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We computed the required actuation range for the telescope design in elog:15357. The result is summarized in the table below. Here we assume we misalign an IFO mirror by 1 urad, and then compute how many urad do we need to move the (AS1, AS4) or (LO1, LO2) mirrors to simultaneously correct for the two gouy phases.
Actuation requirement in urad per urad misalignment
| [urad/urad] |
ITMX |
ITMY |
ETMX |
ETMY |
BS |
PRM |
PR2 |
PR3 |
SR3 |
SRM |
| AS1 |
1.9 |
2.1 |
-5.0 |
-5.5 |
0.5 |
0.5 |
-0.3 |
0.2 |
0.1 |
0.6 |
| AS4 |
2.9 |
2.0 |
-8.8 |
-5.5 |
-5.9 |
-0.7 |
1.3 |
-0.7 |
-0.5 |
0.7 |
| LO1 |
-4.0 |
-3.9 |
11.0 |
10.4 |
1.9 |
-0.4 |
-0.2 |
0.1 |
0.0 |
-1.1 |
| LO2 |
-5.0 |
-3.7 |
15.1 |
10.4 |
8.7 |
0.8 |
1.9 |
1.1 |
0.7 |
-1.3 |
The most demanding ifo mirrors are the ETMs and the BS, for every 1 urad misalignment the telescope needs to move 10-15 urad to correct for that. However, it is unlikely for those mirrors to move more 100 nrad for a locked ifo with ASC engaged. Thus a few urad actuation should be sufficient. For the recycling mirrors, every 1 urad misalignment also requires ~ 1 urad actuation.
As a result, if we could afford 10 urad actuation range for each telescope suspension, then the gouy phase separations we have should be fine.
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Edits:
We looked at the oplev spectra from gps 1274418500 for 512 sec. This should be a period when the ifo was locked in the PRFPMI state according to elog:15348. We just focused on the yaw data for now. Please see the attached plots. The solid traces are for the ASD, and the dotted ones are the cumulative rms. The total rms for each mirror is also shown in the legend.
I am now confused... The ITMs looked somewhat reasonable in that at least the < 1 Hz motion was suppressed. The total rms is ~ 0.1 urad, which was what I would expect naively (~ x100 times worse than aLIGO).
There seems to be no low-freq suppression on the ETMs though... Is there no arm ASC at the moment???
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