Summary: With a lowest resonance of 35 Hz, the new pier avoided the 4-pole motor region at 30 Hz. Reducing payload at the top could at most increase the resonance by 10% to 38 Hz.

We examined the motion of the new leaning optical lever pylon that is grouted to the floor at MY for testing (see photo). A breadboard and translation stage for the photodiode at the top of the pylon appear larger than necessary, weighing about 15.7 kg, so we investigated the maximum gain possible from using lighter equipment by measuring without this load. We also measured with and without the 15 kg dummy photon calibrator.

The figures show accelerometer spectra from near the top of the pylon for the nominal and modified configurations, as well as corresponding spectra for the floor below the pylon. The peak at just below 30 Hz is on the ground and likely produced by a four-pole motor. It is good that the peak for the nominal configuration mostly avoids 30 Hz because such peaks occur at all stations. The transient peak on the ground at 34 Hz is atypical and of unknown source.

The lowest resonant frequencies are tabulated below:
Equipment on pylon                                       Lowest frequency (Hz)

Diode assembly and photon calibrator (nominal).....34.9 
Diode assembly only................................................36.4
Photon calibrator only...............................................38.1
Nothing on pylon.......................................................38.1

We were a little surprised that the lowest frequencies for the pylon alone and for the pylon with the mock photon calibrator in place were virtually the same. We added various masses at different levels and found that mass at the level just above the photon calibrator (see photo, the mock photon calibrator is at the level where the caution tape attaches) moved the peak at 42 Hz, but had little effect on the lowest peak at 38.1 Hz. On the other hand, moving the same mass to the top of the pylon did move the peak at 38.1 Hz down.

Thus, without stiffening the upper stage of the pylon, a lighter diode assembly could, at most, raise the lowest resonance from 34.9 to 38.1, about 10%.

Robert Schofield and Gregorio Tellez