consists of a peephole to be inserted into the focuser in place of the eyepiece. Through a lateral opening, ambient light falls on the brightly painted oblique back of the peephole. Images of this bright surface are reflected by the mirrors or lenses of the telescope and can thus be seen by a person peering through the hole. A Cheshire eyepiece contains no lenses or other polished optical surfaces. The tool was first described by F. J. Cheshire in 1921. It was repopularized in the 1980s and is now mass-produced. Amateur astronomers in particular use them to collimate reflecting or

refracting In physics, refraction is the redirection of a wave as it passes from one transmission medium, medium to another. The redirection can be caused by the wave's change in speed or by a change in the medium. Refraction of light is the most commo ...

telescopes. Some modern models of Cheshire eyepieces in common use include extended sight tubes and are equipped with crosshairs. When inserted into a

Newtonian telescope The Newtonian telescope, also called the Newtonian reflector or just a Newtonian, is a type of reflecting telescope invented by the English scientist Sir Isaac Newton, using a concave primary mirror and a flat diagonal secondary mirror. Newto ...

whose

primary mirror A primary mirror (or primary) is the principal light-gathering surface (the objective) of a reflecting telescope. Description The primary mirror of a reflecting telescope is a spherical, parabolic, or hyperbolic shaped disks of polished ...

is marked in its center, such aids allow the user to adjust the position and tilt of both the secondary and the primary mirror.Carlin, N. O.
How To Collimate Your Newtonian Reflector
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Sky and Telescope ''Sky & Telescope'' (''S&T'') is a monthly magazine covering all aspects of amateur and professional astronomy, including what to see in the sky tonight and new findings in astronomy. Other topics covered include: *observing guides for planets, ...

'', retrieved 2012-01-03. It can also be used to verify focuser alignment.

References

{{reflist Optical devices