[Cheryl V, Giacomo C, Paul F]

Today we tried measuring the OLTF at the IMC 'common mode' servo board. This required locking the IMC of course, which we were able to do at about 1pm without much trouble (as previously). With the settings that we were using for the initial locking, we saw a UGF of about 1.8kHz. To compare with William Korth's LLO alog entry 4182 where a UGF of 14kHz was reported, we increased the common mode gain until we matched the UGF of 14kHz. We required a common mode servo gain of 10dB to reach a UGF of 14kHz, whereas at LLO the corresponding gain was -2dB. The difference in gains required (12dB~ factor 4) was close to what we would expect given the different laser power (200mW here versus 1W at LLO). 

However, there was still some work going on around the chamber at that time, so we decided to go for lunch before recording an OLTF to post. After coming back, we then found it very difficult to lock the mode cleaner again. We could get short locks, but nothing more than 15 seconds. As far as we could tell, nothing had changed in terms of gains or offsets inbetween (we had brought the common gain back to our previous locking value of -4dB). We then spent a lot of time trying unsuccessfuly to adjust gains, offsets, and filters to get a stable lock. We did eventually get some longer locks (~30 seconds) after increasing the watchdog limits for MC2 M2 and M3, but then we noticed an oscillation in the length feedback channel after M2 feedback was applied (see attached plot). 

Some possible causes for the change in the locking situation are:

• Pumping was stopped on HAM2/3
• A high-pass filter button on the IMC servo medm screen is missing a png graphic, and we can't tell whether it is engaged or not. We think this may be a problem because before we increased the MC2 M2/3 watchdog limits we saw the IMC_F time series data making a few seconds long drifts, whereas we think under normal operation this should be quite a high-passed signal. This may have been railing the VCO, and causing the PSL PMC lock losses that we saw too.

We'll keep trying to get the IMC locking up and running again tomorrow so we can look at some WFS signals.