Philippe, Sharon, Anamaria, Robert
We looked for stray light through the 5 viewports indicated on the first page of Figure 1. We saw stray light from the OFI, but, by far, the brightest light appeared to be from the septum window from the end-cap view. I could not see this bright spot from the viewports on HAM5 or the center viewport on the HAM6 endcap, only from the +X viewport on the HAM6 end cap. This view is not available at LLO. Figure 1a-c show photos, and detail the argument that the spot is forward-scattering, at 3-degrees, of light from the beam spot on the septum window. For example, the parallax photographs in Figure 1c are consistent with the origin being at the septum window. In addition, they show that the spot is bright over several inches at the viewport, and thus more likely to be diffuse scattering than a ghost beam.
We could not see all parts of the septum, but, except for the window, we did not see other spots on the septum (this was also the case for the parts of the septum we could see at LLO - https://alog.ligo-la.caltech.edu/aLOG/index.php?callRep=44985).
Impulse injections, along with shaker injections, suggest that the septum is the dominant scattering site, at least in the band above 20 Hz, at LHO and LLO: https://alog.ligo-wa.caltech.edu/aLOG/index.php?callRep=48886. The noise produced by septum motion, estimated from PEM injection results, is consistent with the loss of range due the HVAC at LHO: https://alog.ligo-wa.caltech.edu/aLOG/index.php?callRep=48912. The observation of bright scattering from the LHO window, but not from other examined regions of the septum at either site, is evidence that the septum window could be the scattering site on the septum.
Figure 2 shows a plot from LIGO-T0900269-v2 that is a reminder that we expect scattering from the septum window to be close to limiting. This, along with the visual observation evidence and the evidence from the shaker and impulse injections, all accumulate to suggest that it may be time to consider removing the septum windows at both sites.
