[Sheila Cheryl Keita Georgia]
After discovering the coupling between HAM3 and DARM we were curious about the beams that we see on the PR2 camera and whether they can help us identify sources of clipping and backscatter. We see a few beams in this camera image that we would like to identify.
We haven't zoomed or moved the PR2 camera since we last tried to identify these spots in July. I used Gabriele's script to superimpose the camera image we see now, at 20W, with the visible image. See attached screenshot.
There are 4 spots visible on the PR2 scraper baffle: it looks like 3 at first glance but I think the central spot has a smaller spot slightly above it, the intensity of this smaller spot fluctuates during the lock. To check that these spots were not coming from the MC2 transmission we locked the mode cleaner at 25 W, and did not see these spots with anywhere near comparable intensity.
In the visible image, looking through the hole in the scraper baffle, with think we can see the outline of a round mount which could be the 3" mirror directly behind PR2. Visible in the 3" mirror we think we can see the reflection of a mount that could be the first to the first mirror on the POP QPD path. I'm not sure if any other beams we see on the camera are actually coming from these mirrors though.
There is also some light visible on the balance mass, which Keita and Sheila hypothesize could be a reflection off the AR surface of an ITM, which is wedged such that the thin part is on top, which is reflecting off PR3 and is not getting caught by the scraper baffle. When ALS is locking/locked we also see spots on the balance mass, from both the X and Y arm green. This is shown in the second attached screen shot. The other spots visible in the second screenshot are also green.
On the left we also see some spots which might be coming from the beamsplitter elliptical baffle.
Associated with FRS Ticket 11740.
On Anamaria's suggestion we tried steering the ITM compensation plates and looking at the spots on the PR2 camera.
The spots on the ballast mass originate from both CP's. See attached photos:
1.before the CP move
2. with only the IX CP pitched (-ve)
3. with both ITM CP's pitched (-ve)
4. with both ITM CP's pitched (+ve)
Please see attached pdf for the overlay and identification of IFO items seen in Georgia's alog picture. (Sent previously via e-mail and now posted here for record.)
Eddie and Calum
Again previously on e-mail and adding here for record ...
From Corey Austin:
Those are second order ghost beams from ITM-AR and/or CP (so ITM-AR->ITM-HR->ITM-AR->PR3->masses). The attached slide shows pictures taken at LLO in full lock before/after we added a baffle in front of those masses during our last vent.
LLO has a 'tie-fighter' shaped spot on our camera that we decided was coming from the beamsplitter which may be your 3 beams.
From Alena / Calum / Eddie:
For future use / reference LHO has that baffle Corey refers to (before / after pics attached again) on the shelf at LHO. To see it in more detail (at LLO) see table baffle on pg. 3, Detail 'D' of LIGO-D0900520-v14 (https://dcc.ligo.org/LIGO-D0900520) and yes you have to be careful with placement so as not to clip the main beam!
Hiro, GariLynn and I had been discussing the ghost beams around PR2 and Hiro looked into the BS ghosts due to the wedge, see below and attached.
From Hiro:
The ghost beam around PR2 due to the wedge of BS is 90mm from the center of PR2.
Attaching Zemax simulations for all four conditions: 1 - detector view in front of PR2 baffle before ITM CPs alignment change 2 - detector view in front of PR2 baffle ITMX CP -200 um pitch 3 - detector view in front of PR2 baffle ITMX CP -200 um pitch and ITMY CP -300 um pitch 4 - detector view in front of PR2 baffle ITMX CP +200 um pitch and ITMX CP +300 um pitch The detector is looking in both direction, the model does not have the baffle spacer and ballast masses therefore in zemax we may see more beams than on camera photos. The group of ghost beams highlighted with a dashed line matches the ghost beams seen on the mass in front of the baffle. Adding a simulation of the ghost beams in front of PR2 baffle from an older aLog https://alog.ligo-la.caltech.edu/aLOG/index.php?callRep=30010 [Denis Martynov]
