Q. How can we avoid this instability issue?
1. Use flatter mirrors or at least concave mirrors.
2. Smaller incident angle to avoid emphasis of the RoC in the horizontal direction
3. Use weaker squishing force for mounting of the mirrors
4. Flip the PR3 mirror in the mounting ring by accepting the compromise that the AR surface is in the cavity.
Another possibility is to use a ring heater to correct the curvature. I talked a bit with Aidan about this.