The input beam is most probably being clipped at the Faraday Isolator.
We plan to investigate this further to be sure..
I tried to determine an optimal WFS2YAW offset to be used so that we may avoid clipping.
Initially, I just measured the beam diameter as a function of offset. If the beam diameter would become independent of offset if it is not clipped. However a systematic effect became apparent when I shifted the beam on the detector to a slightly different location. So I repeated the measurements while recentering the beam to the same location everytime (centered at -1650+/- 50 for both H and V directions).
I have attached plots of the scans for both cases, with recentering and without. I have not been able to figure out what is going on since the beam diameter does not become independent of the offset. While the beam profile becomes more gaussian beyond offsets of about 7 or so, the beam diameter does not seem to follow a clear pattern. The measurements are repeatable (within one sigma) so the experimental errors are smaller than 1 sigma.
The photographs below show the improvement of Horizontal beam profile with WFS2Yaw offset. These seem to indicate a good gaussian beam for offsets beyond 7 or so. At offsets more than 12 the MC unlocks.
Will continue tomorrow. Jenne wants to do some IFO locking now.