At least, we don't want to use Al-coated mirrors. We should use multilayer dielectric mirrors.
I popped into the cleanroom earlier today, and all 4 active TTs have dielectric coatings. I'm not sure why the mirror in this photo looks funny, but the actual mirrors installed are correct, at least in type of coating.
I'm not sure if Den wrote down what mirrors are actually in there, and I didn't look carefully - I don't know if they are G&H, CVI, other mystery company?