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When I turned them on, the control signal in Pitch from WFS2 started going up with no stop. It was like the integrator in the loop was fed with a DC bias. The effect of that was to misalign the MC cavity from the good state in which it was with the only length control on (that is, transmission ~2.7, reflection ~ 0.4).
I don't know why that is happening. To exclude that it was due to a computer problem I first burtrestored C1IOO to July the 18th, but since that did not help, I even restarted it. Also that didn't solve the problem.
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At least one problem is the mis-centering of the resonant spot on MC2, which can be viewed with the video monitors. It's very far from the center of the optic, which causes length-to-angle coupling that makes the mulitple servos which actuate on MC2 (MCL, WFS, local damping) fight each other and go unstable.
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I played with the MC alignment for the beam centering. After that, I restored the alignment values.
In principle, one can select the MC2 spot as one likes, while the transmitted beam axis to the IFO is not changed
as far as you are at the best alignment. This principle is almost trivial because the beam axis matches
to the input beam axis at the best alignment.
The alignment solution is not unique for a triangle cavity if we don't fix the end spot position.
In practice, this cruising of the MC2 spot is accomplished by the following procedure:
0) Assume that you are initially at the best alignment (=max transmission).
1) Slightly tilt the MC2.
2) Adjust MC1/MC3 so that the best transmission is restored.
I started from the following initial state of the alignment sliders:
BEFORE TRIAL
| MC1 Pitch |
+3.6242 |
| MC1 Yaw |
-0.8640 |
| MC2 Pitch |
3.6565 |
| MC2 Yaw |
-1.1216 |
| MC3 Pitch |
-0.6188 |
| MC3 Yaw |
-3.1910 |
| MC Trans |
2.70 |
After many iterations, the spot was centered in some extent. (See the picture)
RESULT
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adj. |
| MC1 Pitch |
+3.363 |
(-0.26) |
| MC1 Yaw |
-1.164 |
(-0.3) |
| MC2 Pitch |
3.7565 |
(+0.1) |
| MC2 Yaw |
-1.2800 |
(~ -0.16) |
| MC3 Pitch |
-0.841 |
(~ -0.22) |
| MC3 Yaw |
-3.482 |
(~ -0.29) |
| MC Trans |
2.75 |
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The instability looked cured somewhat.
Further adjustment caused a high freq (10Hz at the camera) instability and the IMCR shift issue.
So I returned to the last stable setting.
Side effect:
Of course, if you move MC1, the reflected spot got shifted.
The spot has been apparently off-centered from the IMCR camera. (up and right)
At this stage, I could not determine what is the good state.
So, I restored the alignment of the MC as it was.
But now Alberto can see which mirror do we have to move in which direction and how much. |