Simon

As there are some doubts about the linearity of the input-polarization, I checked the HWP and can confirm that it is not an accidentally taken QWP (I changed the angle to confirm its periodicity of 45deg).

In order to further increase the linearity, I have put a QWP in front of the HWP and looked for the S-pol minimum (while maximizing P-pol) on the sensors when the mass was out of the beam-path. I could reach a minimum of ~1.2 mV with having ~384 mV on P-pol-sensor. Turning the HWP by 45 degrees, I reached a S-pol maximum of ~312 mV and a P-pol minimum of ~0.9 mV.
Those are much better numbers than we had before so that we can say to have a good linearization now!

With those changes on the setup and setting the initial polarization to S, I started mapping ETMY.