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, or reflection of electromagnetic radiation. Originally meaning only an optical instrument using lenses, curved mirrors, or a combination of both to observ ...

, 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 perfectly collimated light beam, with no divergence, would not disperse with distance. However, diffraction ...

. 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 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 or parabolic shaped disks of polished reflective meta ...

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
''

Sky and Telescope ''Sky & Telescope'' (''S&T'') is a monthly American magazine covering all aspects of amateur astronomy, including the following: *current events in astronomy and space exploration; *events in the amateur astronomy community; *reviews of astronomic ...

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

References

{{reflist Optical devices