Summary: A camera with a flash right beside the lens was used to look for potential backscatter noise sites. Most baffles look good, and there was only one strong backscatter site within the arm cavities: the large gate valve seats by the ITMs.  There were also a few retro-reflective sites outside the arm cavities: for the compensation plates, the HAM3 ISI, and the supports for the elliptical baffles and the TCS mirror 2 were quite retro-reflective. As an aside, I also noticed that the TCS alignment port was partially blocked by a nozzle. In the lower sensitivity input arm, the MCA1 and 2 baffles had bright reflections, as did the HAM3 ISI.

ITMX. In Figure 1, the top image shows approximately what the beam spot on ITMX “sees”. I use these photos to look for potential scattering problems; the camera is placed very close to the center of the optic where the beam spot will be. The flash mimics the wide-angle scattering of interferometer light from the beam spot and lights up regions that will retro-reflect interferometer light back to the beam spot, where it can recombine with the main beam to produce scattering noise. Of course the technique is limited by different angular distributions of scattered IFO light and light from the flash, as well as by color differences, but it gives a rough idea and also shows glints, which are hard to predict. The camera is about 10 cm in front of ITMX, so it sees a little more through the hole in the baffle than the beam spot on the optic will. Nevertheless I expect that the baffle will not block the view of the gate valve seat, the shiny reflecting arc at the right of the view through the baffle aperture. This is the only site found in the high-intensity arm cavities. I think the first step is to double check that the valve seats are actually visible from the ITM beam spots.

The lower image in Figure 1 is the view from the X-manifold spool of ITMX, with the camera located in the IFO beam path (the camera is in the beam path when the flash retro-reflects off of the optic). The arm cavity baffle looks good: the small reflecting rectangle at the bottom center of the baffle is the connection for the safety lanyard and is actually under the bottom side of the baffle and thus should be outside the clear aperture from ETMX. It looks like it is inside the baffle because the far part of the baffle box sags down a little and we are seeing a little of the underside of the baffle.

ITMX RM (compensation plate).Figure 2 shows the view from the beam spot on the compensation plate (the reaction mass). There are strong retro-reflections from the HAM3 ISI. The glints are off of stage 1 of the ISI so there is some isolation of the reflecting surface. There are also glints off of the elliptical baffle, which has one stage of passive isolation like the ISI, and also off of the mount for TCS mirror 2, which is not seismically isolated.

The view in TCS mirror 2 (the copper-colored circle), shows that the TCS alignment port is partially blocked by the nozzle of the port. Thus there is not a complete view of the compensation plate from all parts of the port, which may make alignment more difficult. We also need to watch for backscatter sources on the TCS table.

BS.The first page of views from the beam splitter, Figure 3,  show bright reflections from the frames of the ITMs. I have reported this before (Link) and Mike Smith’s subsequent calculations suggested this would not be a problem, though I don’t think that specular reflection was considered.

Another potential source of retro-reflections to the spot on the BS are the reduction flanges for the tubes holding the gate valves between BSC1 & 8 and BSC3 & 7.

The second page of BS views shows the view back out the input arm. As with the view from the ITMX compensation plate, there are particularly bright glints from the HAM3 ISI (notice the beams of light shining on the floor of the tube), including one from the table edge, just a few centimeters below the path to/from PR3.

PR3. Figure 4; looking toward the BS, there are fairly strong reflections from the electro-polished aluminum baffle, MCA1, as well as from masses on the HAM3 tabletop.

PR2.The worst reflection in Figure 5 seems to be from the edge of the MCA2 baffle.

PRM.  Figure 6 shows a fairly bright reflection from the MCA1 baffle, a wide-angle reflection from the circular baffle, and reflections from objects on the HAM3 table.

MC1 & 3. Figure 7 shows that the MCA1 baffle again seems quite reflective and that there are also reflections from objects on the HAM3 table.

MC2. The reflection from the MCA1 electro-polished surface in Figure 8 does not appear as bad from this side, possibly because of the greater distance.