Setting up and using a Bath Interferometer (to test telescope mirrors)

This video is many months in the making!
Once you have your Bath interferometer, here is how to use it. How to get everything aligned, common mistakes, and how to get good interferograms.
+Xtilt means right side of mirror is closer
+Ytilt means top side of mirror is closer
+Defocus means outside of focus
Helpful information at gr5.org/bath/