[Jenne, Kiwamu, Vaishali with help from JimW and JeffK]
Continuing the locking effort from yesterday (36197), we managed to get the mode cleaner to lock. Here's a roll down of the events from today:
1. We (Jenne and I) first aligned the MC2 REFL camera because we were almost on the edge of the PD.
2. As this didn't fix the not locking problem, we asked Kiwamu for help and then we looked at a bunch of parameters like filters, gain thresholds, ramp times. We also looked to check if the suspensions were behaving correctly and then found the mirror which had been turned off. This button (MC2 M2) was not in use at all. Maybe we should have double checked the sdf differences but we know better now.
While we were aligning the MC2 REFL we noticed that MC2 Trans wasn't looking like what it used to. We tried to trace the beam and found that the camera was being illuminated by a ghost beam and not the actual transmission beam of the MC. We found a bright spot by looking into the light pipe and then found that the beam wasn't coming onto the telescope at all. As we couldn't see anything on the viewer card, we turned up the laser power to 10 W and found the beam again in the light pipe only with the IR viewer.
Jenne then tried to gently tap the light pipe (this is the same pipe that had problems yesterday and was fixed) with me looking at the bright spot and it didn't move at all which leads to us believe that the beam might be hitting the edge of the table somewhere.
After hypothesising out loud that this might have been because the tables hadn't returned to the correct positions, we were corrected by JimW who told us that the ISIs return back to their positions on their own.
We then tried to change the axis of the modecleaner in order to redirect the beam in the light pipe but we weren't too successful.
Not having solved this mystery of what happened to MC Trans, we concluded the work for today with a modecleaner that locks at 2 W and 10 W.
Jenne will correct me if I have missed something or used wrong names of mirrors in comments !
I'm hoping that we can talk to someone today with some memory of how IOT2L used to look, because it seems pretty bad right now. The beam that we suspect is the real IMC trans beam (which comes from the transmission through IM1) seems like it's hitting inside the light pipe, or the wall of the enclosure, but it's nowhere near the top periscope mirror.
There is only one mirror on HAM2 to steer the beam transmitted through MC3 and IM1 out of vacuum, and it's on a standard fixed mount, so it doesn't seem like it should have the slip problems that we suspect exist with the IMC REFL path. Since the HAM ISI tables restore their DC positions, the beam really ought to be coming out of the vacuum in nearly the same way it went in.
We tried putting offsets in the IMC WFS loops (both DOF4, the uncontrolled degree of freedom and DOF1) to move the Trans beam around a bit, but to make any significant change to the ghost beam's position (and therefore, presumably, the actual beam's position) we were clearly misaligning the IMC.
Anyhow, right now the IMC locks just fine. We cranked the digital gain of the trans PD so that it looks like the ghost beam's power is similar to the actual beam, so that some of the filter module triggering works, but otherwise we don't really need IMC trans, so maybe we should move forward with the IFO rather than spending too much time with this.
After talking with Cheryl this afternoon, she points out that the path the beam must take through the light pipe / ducting between HAM2 and IOT2L is extremely tight, much tighter than I had realized. That ducting was dislodged on accident earlier this week when it was bumped, and when it was reattached it likely didn't get put back in exactly-exactly the right way. So, probably the solution will be to scootch the middle part of the flexible ducting so that it's farther away from where the IMC Trans beam path needs to be.