Bright beam spot on HAM3 ballast mass baffle moves with ITMY compensation plate yaw motion

We have been looking for the source of the scattering noise that varies  with the ITMY compensation plate yaw setting (80499). I recently searched for beam spots in the HAM3 area that move as CPY is yawed, using movies that I took from both the MC2 camera viewport and the PR2 camera viewport near HAM2.  Anamaria and I had done some of this before (77631) but this time I recorded the CP movements on the audio track of the movies for a more precise correlation. And I also modified the movies in iMovie to increase visibility of faint spots. Figure 1 shows that I did find a spot that moved precisely with CPY Yaw settings (link to movie clips: https://youtu.be/FDdNDPoQadU ). The spot appears to be on a ballast mass baffle (see Fig. 1), which is not angled as much as the scraper baffle (which I think the beam is supposed to fall on (Alena's slides)) and may thus retroreflect more light.

Mystery  beam spot on HAM3 spool piece comes from ITMX direction, not HAM2 direction

I had previously misinterpreted the pattern of light on edges and bellows of the HAM3 spool piece as suggesting that the mystery beam spot (78192) was coming from the HAM2 direction . More recently I found that there was light on the HAM3 side of the MC baffle that had a similar interference pattern and was consistent with being part of the mystery beam spot (Figure 1). The light on the HAM3 side of the baffle was visible when looking through the viewport for the PRM camera at a high angle. Thus, the beam is most likely coming from the ITMX direction, travelling close to the –Y wall of the beam tube.

One possibility is that it actually comes from ITMX, either scattered light from ITMX or scattered light from the back side of MC2 reflecting off of ITMX. Figure 3 shows these suggested paths and a picture taken from the point of view of the beamspot on ITMX that shows that there is a clear path to the site of the mystery beam spot. I think that the cartoon shows that it would be worth using a real model to determine if these paths are possible.

I checked to see if the beamspot on the eye baffle moved or modulated when I actuated the compensation plates. I made movies as I moved the two compensation plates more than 600 microradions in pitch and yaw, but, unlike the spot on the ballast mass baffle, I did not see any modulation or motion of the spot on the eye baffle.