After bringing the IMC down this morning, we had trouble getting the IMC back up. The alignment was bad enough after recovering the seismic and sus that the wfs would walk out of alignment. Keita recovered the alignment by ramping the WFS gain down (on IMC_WFS_MASTER screen (under the IOO dropdown), lower left, it's nominally .1) and moving MC1 and MC2 to maximize the power on MC2_TRANS_SUM and minimize IMC_REFL_DC_OUT, while watching them in dataviewer. He then tweaked MC2 to center MC2_TRANS P and Y to near 0 (~.001) on the PD (on the IMC_CUST_OVERVIEW, top right, on the little pd graphic right of MC2). The gain on the WFS was then ramped up to some small number (I think ~.004? maybe .04) and the IMC watched to make sure it didn't go unstable. When it looked stable, the gain on the WFS was ramped back up to .1.
We wasted a bunch of time trying to retrieve earlier alignments, clearing wfs histories and moving the PZT (a strange land where pitch is yaw and yaw is pitch), before Keita decided to just do the realignment by hand.
Note that the manual alignment was done after everything was brought back to old numbers (MC1, MC2, MC3 using witness BOSEMs, PZT offset using the output of the PZT before the maintenance). Even though the PZT change was not huge (as seen in the MC REFL position), I cannot claim that the change was nothing.
Also, at first I was confused by the fact that the MC trans camera looks as if the beam is split (01 mode) even though MC is locked to 00. Kiwamu told me that the only way to make sure is to look at IFO cameras like IFO REFL and AS.
PSL FSS oscillation precluding IMC locking
It appears that FSS oscillation was wreaking havoc with the ISS and this was the cause of the IMC not locking.
Reducing the FSS Common gain to zero then bringing it back up to 26 dB stopped the oscillation, as seen on the "PZT MON (FAST)" trend display on the PSL FSS screen.
It the display shows a black region (rail to rail oscillation), then the FSS is oscillating. In the nominal state, it should just show a think black horizontal line.
The FSS was tuned up this morning and is now operating as designed ~ 500 kHz UGF, 60 deg phase margin, no features (peaks) up to 5 MHz. However, it may not be as robust against kicks from the IMC as it uses the FSS as its frequency actuator.
We did not take time this mornining to investigate the stability of the FAST/Pockels Cell crossover. I was somewhat surprised to see the FAST gain at 5 dB. It may be that the crossover is not stable.
We used to run the FAST gain at 15 dB. Seems that it was turned down to 5 dB on April 15, 2015.
Next opportunity will check the crossover by looking at the mixer monitor noise spectrum in the 1-50 kHz frequency range as we adjust the fast gain - optimize the tradeoff between FAST gain and noise peaking at the crossover.
JimW is generating a request to TJ to add an alarm for the FSS range to the Guardian.
FSS oscillation was fixed while I was tweaking the MC2 trans position. Before that, IMC locked to the correct mode but the IMC WFS ran away.
So it seems like the alignment thing was a red herring.
The FSS went into oscillation again, after talking with RIck on the phone we turned the common gain down from 26dB to 23dB.