I made a couple of ancilliary investigations while I was in-chamber helping adjust CPY. First, I shined a flashlight through the ITMY elliptical baffle towards the BS. Figure 1 shows that this produced a 45 degree annular beam, similar to the one observed during lock that I noted here ( 83050 ) , consistent with the hypothesis that it is a reflection from the BS cage (see linked alog).

Second, our entry through BSC8 also reminded me that LHO has some unique potential scattering sites that LLO does not have.  Figure 2 shows that several of the blanked off nozzles in BSC8 act as corner retroreflectors visible to the beam spot on ITMY, and there is a reflection from the chamber, just below the beam.  A look at my compilation of beam spot photos from several years ago ( 41142-Figure3 ) also shows these issues at ITMX (second page of Fig. 2). We should probably put in nozzle baffles next time we are in-chamber near BSC8 and 7.