Foucault Knife-Edge Test - How to build / Tutorial

This Foucault Knife-Edge Tester, originally developed by Léon Foucault in 1858, is used to check optical surfaces, in my case to check the accuracy of a parabolic telescope mirror's surface. Even it's based on a simple optical principle and ,even with lower machinery than i used, to build, this Foucault Tester allows you to measure very accurate with a variance of up to 1/10000 of a mm. It is simple to use and build, inexpensive and more cost-effectivley than most other testing techniques. I hope this video fulfills its destination and helps some people who are grinding/polishing a telescope mirror and want to test it during the polishing process. If there are open questions left, just post them in the comments, or send me a message.