This might be one of the main issues we can take care before O4b. A rought list of the sub tasks would include

• Soon measure the block by block transfer functions among the stacked blocks and/or the top optical table, to check if these touching screw heads (and/or too long screw legs between the stack block and the optical table) would cause some non negligible issues ot not.
• Depending on the results of the measurement above, insert/locate (vacuum compatible) dampers between or on the top optical table and/or the stack block(s). The damper material can be the same damping metals as added for the OMC body (M2052? according to the drawing). The main target frequency candidate might be ~30 Hz peak seen in the invac geophone spectra, or ~220 Hz excitation behavior seen in the invac geophone time series when one foot-tapped the ground outside of the OMC cleanbooth.
• I tried to consider the way to erase the "sitting" screws and make the head thinner, but so far I do not have good idea other than the most direct way: taking out all, and do stack from the first. Such brute thing should be a thing after O4b...

Well, generally speaking, "not-reviewed installation" is mere putting stuffs but does not mean "installation completion".

I think we should also check the stacks for OMMT, IMMT1 and IMMT2 by the scope. IMC was already closed.

Or do you (or the PEM team) have any data for their isolation function? 

In our previous study, it was not so precise but, the oplev signal and the on-table ACC signal were smaller when I tapped the lower part of the stacks.

https://klog.icrr.u-tokyo.ac.jp/osl/?r=28943

[Takahashi, Ikeda]

We took pictures of all the space between the SUS blocks with the fiber scope in the OMC chamber. The conditions are summarized in the table.

O: Small gap, X: No gap, Δ: Touching or very close

If so many positions are contacted, I am afraid that the optical table rotation angle may change because of the friction forces at these contacting points.

[Takahashi, Ikeda]

We took pictures of the space between the SUS blocks with the fiber scope in the OMM chamber. We could see gaps around all the bellows that we could access. We could not access the 2-1, 2-3, and 3-3 bellows.

[YokozaWashimi, Tanaka, Ozaki, Sudo]

Today we tried to evaluate the seismic isolation of the OMC stacks, using a 3-axial accelerometer (S2315344) and an impact hammer (G1910656).
This is a quick report.

1. Locating the ACC on the OMC vac-table (Fig.1, Fig.2)
2. Tapping each stack (top, middle, bottom of stack1, stack2) with the impact hammer (direction: Fig.3)
3. Picking up each pulse (-0.2/+0.8 sec) and calibrating its transfer functions from impact [N] to acceleration [m/s2] automatically (example: Fig.4, Fig.5)
4. Calculating averages of the events (Fig.6, Fig.7)

[YokozaWashimi, Ishikawa, Ozaki, Sudo]

We performed hammering tests (vertical tapping) for the OMC in-vac table.
The tapped points are the table, the base plate, and the ground near the stack1 or 2.