[Elenna, Gabriele]
I did another test of the PRCL offset today, mirroring previous tests (76814). The motivation for this test is first the large coherence of DARM with LSC REFL RIN, indicating that we are locked with some offset in PRCL. Next, there is large coherence of PRCL and CHARD Y to DARM, and previous tests have shown some PRCL is coupling through CHARD Y, and that coupling can be reduced by adding a digital offset to PRCL, which counteracts whatever locking offset is present in PRCL.
Once again, a PRCL offset is shown to reduce PRCL/REFL RIN coupling, and PRCL to CHARD Y coupling. The minimum occurs with an offset of about -58 (compare to April's best offset of -62). This coupling indicates an offset somewhere between 25 pm and 50 pm (quoting Gabriele 76810). The digital offset reduces REFL RIN/PRCL coupling by 40 dB. PRCL and LSC REFL RIN coupling plot
The offset reduces the noise in CHARD Y by reducing the PRCL coupling by a factor of 10. PRCL and CHARD Y coupling plot, and noise reduction in CHARD Y, CHARD Y noise
The PRCL to CHARD P coupling does not change, PRCL and CHARD P coupling plot (I was lazy and did not properly label this plot, but each trace is a different offset count similar to the other plots). However, the offset worsens the noise in CHARD P, because the offset effectively changes the amount of HAM1 noise coupling into CHARD P, CHARD P noise. We only checked CHARD P, but it likely changes the noise in INP1 P and PRC2 P as well. With this PRCL offset, I ran a HAM1 FF off time, so we could get data to retune the feedforward. (start: 1409940778, stop: 1409941096).
I think that with this PRCL offset, and with an updated feedforward, we will see some improvement in the low frequency noise. The last time we did this test, we did not check the effect on the feedforward, and this is possibly why the sensitivity did not get better, even though we are seeing the exact same reduction in CHARD Y noise as before.
Overall, we can convince ourselves that this result makes sense because the offset is changing the amount of 00 mode present at the REFL WFS.
We have made no changes yet, but I will fit new feedforward so that we can retest this offset + new HAM1 FF at the next commissioning time.
Overall, the PRCL coupling to DARM did not change much with this offset, which contradicts the idea that most of the PRCL coupling to DARM is occuring through CHARD Y.
I have fit new HAM1 FF filters that can be used when the PRCL offset is engaged. I fit new filters for INP1 P, PRC2 P, and CHARD P. There is very little to subtract for the DC1 and DC2 loops, so I recommend those FFs are turned off when the PRCL offset is engaged. All the new filters are saved in FM10 of the filter banks, and are labeled "PRCL_offset".