I tried to investigate which mirror is suspended in the NM2 chamber.
According to the records it should be a LISM mirror with the two faces flat.
So I reflect a green pointer on the mirror and collect the two reflections
from the two sides of the mirror. I let the beams propagates over about
15 m. I can't see any difference between the beams along the propagation.
So the two sides of the mirror are both flat (or both curved with a large
curvature). The distance between the two reflections is about 2.7 cm and does
not change in a significant way along the 15 m, so the mirror does not
have a large wedge.