I suspect that the ~250 Hz peak in the LLO DARM spectrum, and, I would guess, soon to appear in LHO’s DARM, is in part due to the piezo actuator/mount connection on the periscope (https://alog.ligo-la.caltech.edu/aLOG/index.php?callRep=14971). The last figure in the linked log is a photo of the mount on the LHO periscope. I set up an isolated version of the actuator/UA 200 to test my guess that the frequency is due to oscillation of the piezo actuator in the UA 200 mount. Figure 1 shows that a gentle tap on the actuator produces a large peak near 250 Hz. Figure 2 is a photo that shows the setup and, in the background, a picture of the actual setup on the LHO periscope. I eliminated several possible sources of the resonance, other than the mounting.  To test that it wasn’t the large optic with the piezo “flexure” acting as the spring, I epoxied an iLIGO version of the actuator with a large optic directly onto a plate. The lowest resonance was about 800 Hz. To check that it wasn’t the entire package oscillating on the connection to the table, I added weight to the mount (not the piezo actuator) and did not change the resonant frequency. I also attached the accelerometer directly to the mount. The peak was much larger, as expected.

There are two paths that I have been discussing with Rick and the Florida group. The first path is to design a new mount that holds the actuator at more than one point along its long axis and reduces moment of inertia. And the second path is to move the piezo actuator to the table and put a normal mirror in a UA 200 at the top of the periscope (since the periscope has a broad 150-300 Hz resonance, the piezo/mount resonance is amplified by being on the periscope). I think we should do both.