Keita, Rana, Evan
After refocusing the HAM5 camera, we see in full lock that there are at least two ghost beams hitting the SRM composite mass. These ghost beams move when CPY (but not CPX) is moved.
The attached plots show the DARM spectrum for different CPY alignments. There is no obvious sweet spot, but perhaps we will find one by looking at some long-term DARM BLRMS.
This image shows the HAM5 camera with the nominal CPY alignment (-150 µrad in pitch, 0 µrad in yaw). The two bright, vertically aligned spots on the left-hand side are the CPY ghost beams.
Also, the take snapshot button for camera 17 just saves blank images.
Here are 2 other old alogs that are relevant if you are worried about scatter from CPs:
Robert saw scattering from CPY in the PR2 camera: 31243
I saw that 6 times higher drive was needed on CPX than CPY to make noise show up in DARM, and the noise that did show up was clear fringe wrapping shelves for CPX and a broad shelf for CPY. 30979
CPY scattering seemed like it is not an immediate problem since the drive reuiqred to see noise in DARM was 100 um at the error point of the damping loop at 0.1 Hz. I don't know the gain of the damping loop, but assume that it is not less than -20 dB at 0.1 Hz, so that we are pushing the mass at least 10 um, probably more. This should be larger than the normal path length modulation. It would be good to look at what the gain of the damping loop actually is to see if this is really a much larger path length modulation than what we would normally expect.
SRC model shows that CPY is installed parallel to HR surface of ITMY (misalignment angle of 0.07 degrees). This number gives a vertical offset of the ghost beam on SR2 of 2 cm and on SRM of 10 cm. This is how I read the camera image. One might also suggest that the plate is misaligned even further from the nominal position — by 0.14 degrees. In this case ghost beams swap and we can’t tell a difference.