Sheila suggested we adjust the top mass damping to improve the step response, 91808. I created a 20 Hz elliptical low pass so we could raise the gain even further. However, this has little effect on the 0.45 Hz ringing in the step response, because this is not coming from the top mass. This is evident in the OLTFs, see pitch below. There could be cross coupling with length, but more likely this is some pitch mode between the lower stages that is not witnessed in the top mass osems. Therefore, damping it will have to occur with a lower stage damping loop using the oplev or M3 wit.

As it stands, we're getting plenty of suppression in LPY with the top mass damping. With the updated model, 91834, we can revisit lower stage damping.

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I wanted to add some details about a challenge we are having with the beamsplitter. In short, if the BBSS is kicked a large amount, this saturates the QOSEMs to the point that the top mass damping loops cannot be used, and we have to put the beamsplitter in safe until the suspension freely damps enough that we can reengage damping.

This happened four times today, and has happened at least twice before. The first time occured today because I was adjusting damping gains and put a gain that was so large it rung up the loop. It took about 10-15 minutes to damp down. This kick also trips the ISI.

The other three times occurred today while we were trying to lock MICH dark during initial alignment. I had put the beamsplitter into what I thought was a decent enough alignment to lock MICH dark- clearly it was not because the MICH loop engagement kicked the beamsplitter significantly. I tried twice more after touching up the alignment further and the same problem occurred. From a quick glance, it looks like the MICH and BS lock settings were correct, but I with carefully check tomorrow one more time before we try this again.