Summary: To help determine priorities for stray light mitigation during the O3-O4 break, I measured the residual coupling in the HAM3 area, and found it to be low. So I don’t think that HAM3 work to reduce light on the chamber walls is highest priority, though it would mitigate risk associated with alignment changes.

In the fall, we used impulse propagation delays and location-dependent beating from separated shakers to localize the source of 48 Hz scattering to HAM3 door areas (https://alog.ligo-wa.caltech.edu/aLOG/index.php?callRep=52184). We removed the 48 Hz peak from DARM by inserting black glass in the illuminator port, but we didn’t get the chance to measure the residual coupling, which had the potential to be fairly large because the light from the scraper baffle illuminated more of the door and nozzle than just the viewport (see above link, Figure 2).

 The figure below shows that I had to increase the vibration level at HAM3 by a factor of 10 between 35 and 50 Hz in order to increase the DARM noise floor. Also shown is the large broad 48 Hz peak and harmonics that appeared when the black glass was briefly removed without changing the settings.

These results suggest that blocking the remaining light from the PR2 scraper baffle and other stray light on HAM3 is not the highest priority (I would say HAM5-6 noise, likely from the septum, is). However, work on HAM3 would mitigate the risk of a coupling increase with alignment changes.