Around 7am, Daniel came in to check on H1. He was seeing the same issue I had, and I heard him mentioning how the y-arm was locking on the wrong modes & he couldn't tweak optic around to remedy this. So he was looking into locking to see what Guardian might be doing.
Eventually he finally got the 0:0 mode back! And it turns out it was a gain change.
He mentioned that the Y-arm thought it was locking the right mode, but in reality it was not. So a lower order mode was continually being locked (& guardian might have even been trying to enable WFS on this bad mode---which is not good!). Looking over Daniel's shoulder I saw that he had the PDH Locking medm open and here he was looking at the Acquire Gain.
- Acquire Gain at -4 (orignally)
With this high gain, Y-arm was thinking a bad non-0:0 mode was what we want, but we had an ugly 1:0 mode trying to lock. 
So, Daniel lowered the Acquire gain so we would no longer be fighting autolocking.
- Acquire Gain changed to -20 (by Daniel)
Then he unlocked the Y-arm and was now not fight the autolocker which was trying to lock a 1:0 mode. So, now he could move in yaw (I belive he said ETMy about 1.0 counts) beyond this 1:0 mode and was then able to get the Y-arm back into 0:0 territory/good alignment. And just like that Y-arm was back.
Attached is a screenshot of the PDH Autolocker for the Y-arm and you can see the Acquire Gain on the lower right.
(Daniel is adding a more thorough alog now.)
The Y arm has had bad mode matching since our last big vent, which often means that the automation won't work unless the arm is well aligned. Jason has been working on the mode matching problems for the last several Tuesdays. Normally the situation can be handled by taking the Y arm guardian to unlocked, watching the flashes and manually adjusting the alignment until the flashes reach the normalized value of 1. Then returning the guardian to the state you found it in will usually allow you to keep moving. Was this situation different?
After the lockloss, I did the following:
- Since ALSy would not LOCK, I took the ALSy Guardian to UNLOCKED
- Tweaked the ETMy & TMSy to maximize the green power to over 1.0, but all I could manage was 0.7 or so no matter what I tried for about 20min.
- Thinking the ITMy might add more adjustment options, I decided to run an Initial Alignment.
- Now adjusted ETMy, TMSy, and now ITMy, but
- Unfortunately, I still could only muster about 0.7-0.74
- NOTE: I saw most flahses showing a TEM 01 mode, so I would try yaw moves, but still no luck.
- The 01 modes would lock with the higher power and the TEM 00 modes would lock in limited frequency, but at much lower power than the 01 locks.
- Summary: The usual protocol of looking at the green power signal on ndscope didn't help me.
Daniel noticed the same occurrence of the TEM 01 flashing being more prone to lock and with higher powers than the TEM 00 flashes.
So this led Daniel to changing the Acquire gain. Then he made yaw adjustments to get us out of the TEM01 yaw misaligment state and (fairly quickly) he was able to get to an alignment where the TEM 00 would lock at powers approaching 1.0 (vs TEM 00 locks which I tried had values of 0.2 or less).
This was definitely different my usual experience of tweaking a green arm back to a good alignment. Not realy sure what made this situation different than the normal y-arm tweaking we've been used to for the last few months.
I wonder if running the baffle align scripts would have been helpful in this case. Hopefully they would have helped put the optics in a slightly better place for catching a good mode.
Oooo. Yeah, that would have been something worth trying, but I actually have not run them (I recently looked over Jeff K's shoulder as he ran them on a recent Maintenance). How does one run a baffle alignment?