From what I understand the coherence length determines how far out of alignment the mirrors can get and still show fringes. Does it also affect the results when the mirrors are close to equal? The small laser is about 6”. I am still trying to figure out why it wouldn’t go black. I assumed the problem might be coherence length, beam spread, or it being randomly polarized. In the video the guy was using a 35mw polarized spectra physics 127 hene laser. The laser I got to go dark was a 24” polarized hene and the beam didn’t spread like the small one.