J. Kissel, R. Kumar, B. Weaver, S. Dywer

Context
After the SQZ team felt done with OPOS suspension the re-install of the OPO (with an upgraded translation stage) we're following up on
    - Ryan's first look of the suspension's health (LHO:88851) 
    - After restoring the OPOS to normal control system configuration (LHO:88872) 
    - After recentering the AOSEMs (LHO:88910)

Measurement
I gathered more standard health check TFs. In doing so, I spruced up the excitation frequency response and data gathering templates to ensure good coherence. I also ran the TFs with all the damping loops besides the excitation degree of freedom ON to reduce incoherent motion from cross-coupling. The refreshed templates are 
    /ligo/svncommon/SusSVN/sus/trunk/OPOS/H1/OPO/SAGM1/Data
        2026-01-26_2054UTC_H1SUSOPO_M1_WhiteNoise_L_0p02to50Hz.xml
        2026-01-26_2054UTC_H1SUSOPO_M1_WhiteNoise_P_0p02to50Hz.xml
        2026-01-26_2054UTC_H1SUSOPO_M1_WhiteNoise_R_0p02to50Hz.xml
        2026-01-26_2054UTC_H1SUSOPO_M1_WhiteNoise_T_0p02to50Hz.xml
        2026-01-26_2054UTC_H1SUSOPO_M1_WhiteNoise_V_0p02to50Hz.xml
        2026-01-26_2054UTC_H1SUSOPO_M1_WhiteNoise_Y_0p02to50Hz.xml


Results
Attached is comparison of 
    - the opos dynamical model (Equations of Motion matrices: ssmake_voposus.m rev 12322; parameter set: oposopt_h1susopo_fit rev 12322)
    - the most recent, best at-vacuum, measurement of L1's OPOS
    - the last, best, at-vacuum measurement of H1's OPO prior to moving from HAM6 to HAM7
    - the last, bext, at-vacuum measurement of H1's OPO after moving to HAM7 (with a payload almost identical to what's installed now)
    - yesterday's measurement described above.

Focusing on page 6 first, one can clearly see that YAW to YAW transfer function is wildly different, implying a significant change to the dynamics of the suspension.
There's more supporting (but less signficant) evidence in 
    - pages 1 and 2 which shows and increase in the primary L / T resonance from 1.25 Hz to a split 1.32 Hz / 1.39 Hz
    - pages 1 thru 6 (all diagonal elements of the TF matrix) and pages 7 thru 20 (most off-diagonal elements) all showing a huge (tho incoherent) resonance in yaw at 4.98 Hz that was not there before
    - Nominally an oversight in the design, the expected large V to Y coupling looks quite impeded.

Discussion
After conversing with Sheila about what might have changed, or what might be impacting the dynamics, she reviews the changes:
    - The OPO cavity assembly (D1500296) has only a few changes -- the translation stage within is new, and there are now thin remote-control cables coming out of the assembly that weren't present before.
    - Those new remote control cables have been bundled into a pre-existing bundle, potentially making the system stiffer. We guess that this is the likely culprit of our problems.
    - Further the re-placement of the OPO assembly on the OPOS was not precise. It's plausible that the physical location of the cavity is now *slightly* different position, *potentially* changing the balance and mass ratio of how much moving mass is in which translational DOF. Maybe the source of L / T resonance shifts, but I doubt it. 

Also, conversing with Betsy and Rahul about the state of cables they found while centering the OSEMs:
    - They see a collection of cables (OSEM and SQZ) on the "far" (+Y) H3/V3 side of the OPOS that are resting on the platform without a good, dressed, soft connection between the platform and where the cable stress is relieved on the base.

Conclusion

We need to address / free-up the dynamics of this suspension before closing up the chamber.