the mode shape @ 5-15kW is very elliptic (50% of ellipticity : large size = 2x small size).

we only changed M1 and did not move M2,M3,M4.

what are the possible reasons for the ellipticity:

- M1 misalignment (cavity axis if off the mirrors center) :
we changed the M1 orientation in X and Y => no effect on the mode shape

- cavity too close from instability region :
we changed the distance between spherical mirrors (S3 moved by almost 700µm ) => no effect on the mode shape

- thermal effect changing the mirror curvature :
we observed mode at 5 or 15 kW => no effect on the mode shape

- bad alignment of the lens in front of the camera :
we observed elliptic and round shape beams with exactly the same centroid on the camera

- thermal effect changing some stress on the mount holding the mirror :
we observed mode at 5 or 15 kW => no effect on the mode shape

- default in the mirror substrate polishing :
why not, but it's strange that with the previous mirror we saw the same kind of ellipticity at low power and a perfect round shape beam at high power.
(with this mirror, we never succeded to overcome 15kW in FPC)

- how strong the small screws holding the mirror are tightened, which could induce some stress :
but why did we saw a perfect round shape of the beam at high power with the previous mirror ?
and it would be strange to get almost the same orientation of the ellipse with the 2 mirrors.

- other ideas ???