I measured the spot position on PRM by using the usual angular dither technique (see for example LLO alog 5010) with PRY locked. It turned out to be pretty good.
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measured vertical off-centering = too low by 2.0 mm
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measured horizontal off-centering = off toward PR3 by 1.0 mm
(some settings):
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ITMX and SRM are intentionally misaligned.
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PRY is locked on the carrier light by REFL_A_RF45_I with a demod phase of 6.9 deg
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No whitening gain, but whitening stage 1 and 2 are engaged to obtain better signal-to-noise ratio.
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Feedback on PRM and PR2 as usual
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servo gain in LSC = -50 which should give a UGF of 30-60 Hz.
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Excitation in SUS-PRM_LOCK_P(Y) with an amplitude of 1000 counts at 10.1 Hz
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I used dtt to take a transfer function of (REFL_A_RF45_I) / (PRM_LOCK_P(Y))
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Before the measurement, PRM was tweaked to maximize the intracavity power
(results)
See the plot below.
(some notes)
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At the beginning of the measurement, the beam on REFL_A was at the edge of REFL_A. This resulted in bogus data because it introduced some kind of wave-front-sensing effect as the PRM angle was modulated.
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This was fixed by moving the tip-tilt mirrors in HAM1, namely RM1 and RM2.
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I used a conversion factor of 42.2 mm for converting the imbalance alpha (see 40m elog 2863 for the definition) into the position of the rotational nodes.
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According to D1101377-v2, I used the following numbers to derive the conversion factor:
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radius of the mirror: R = 75 mm
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mirror thickness: L = 75 mm
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magnet separation D = 95.45 mm