We installed a 160MHz four-mirror OEC with two planar mirrors and two spherical mirrors. The radii of curvature of the spherical mirrrors are 0.5m.

The design distances are the following:

- Cavity length: 1.87m

- M1-M2 (planar mirrors): 411.62mm

- M3-M4 (spherical mirrors): 520.00mm

- axis: 70mm

- M1-M3 and M2-M4 (diagonals) : 471.04mm

- incidence angle : 149.16mrad

The design implementation in the vacuum chamber is the following:

- wall-M1 : 253.82mm

- wall-M3 : 199.63mm

The mirrors we used are:

- M1: C23017/1. Substrate: Fused silica. Absorption: <0.6ppm. Scattering: 2.5-3.5ppm. Transmission: 115ppm. Total losses: 117.5-119.1ppm. Radius of curvature: planar.

- M2 : C23018/4. Substrate: ULE. Absorption: <0.6ppm. Scattering: 3-7ppm. Transmission: 3ppm. Total losses: 6-10.6ppm. Radius of curvature: planar.

- M3 : C23018/11. Substrate: ULE. Absorption: <0.6ppm. Scattering: 3.8-4.5ppm. Transmission: 3ppm. Total losses: 6.8-8.1ppm. Radius of curvature: 0.5m.

- M4 : C23018/12: Substrate: ULE. Absorption: <0.6ppm. Scattering: 2.8-4ppm. Transmission: 3ppm. Total losses: 5.8-7.6ppm. Radius of curvature: 0.5m.

Calculating R_i=1-(A_i+S_i+T_i) with i=[1, 4] and R=√(R_1*R_2*R_3*R_4), the expected finesse is: F=π√R/(1−R)=43211-46164.

The laser source is : NKT Photonics Koheras Adjustik + collimator. The laser beam diameter at 1/e^2 after M2 is : 1.62mm in the x direction and 1.58mm in the y direction (see attachements).