Kiwamu, Hang
We did an estimation of the beam position on test masses based on the a2l gains that gave us best decoupling.
The result was shown in the first image attached. The blue line indicated the boundary of L3 and the red spot the position of the beam on the test mass. It seemed that the beams were <~1 cm off center.
| Trans. [mm] | Vert. [mm] | |
| ITMX | 3.0 | -7.3 |
| ITMY | -6.3 | -3.5 |
| ETMX | 5.3 | -4.7 |
| ETMY | -2.7 | 4.1 |
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In case that you are interested in how we obtain the results:
The basic idea is to excite the pitch (yaw) motion of L2 stage, and let this excitation go through both
1): the L2->L3 P2P (Y2Y) path, and
2): L2->L2 P2L, and then L2->L3 L2L and L2P.
The ratio of L3's L over L3's P will then give us the vertical position of the beam on test masses. See the second picture for a graphic representation.
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The code to re-run this analysis is available at:
/opt/rtcds/userapps/release/isc/h1/scripts/a2l
You can do the analysis by entering
./run_GrabBeamSpot.sh
in the command line
But isn't there a static component of L2P -> L3L that we have to worry about? If there is something like that it seems like it would be static, but it might shift the absolute beam position by some amount.