J. Kissel

Recently, Shivaraj, Arnaud, and Joe have noticed that during large tidal excursions, the actuation strength of the test mass ESD system changes (see LLO aLOG 31460). The mechanism is as such: 
    - LLO feeds its low-frequency tidal signal to the ETMs at the M0/TOP stage (LHO feeds its tidal signal to L1/UIM) before offloading very-low-frequency control signal to HEPI.
    - This actutation force displaces the main chain from the reaction chain (the reaction chain does not move), relative to the QUAD's cage because that's M0 stage OSEMs actuation reference (as opposed to the L1/UIM stage, in which the actuation reference is the reaction chain).
    - The ESD actuation strength depends on the distance between the test mass and reaction mass. Thus, as the tidal control forces increase the distance between the chains, the actuation force of the test mass actuator decreases.

After hearing of this, and knowing the difference between H1 and L1 tidal control, I'd asked "have we seen similar effects at H1?"

Yesterday's tidal excursion (see LHO aLOG 34711) is an excellent test bed... and no similar change in actuation strength is seen when the Tidal control has this 80 minute excursion . However, there are are changes in strength that appear at the beginning of each lock stretch. These *might* be tidal, but they also might be correlation / covariance with the as yet unmeasured SRC detuning parameters...

I attach several pre-made plots to kick-start the investigation:
    - Jim's plot of the tidal control signal sent to the test masses
    - Relative actuation strength of the ETMY test mass, as continuously measured by calibration lines.