I have recently reported that the “mystery” beam on the spool piece wall near HAM3 was coming from the direction of ITMX (82252). To further this investigation, I started photographing the area around the ITMs and BS as best I coluld through our viewports (there is not a good view towards HAM3).  I found several unexpected distributions of light in the vertex:

1. 20 degree conical annular beams from ITMs

Both ITMX/CPX (Figure 1) and ITMY/CPY (Figure 3) cast an expanding annular “beam” towards the BS with a cone half angle of roughly 20 degrees from the main beam. My best guess is that it is produced by arm cavity light hitting the bevel of the ITMs (see cartoon in Figure 1). A good test of this would be to install the new test mass cage baffles at one or more of the ITMs at LLO (presuming Anamaria finds this beam at LLO) this upcoming break. The baffle should hide the bevel and eliminate the ring of light.

2. 45 degree conical annular beams from BS

The BS appears to cast an expanding annular “beam” with a cone half angle of 45 degrees, centered around the -X, -Y direction (Figure 2), and likely another annular beam, also with a half angle of 45 degrees, centered around  the  -X, +Y direction (evidence in Figure 3). I tried to find a geometry where the bevels were also the source of these beams but didn’t. My best guess is that the annular cone is produced by reflections of light from PR3 and the ITMs off of the inner surface of the circular cage around the BS, or the inside surface of the circular barrel of the BS itself (see drawings on third page of Figure 2). 

3. Reflection of BS in ITM elliptical baffles likely visible at ITMY and HAM3

The BS beam spot is reflected towards ITMY by the slanted piece of the ITMX elliptical baffle (Figure 3). While the actual beam is not reflected towards ITMY (it isn’t clipped), the baffle reflects light towards ITMY that is scattered out of the main beam by only a few degrees.