Terra, Evan, Stefan
With the spot move script from last alog we could reliably maximize the recycling gain. We reached 30 (as measured by LSC-PR_GAIN - Evan claims this is more like 35).
The instability in AS90 could be removed by manually aligning the SRM - but POP18 got worse when doing so.
Interestingly - the same move that fixes the recycling gain at 50W, has almost no impact on the recycling gain at 2W.
For reference, the last lock loss was at about 6:15 UTC on Aug 1st.
We had to adjust the soft loop gains to
ezca['ASC-DSOFT_P_GAIN'] = 0.5 # 3 killed it
ezca['ASC-DSOFT_Y_GAIN'] = 3 # oscillates at lower UGF when much lower
ezca['ASC-CSOFT_P_GAIN'] = 0.5 # designed differently
ezca['ASC-CSOFT_Y_GAIN'] = 3 # oscillates at lower UGF when much lower
Estimating the spot mosition move required to regain the recycling gain.
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Conclusion: The corner spot moved by about 8mm in pitch and yaw.
The move direction (to improve the recycling gain) in each arm is a soft translation of the beam axis towards the other arm (yaw), as well as up (pit).
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We used two methods to estimate the spot move on the ITM/BS/PR3.
A) using IM4 angle readback:
Since PR2 doesn't move, looking at how much IM4 moved allows us top simply project the move on PR3. The effecive lever arm (incl. the PRM and PR2 lens effects) is 151.4m.
To regain the best recycling gain we moved IM4 26.2urad in pit and 25.5urad in yaw. This corresponds to (note the factor of 2 for beam steering vs optic angle)
IM4 PIT projection on PR3: 7.9337mm
IM4 YAW projection on PR3: 7.7217mm
B) using the test mass oplevs:
The move recorded by the test mass oplevs (in urad) are (EX,EY,IX,IY) are:
PIT:
1.8700
1.6500
2.5000
2.0600
YAW:
2.7000
2.8200
3.3200
3.2600
Since it is always hard to get a clean soft that is not masked by residual hard, we project out any spurios hard move.
i) Without removing the hard:
ITM:-1.9858mm, ETM:12.091mm
ITM:0.91049mm, ETM:7.2637mm
ITM:2.4191mm, ETM:10.6978mm
ITM:6.315mm, ETM:6.329mm
ii) removing the hard by forcing the ETM and ITM spot move to be the same:
i.e. we force the spot move to be the same on ETM and ITM:
ITM:4.5198mm, ETM:4.5198mm
ITM:3.8466mm, ETM:3.8466mm
ITM:6.2452mm, ETM:6.2452mm
ITM:6.3215mm, ETM:6.3215mm
iii) forcing the spot to be zero on the ETM (that's how we think we tuned the SOFT loop sensors):
ITM:8.4743mm, ETM:0mm
ITM:7.1944mm, ETM:0mm
ITM:11.674mm, ETM:0mm
ITM:11.7903mm, ETM:0mm
The reality probably lies between ii) and iii)
FInally, note that the control signal recorded in alog 28764 can't be used because of all the radiation pressure torque effects.
Attached is a snapshot of the arm oplev motion.