Alexa Dan Fil Keita Koji
This afternoon we brought the second of Koji's three OMC children under control.
After working in the morning to understand the quadrant layout of the OMC QPDs, we were able to close simple diagonalized versions of the OMC QPD alignment loops to stabilize the beam in the chamber. With Fil's help we enabled the high voltage to the PZT driver (in the process finding a cable that was plugged into the wrong slot on the AI chassis). Before fiddling with the high voltage we checked that the HV readbacks were working as advertised. We were able to drive the high-voltage PZT in a triangle wave across its entire range (0 -> 100V) and see flashes in the OMC. We tried to improve the alignment by watching the flashes and searching for a clear 00 mode, but this turned out to be fruitless, probably because the beam is moving too much.
We proceeded to align the rest of the beam paths on the HAM6 table: OMC REFL QPDs, AS WFS, and the OMC REFL beam through the viewport. For a while we were confused by a mirror on a too-short pedestal; Keita showed up at a lucky moment and fixed it. The beam has been aligned onto all of the QPDs in the vacuum.
We should be able to lock the OMC tomorrow and study the alignment (and, possibly, take a first stab at the mode matching). At the moment, the beam is moving a lot in pitch; this motion is due to something upstream of HAM6. We suspect SR3 and the ISI table in HAM5, but it could be something else. This motion really needs to be fixed before any robust OMC locking can happen, either by stabilizing the source or constructing a more robust OMC QPD servo. While we flashed the OMC Koji was able to visualize the modes inside the cavity, and the 00 mode was there.
Before leaving the chamber we relieved the yaw and pitch bias on the OM tip-tilts and centered the OSEMs, and then clamped the tip-tilts to the table. The TTs are in good shape. One of the OM3 OSEM cables can potentially slip and block the beam to the AS WFS, we should find a way to clamp this cable in a safe place.
Tomorrow we need to install the kapton washers on the pico drivers. We still need to align the OMC TRANS path, but we probably have to lock the OMC on a fairly strong 00 mode in order to see the beam. In principle OMC TRANS can be aligned after the north door is installed.
Now that we have a beam we can do RF balancing of the AS WFS, if that's necessary.
We'd like to try to put the north door on HAM6 by the end of the day tomorrow. Two other things need to happen before we can attach the door: cable dressing and table balancing by the SEI crew, and a check for ground loops. (These steps may need some iteration if the ground loop situation is bad.)
For now the purge air has been turned off to give the mirrors a rest.
SR3 appears to be moving around a lot more than the other large triples (judging from the BLRMS channels that are used by the watchdogs). The gains for the roll, pitch, and yaw damping filters on the M1 stage are lower than longitudinal, transverse, and vertical gains. It doesn't look like these gains can be increased indefinitely, but I increased pitch by a factor of ten (-0.002 to -0.02) and things got a little better. These loops need some love.
Here are the current gain settings (only pitch has been changed):
L: -3.0
T: -5.0
V: -1.0
R: -0.02
P: -0.02
Y: -0.02
Note: Although T1300611 suggested that the HV power supply was to be 200V and the nominal output voltage was 100V,
we uses 100V for the power supply and thus the nominal output is 50V. This is due to decision after the failure of
one of the PZTs in L1 OMC.
From the point of view of the beam the OMC QPDs segments are arranged as follows:
3 2
4 1