Yesterday I did a cavity scan with IR while holding the Yarm with green.
ALS servo tuning:
The gain of the loop is set such that BEATY_FINE_Q_ERR x GAIN = 120k. This is a kind of "empirical low" in order to have the UGF around 1kHz.
Start with FM5 [1000:1] enabled, determine the sign of the gain increasing it in small steps and making sure that the mirror doesn't get a kick. Then gradually raise it while looking at the BEATY_PHASE_OUT power spectrum.
Enable FM7 [RG16.5], FM6 [RG3.2], FM3 [1:5], FM2[0:1], FM10 [40:7].
Plot 1 shows the power spectrum of BEATY_PHASE_OUT (calibrated in Hz).
Offset setting and cavity scan
The C1ALS_OFFSETTER2 was used to set an offset for ALS scan.
Many scans have been done to find the optimal offset conditions, I only attached one (Plot 2).
I also misaligned the END mirror in pitch to enhance the HOMs peaks, but it turned out that it was not enough, because I didn't see a very big difference between the "aligned" and the "slightly misaligned" measurements (Plot 3).
Increase the cavity misalignment both in pitch and in yaw and repeat the measurement.