Summary: The QOSEMs on the BBSS will see a vertical sag of ~50um. The LF and RT QOSEMs should be offset 50um low of center in there x axis, and the F1, F2, F3, SD QOSEMs should be offset 50um low of center in there y axis; such that they will be well centered when under vacuum.

In T2600176 (write up to come) we calculate the expected vertical sag of the BBSS suspension chain to be 58um at M1. This is slightly larger than the 50um predicted for the BSFM (T1100616), which is consistent with the fact the payload is lighter. To verify these calculations, we pulled OSEM data from the last pump down of the BSFM (April 26 2025 = 1428872333)), and see that there was a vertical sag on the LF and RT BOSEMs of ~35um. See the figure attached.

This is about 10um less than predicted by the calculations for the BSFM, a trend which is also seen on the Quads (LHO alog 15887). As such we suggest that the BBSS should have its M1 QOSEMs offset by ~50um low of center, also about 10um less than the theoretical value. The M2 and M3 OSEMs do not need to be offset, as they don't measure in the vertical direction.