Wanted to check the clipping in Green path at EY. Since EY doesn't have WFS ready, the only thing I can do is to see the QPDs, which is not that interesting, but I asked EX people to do this kind of thing for EX using QPDs and WFSs to check the full green REFL path.

What was done for EY:

• Center the QPDs using QPD servo, and disable the servo (though I could have done this with the servo engaged).
• Hold the SUM output of the QPDs.
• Dither PZT1 or PZT2 in PIT (or YAW), measure the TF from QPD PIT to QPD NSUM.

If there's a clipping there's a linear coupling to NSUM, but it's difficult to get a sense of how large the excitation is. To get that sense, we look at QPD PIT signal itself. Apparently, PIT signal depends on where the QPD is placed (e.g. QPDB is mostly insensitive to PZT1 while QPDA is less sensitive to PZT2 than QPDB is, and this is by design), so we need to pick a good QPD for a given PZT. I didn't mix QPDs as using one QPD looked good enough.

Anyway, this is only with QPDs and thus things are simple, you need to look at QPDB for PZT2 (left top, blue) and QPDA for PZT1 (right top, red). 50dB difference between the beam displacement on the QPD and the NSUM signal. This might be some small clipping or inbalance in the QE or in the transimpedance or maybe some coupling via cable, but the point is, this looks good.

As I wrote above, you should be able to do this using WFS and check the full green REFL path.