Alastair, Aidan.

We tracked down the source of the cross-coupling between the HWSY probe beam and the HWSX sensor. The offending beam was coming from the HWSY BS (we think). By blocking the Y-beam at various points we were able to figure out where it was coupling across the HWSX and we subsequently placed a dump in front of it before that point (between M8 and M9 in the Y-arm optical layout - see the attached photo).

One more thing, I checked the following configurations to look for any other stray beams:

• Viewed HWSX camera with HWSX probe beam OFF and HWSY probe beam ON: no visible signals on the camera
• Viewed HWSY camera with HWSY probe beam OFF and HWSX probe beam ON: no visible signals on the camera