oculaire de collimation
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At first sight this looks just like a normal eyepiece externally. The difference is that it has no lenses, but only a small hole in the centre and a large opening at the side. A silver reflective surface can be seen there, which is angled at 45° to the eyepiece and which also has a small hole in the centre. The eyepiece is inserted into the focuser of your refractor, which must be so set up that the opening at the side of the Cheshire is directed at a brightly lit area or a light source. The front aperture of the refractor must be covered. Now take a look through the small hole in the eyepiece.
You will see two reflections, which come from two convex surfaces of the objective lens. You must now carefully loosen the screws holding the front lens in place and tip the lens so that the two reflection points are brought into alignment. Now carefully retighten the screws, avoiding as far as possible changing the orientation of the lens again. The telescope is now perfectly collimated.
Of course the Cheshire eyepiece can also be used to collimate Newtonian telescopes.