J. Kissel

Interested to see how the projected ISI channels compared with Kiwamu's assessment of PR3 (see LHO aLOG 15048), I've plotted 
(a) The input noise budget for the Suspension Point Longitudinal Motion, and
(b) The Transfer function between suspension point and L, P and Y of the optic using whatever local sensor was available.
for SRM, SR2, and SR3. Remember that the damping loops for these suspensions is still the "hand-tuned" dumb velocity damping. Also note that SR3, Oplev to M2 Pitch damping is ON for this measurement.

A couple of conclusions:
From the Susp. Point L budget --
(1) For all three SUS, with the current HAM ISI blend design, we're limited by the HAM4 / HAM5 Y motion at just about all frequencies. Where there has been effort to carve out performance at 1-2 [Hz] in Y in the ISI, RX of the ISI limits the performance.
(2) Some how, even though RZ appears WELL below the limiting noise source for L, it's still somehow ~70 to 80% coherent between 0.1 and 0.6 [Hz]. Huh??

From the Susp. Point L to Optic TFs --
(4) Susp. Point Longitudinal is the dominant contributor to Pitch at the optic, by a long shot.
(5) The OSEMs, which are relative motion sensors between the cage and suspension, fall off in response below the SUS's first resonance as 1/f^2 as expected. 
(6) Perhaps unexpectedly, the OSEMs are still *coherent* below the first SUS resonance, down to quite low frequencies.
(7) Susp. Point L, some how contributes a great deal of motion to Yaw as well as Pitch between 0.7 [Hz] and 1.5 [Hz], amplifying input motion as much as a factor of ~100. 
(8) The difference in damping between SRM and SR2 seems to have significant affect on the Susp. L to Optic L TF at the first and second resonances (0.7 [Hz] and 1.53 [Hz] for the HSTS).

Recall that for angle, SR3 contributes the most to *cavity* angular fluctuations.