Since Monday, Jason, Travis and I have been working on (amongst other things) the BS elliptical baffle rebuild and realignment.  Once all parts were in hand, the rebuild was straight forward.  Travis used the removed HR and AR baffle assemblies to line up and preassemble the mounting brackets so that the new baffles went in roughly where the old ones used to be positioned on the BS suspension structure.  However when we put the target on the HR panel, we discovered one of GariLynn's earlier worries - the baffle was off center by many millimeters.  Looking at the baffle relative to the structure seemed to indicate the same thing - the beam was left and down when looking at the HR surface target (meaning, the baffle was potentially too far up and right relative to the simulated IFO beam).

Unfortunately, this target-mounted-to-the-elliptical-baffle is what was used to align the entire BS SUS/ISI cartridge in the chamber install in ~2013, as per IAS procedure E1200795.  This fact lead us to spend a day or 2 sorting out how to tell if the BS optic is misaligned in the chamber (without any IAS measurement ability), and how we possibly could have installed the elliptical baffle out on the cartridge in the first place many years ago.  During this we went back in-chamber to make "precise" measurements.  We used Class B rulers to measure where the IFO simulation beam was hitting the BS, how well the BS is centered in the structure, and exactly where the beam was hitting the target.  Attached are actual sketches and pictures with detailed numbers.

Here's what we were able to measure to within ~1mm error since it's by-eye and other knowns:

1) The beam is hitting the center of the BS optic 2mm to the left of the optic centerline.

2) The optic is suspended in the structure to within 1mm or less of error.

3) The beam is hitting the HR target baffle 5.5 mm left of the target center.

4) Our Y-arm IFO simulator beam which runs between the ITMy center and the SR3 center ran dead center on the ITMY elliptical baffle (prior to it's removal) and the HWSY path a few weeks ago when HAM4 work was checked.

5) Per IAS procedure E1200795, the BS suspension to ISI locations were performed via more direct methods which did not involve the Baffle target, so we believe the BS to ISI mounting is correct.

Point 1) and 2) give us some confidence that the error of the BS optic placement is not as bad as how the baffle target indicates.

Some consult with GariLynn and Jenne, to understand what centering is most important from various aspects (including looking at some modeling done by Hiro in T1400055) and we have come to the conclusion that we will carry on as planned.  Namely, to first order it is best to have the baffle aligned very well to the beam.  Our next steps will be to recheck the IFO simulation beam pointing again, align the HR baffle to it, and take a series of pictures to ensure that the edges of the baffle are lined up correctly with the edges of the BS optic from the beam POVs.

Also during our walkabout over the last 2 days, we re-interpreted Keita's BS centering/beam clipping alog 28196, pointed out by Sheila.  Because it is difficult to see the edges of the BS optic when the baffle is installed, it seems Keita made some geometrical analysis of the HR and AR baffle positions to determine the position of the BS center.  However, since we believe the baffle was mounted in the wrong position by a few mm, his centering numbers likely are not correct.  Still, his numbers indicate the same directions of error as we see - the beam is hitting left and low of the baffle/optic.  I think we can repeat this measurement in the future by scaling the known diameters of the baffle holes and BS optic size on the camera view pictures.  Will think on this more.