Last Thur at 4pm local, the ETMX Oplev took another jump - this time ~70uRad in PITCH, negligable in YAW. At the same time, the PIT, YAW, and SUM signals became noisy and have been ever since. According to the THUR Mar 08 Ops summary alog 40914, the only crew was the in-chamber PCAL baffle crew, who were not around the OPLEV piers outside of the chamber and down the tube a bit. There apparently were no reboots. The hash on the signals is an apparent sign that the oplev laser is dying however. Jason reports that the PIT and YAW signals are normalized by the sum. There is a very small shift on the SUM signal at the jump on THUR but I'm not sure how it adds up to the PIT-only 70uR shift.
Due to this mystery and the PCAL misalignment (alog 40968), we launched Keita to check the ALS pointing at the table at EX. He found things somewhat well aligned using ~50uRad of bias on the ETMX/TMSX. So, at least the ALS beam and the hashy, twitchy OPLEV beam pointing monuments agree that the ETMX is still pointed to within ~50-100uR of the last good arm pointing (recall that the vent to air contributes to some pitch mis-pointing as well, see alog 40714). The PCAL reports a much larger magnitude of ~1mR of alignment error.
In the morning, Travis and Rick intend to remount the PCAL target on the front of the ETMX and see where the incoming PCAL beam is - if it's off, the whole periscope has been misaligned. If it's hitting the ETM at the right place, Travis will look closer at the ETMX itself. (Note, we have not yet touched the ETMX at all since the vent from last week - only the PCAL baffle crew took parts past it and into the manifold to work. The ISI has been locked however, but the PCAL was checked for good alignment before the start of the baffle work and after the ISI lock. alog 40892)
Also, Jason plans to swap the laser for a healthy one first thing tomorrow.
Hmmm... looking closer at the attached plot I see that at the end of the plot when we asked Corey to return the ETMX to zero mid-morning, he only had to use the ETMX PIT slider but I see a step in both the PIT and YAW trends of the OPLEV...
TBC... Anyone volunteer to sort this mystery?
Today, Hugh pointed out that the EX vea temp has been on the rise for the last few weeks due to all of the cleanrooms running. This isn't a huge surprise but it got us wondering if the steps in pointing of the ETMX as viewed by the OPLEV PIT are actually the suspension sagging and brushing something mechanical. (The steps account for ~100uR of unaccounted mispointing.) Indeed the top BOSEM sensors see the steps. However, a couple hours of staring at trends gets us no closer to this theory as the actually problem, for the following reasons:
1) Bringing the BSC9 volume up to air lifted the suspension, while the temperature increase is dropping the suspension, restoring it towards its original vertical position.
2) The temperature is hotter now than at the time of to step events (purple lines on the plot), yet transfer functions of ETMX V and P look healthy to me.
So, the agreed apon sentence from late yesterday still holds:
We will rezero the OPLEV now, with the biases from the arm lock - this should be good enough to find the arm again after pump down to within ~100uRad.
Recall, ALS pointing on the table yesterday with these slider settings looked pretty good.
Bias sliders:
PIT 7.3
YAW 40.2
