Is it because of the change in the resonant frequency of the BS-PRM stack? How much the load on BS-PRM changed?
Or is it because of the change in the resonant frequency of PR2/PR3
I claim that neither of those things is plausible. We took out 1 PZT, and put in 1 active TT onto the BS table. There is no way the resonant frequency changed by an appreciable amount due to that switch.
I don't think that it is the resonant frequency of the TTs either. Here, I collate the data that we have on the resonant frequencies of our tip tilts. It appears that in elog 3425 I recorded results for TTs 2 and 3, but in elog 3447 I just noted that the measurements had been done, and never put them into the elog. Ooops.
Resonant frequency and Q of modes of passive tip tilts.
Notes: "Serial Number" of TTs here is based on the SN of the top suspension point block. This does not give info about which TT is where. Pitch modes were all too low of Q to be measured, although we tried.
Tip tilt mode measurements were taken with a HeNe and PD shadow sensor setup - the TT's optic holder ring was partially obscuring the beam.