Judging the concentricity of the outer diffraction rings and the inner dark circle is not easy. This method employs an overlay on the webcam image which makes it much easier to see when collimation is perfect. This example looked fine to the eye, but it can be seen that the inner dark circle is slightly displaced upwards.
![](https://s2.save4k.ru/pic/5l1Y7AbZB6k/mqdefault.jpg)