evan.hall@LIGO.ORG - posted 21:06, Tuesday 25 April 2023 - last comment - 14:20, Wednesday 14 February 2024(69026)
Driving PR3 pitch and yaw at low frequency
Sina, Elenna, Kevin, Evan
Similar to Anamaria's measurements at Livingston last year (LLO:58623), we drove PR3 sinusoidally in pitch and yaw at low frequency to try to ascertain the amount of beam spot motion coupling into the arms. The following times are all 2023–04–12 UTC:
- 17:10:00 to 17:16:00 — quiet time
- 17:19:45 to 17:29:45 — PR3 pitch, 0.34 Hz, ~0.02 µrad
- 17:32:00 to 17:38:00 — PR3 pitch, 0.34 Hz, ~0.04 µrad
- 17:39:46 to 17:45:00 — PR3 yaw, 0.34 Hz, ~0.02 µrad
- 17:49:00 to 17:55:00 — PR3 yaw, 0.11 Hz, ~0.02 µrad
- 17:55:30 to 18:01:30 — PR3 pitch, 0.11 Hz, ~0.04 µrad
At 0.34 Hz, the transfer functions from oplev to oplev are
The absolute values are indicated because the measured phases are neither clearly around 0° nor 180°. The coherences are above 0.85. (At 0.11 Hz, the coherences are somewhat lower so I omitted those values.)
Livingston also saw order-unity transfer functions like this: LLO:59846.
A series of finesse simulations (G2400278, G2400281) indicated that by itself, PR3 cannot drive arm cavity motion at this level since (in the absence of any angular feedback control) the arm cavity finesse suppresses PR3-induced spot motion by a factor of order 100. On the other hand, PR3 motion can appear in the reflection wavefront sensors with an optical strength similar to or even greater than cHard motion, meaning that if PR3 has not been carefully diagonalized out of the sensing matrix then its motion can easily be impressed onto the test masses with an order unity ratio.
The attachment shows coherences between PR3 oplev and test mass oplevs from the end of O4a. Ignoring the microseism region, there is coherence between PR3 pitch and EY pitch from 0.4 to 0.8 Hz. One might guess this coherence arises because of LVEA ground motion, but there is no coherence between the PR3 oplev and either the HAM1 Z L4C channel or the HAM2 Z ground motion.