ETMX Y Initial: 4.9 Final: 4.85 Diff: -0.05

ETMY P Initial: 4.48 Final: 4.48 Diff: 0.0

ETMY Y Initial: 1.13 Final: 1.13 Diff: 0.0

ITMX P Initial: -1.0 Final: -0.98 Diff: 0.02

ITMX Y Initial: 2.87 Final: 2.88 Diff: 0.01

ITMY P Initial: -0.39 Final: -0.36 Diff: 0.03

ITMY Y Initial: -2.45 Final: -2.47 Diff: -0.02

 

Then, I ran broadband injections of DHARD Y and CHARD Y and adjusted the ITMX and ITMY A2L gains by hand. First, I made differential adjustments in the ITM gains while injecting in DHARD Y. I found that the gains needed to be closer together to reduce the coupling.

Best DHARD Y coupling ITMX Y: 2.5, ITMY Y: -2.1

I knew this was the best minimum because the sign of the transfer function flipped.

Noticably, the best effect occured above 20 Hz, while the coupling below 20 Hz only seemed to get worse with the gain adjustments.

The new A2L values for the ITMs have been saved and SDFed.

 

Next, I injected in CHARD Y while making common changes. I found that the gains needed to be more positive to reduce the coupling. The coupling got better or worse everywhere when I made changes, unlike DHARD Y.

Best CHARD Y coupling ITMX Y: 2.9, ITMY Y: -1.7

I again knew this was the best minimum because the sign of the transfer function flipped.

 

I then reran the DHARD Y injection and noticed that the coupling was still about the same. In both cases, I reduced the coupling by an order of magnitude.

Notice that the ITMX gain value chosen by the script is about the same as my answer, but the ITMY gain is very different.

 

Then, I moved to adjusting the AS WFS offset while injecting in DHARD Y. In the past, we have found that the WFS offset reduces the low frequency DHARD Y coupling. Currently, the offset is set to -0.15. Changing the offset to zero increases the coupling by about 3 dB. I then tried making the offset more negative, and found that the minimum occurs when the offset is -0.3 (again, where the sign of the TF flips). This reduces the lwo frequency DHARD Y coupling by an order of magnitude and has little effect on the high frequency coupling, although it does make it marginally better.

 

The A2L gain changes only improve DHARD Y coupling about 20 Hz, and seem to only make it worse below 20 Hz. The AS WFS offset improves the coupling below 30 Hz, and has little to no effect on the coupling above 30 Hz. When changing the A2L gains, the minimum is achieved when the transfer function flips sign around 20-30 Hz. When the minimum coupling is achieved in the AS WFS offset adjustment, the transfer function flips sign between 10-20 Hz. Meanwhile, the minimum CHARD Y coupling occurs when the sign of the transfer function flips around 20 Hz.

 

In the plots below, I show the transfer functions from the measurements. In the A2L measurements, the WFS offset was set to the nominal value of -0.15. In the WFS offset measurements, the A2L gains were set to the new, better values of 2.9 for ITMX and -1.7 for ITMY.

 

Overall, these changes have improved the low frequency region of DARM. We are keeping the new A2L gains, but have not changed the AS WFS offset to the "better" value. We need to consider the effect of the offset on the SQZ ASC before changing it.

Since these results are giving different values than the A2L script, tomorrow we should run the same injection tests to adjust the pitch A2L gains for the ITMs. We can also check the ETM values.