Summary:

The IMC mode matching lens, IO_MB_L2, has been rotated multiple times over the last few years, as evident by having to move the beam dump for the reflection from it's back surface.  I have looked but not found any records of such deliberate lens rotations, and without this record, my concern is that the lens may have also move along the beam path, altering the mode matching into the IMC.

The mode matching from the PSL to the IMC is done in the IO path with the lens pair IO_MB_L1 and IO_MB_L2.  Since the initial install of these lenses, the lens IO_MB_L2 has been aligned in angle with respect to the main beam at three different angles.  Changes along the beam path cannot be ruled out, which suggests the mode matching into the IMC be measured.

Details:

• 2012, initial install, IO_MB_L2 was aligned so that the reflection from the back surface was located around 15mm in the -Y direction from the main beam at the output aperture of the EOM.  A razor blade beam dump was installed to dump the beam in that location.
• late 2015, the IO_MB_L2 reflection from the back surface was not on the 2012 razor blade dump.  It was found on the +Y side of the main beam.  The IO_MB_L2 lens was observed to have been rotated about 30 degrees in the clockwise direction.  A new razor blade beam dump was installed to catch the back surface reflection, and this beam dump is near ALS-L1 in the ALS path .
• early 2018, IO_MB_L2 was observed to have been rotated in the counterclockwise direction, closer to perpendicular to the main beam.  Before the PMC swap, I asked Ed to look for the back reflection, and he did not find that beam on the 2012 beam dump, or the 2015 beam dump, but did find one candidate beam that was hitting an optics mount on the +Y side of the main beam (+Y side of the EOM).
• it's unknown if these changes have had any effect of the position of the lens along the beam path, and the motivation for these changes are unknown

Images:

• Image group 1: looking from the North end of the PSL table to the South, toward the EOM and IO_MB_L1.  The images are described in their names, which can be seen at the top. 
  • the image on the left is from 2012, and the reflection from the back surface of IO_MB_L2 is on the beam dump that is closest to the output aperture of the EOM
  • the image on the right is from 2017, and it shows the 2012 beam dump near the EOM output aperture, with no beam, and the 2015 beam dump on the left, with 2 beams
• Image group 2:
  • the image on the left is from 2012, and the reflection from the back surface of IO_MB_L2 is on the beam dump that is closest to the output aperture of the EOM (same image as group 1)
  • the image in the center is from 2015, and shows that the beam dump in from of the EOM is still in place, and there is no beam dump near ALS-L1
  • the image on the right is from 2017, and it is taken from the NE looking SW, and shows the beam dump near ALS-L1