Autocollimation 

This video dives farther into the autocollimation test. It is a superior test used by several amateur mirror making workshops including this one and has a long history of being used by professionals and amateurs alike. This video covers a brief history of the autocollimation test, discusses the minimum requirements of a flat, describes suggested features of a test bench, and reviews some of the avoidable pitfalls one can encounter in their attempts to use the test. Finally, it shows how the test can be used to test entire optical systems including optics already installed in a telescope.