georgia.mansell@LIGO.ORG - posted 18:30, Monday 17 May 2021 (58994)
OMC ASC control signals on dither alignment vs QPD alignment
I was wondering why the OMC ASC noise projection measurement attempts were so different when we’re on the dither alignment vs the QPD alignment, so I poked around the OMC control screens and made spectra of the control signals while we’re on each ASC scheme.
The OMC length dither is 4.1kHz, the ASC dithers are all around 2.1-2.2kHz. The first attachment is the output DOF2TT matrix, a quick alog search shows that this was set back in 2015.
The second attachment is the spectra of the control signals while we’re on the dither vs QPD alignment. The times I used are from Friday when Jenne and Varun switched between the two schemes.
1304973938 - time with OMC ASC on dither
1304964517 - time with OMC ASC on QPD
There are a few things that jump out in these spectra:
- The QPD signals are noisier at low frequency. This checks out with what Varun experienced on Sunday - the dither loops were extremely low bandwidth - he had to wait 10+ mins for them to converge, while the QPD loops converged much faster. Is this by design or do we just not have enough gain when we switch to dither?
- The QPD signals are sensitive to the actuator calibration lines (the 4 lines between 15.6 and 17.6 Hz), but the dithers are not. I checked the calibration line monitors and they were on for both these times. Presumably this is also because the dither loop bandwidth is super low
- The sensing noise at high frequency is lower for the QPD loops - except for ANG_X which is for some reason the same?
At the site commissioning meeting today Peter F asked about the OMC suspension state. While the suspension is in its nominal aligned state there is an analog low pass engaged (state 2 on the BIO control).
Images attached to this report
