[Keiko, Terrence, Eleonora, Matteo (remotely), Naoki (remotely)]

Yesterday we verified that the actuation range of PZT for WFS DC centering loop on REFL bench is too small, so we redesigned the optical layout to increase the lever arm.

We found a good solution by using a 2000mm lens but unfortunately we couldn't find such lens and it takes ~ 1 week to be delivered.

We found another solution with the lenses availables. It uses 3 lenses (f = 125, f = 74 and f = 1000). The evolution of beam dimension and Gouy phase is shown in attached pic 1 and the new layout is shown in pic 2. Datatips account for the two WFS position. The guoy phase separation is set to 90 deg.

We prepared all the optics and we will install them tomorrow.

Some useful numbers: the q-parameter of the beam at the REFL viewport is  3.0348 +16.8565i, the actuation range of the PZT is 500 urad, the dimension of the QPD sensor is 3 mm.

In case we will need even larger lever arm we will go for the 2000mm lens (we ordered it anyway). Beam evolution and a tentative layout in this configuration are shown in pic 3 and 4. About this design I'm concerned that the 1 inch PZT mount would be too small.