Robert, Sheila, Eric, Daniel D
Yesterday morning we were able to identify the source of the scatter in HAM7 as being SFI2, and notice that the beam is very off center on the polarizer on the ZM4 side of SFI2, B:P2.
We locked the OMC on a single bounce beam with 50W input to the IMC, and turned on the ISS second loop, and got 67mA DCPD sum. With the viewport on the HAM5 gate valve and a 1 Hz excitation of 1000 counts amplitude on ZM5 TEST, we could clearly see a fringe wrapping shelf at 140 Hz. We used a piece of black glass to block the beam and could see the scatter shelf go away when the black glass was after B:P2 and reappear when the black glass was before the rotator.
Robert got some photos of the location of the beam on B:P2, it is clearly off center and near the +Y edge of the polarizer aperture. We attempted to steer the ZMs to bring it close to the center, but we clipped the beam on the aperature attached to B:L2 before we could bring the beam close to centered. This indicates that this path was aligned with the beam poorly centered on the Faraday.
Moving the SFI in the +Y direction by 0.17 inches would center the beam
I calibrated 3 photos, two IR and one cell phone photos, using the 1 inch width of the wedge mount, D2000233_A+_SFI_KTP_Wedge_Mount.pdf. I took the photos approximately face on to the SFI as Sheila held the card. The three values for how far the beam was off center were 0.16, 0.17 and 0.18 inches.
