Collimation eyepiece for refractors and Newtonians

An adjustment eyepiece for lens telescopes, simply exchange it for a normal eyepiece and look through it.

The adjustment eyepiece has a shiny metallic, sloping surface with a hole in the middle. The front lens can be seen through the hole. Cover the lens (usually a lens cap is at hand, if necessary a dark cloth or something similar). Now you can see the reflections of the light in the lens, which hits the slope and is directed into the tube. Ideally, you will see two reflections that are on top of each other and are in the middle of the picture. If this is not the case, adjust the lens barrel until the collimation is satisfactory.