TS Optics 1,25" Cheshire collimating eyepiece for Newtonians and RefractorsFor a precise collimation of Newtonian optics and of refractors
The Cheshire eyepiece has a small pinhole to look through and a crosshair eyepiece at the opposite end. Light enters the system through a porthole on the side and is reflected diffusely into the telescope by a blank aluminium surface. This allows you to precisely see whether your Newtonian or refractor is collimated properly.
The Cheshire eyepiece has a broad rim on which it rests when it is inserted into the telescope's focuser. This provides an exact alignment of the Cheshire so that the collimation result will be accurate even if there is play between the Cheshire and the focuser.
When collimating a Newtonian telescope we recommend to rotate the telescope until the focuser looks straight up, and then to insert the Cheshire without locking it. This prevents the Cheshire from getting tilted slightly out of axis. Turn the porthole on the side towards a light source and look into the pinhole of the eyepiece. Once the crosshair and the reflection of the secondary mirror are concentric the telescope is properly collimated.
When collimating a refractor you should put the Cheshire directly into the focuser. Do not use a star diagonal, which in itself might be slightly miscollimated. You will need to keep the lens cap on, else the image will be too bright and you will not see the much fainter reflections. When looking through the Cheshire you will see as many reflected discs as there are glass-to-air surfaces. (Example: An air-spaced doublet refractor will show four reflected discs, of which usually two are brighter.) Once all these rings are concentric the telescope is perfectly collimated. A slight misalignment usually is no problem, however if the rings are drastically eccentric then the telescope should be recollimated.
1,25" Cheshire collimating eyepiece
The classical collimation tool for a precise collimation