The SQZ filter cavity version of this: 86624

The plots from LHO:86253 show the before vs. after M1 OSEM noise performance for 
    MC1
    MC2, and
    MC3.

Comparing the page 3 summaries of each of these .pdfs show that 
    - We only expect change between ~0.2 and ~8 Hz. So, any improvement seen in the 7 to 9.5 Hz region is very likely *not* related to the sat amp whitening change.
    - For MC2, the LF and RT OSEM degrees of freedom --  both the before and after traces show that L and Y are well above the expected noise floor below for the entire region below 10 Hz.
    - The change all DOFs is a broadband change in noise performance; not only is there nothing *sensed* by these OSEMs above ~5 Hz, but there's also no change and thus any resonance-like features that change in the IMC signals will not be related to the sat amp whitening filter change.

We should confirm that the data used for the MC2 "before vs after" were both taken with the IMC length control OFF (i.e. the IMC was OFFLINE).

Back to the IMC metrics -- 
    (1) Only IMC-L and the MC TRANS signals appear to be dominated by residual seismic / suspension noise below 10 Hz. Hence
        (a) I believe the improvement shown in the MC TRANS QPD, tho I would have expected more.
        (b) I believe that we're seeing something that is limited by seismic / suspension noise in IMC-L, but the fact that got worse doesn't agree with the top-mass OSEM's stated improvement from LHO:86253, so I suspect the story is more complicated.
    (2) it's clear that all of the DC "QPD" signals from the IMC WFS are not reading out anything seismically related below 10 Hz. Maybe this ASD shape is a function of the beam position? Maybe the real signal is drown out by ADC noise, i.e. the signal is only well-whitened above 10 Hz? Whatever it is, it's drowning out the improvements that we might have expected to see -- we don't see *any* of the typical residual seismic / suspension noise features.
    (3) WFS B RF signals appear to also be swamped by this same color of noise. WFS A RF signals show a similar color, but it's less in magnitude, so you see some residual seismic / suspension peaks popping up above it. But it's not low enough to show where we might expect the improvements -- the troughs between resonances. So given the quality of this metric, I'm not mad that we didn't see an improvement.

So -- again -- I really only think that metrics (1), i.e. IMC-L and MC2 TRANS are showing noise that is/was dominated by residual seismic / suspension noise below 10 Hz, so that's why we see no change in the IMC WFS signals; not because we didn't make an improvement in what they're *supposed* to be measuring.

We'll have to figure out what happened with IMC-L between June and Sep 2025, but the plethora of new high-Q (both "mechanical" and "digital") features between 3 and 10 Hz suggests that it's got nothing to do with the sat amp whitening improvement. The slope of increased broadband noise between 1 to 3 Hz doesn't match what we would  expect, nor does it match the demonstrated improvement in the local M1 OSEM sensors. We should check the DC signal levels, or change the window type, to be sure this isn't spectral leakage.