I measured the AC coupling loop OLTF of the ISS 2nd loop with the same setting as O2 and the phase margin was less than 3 degrees (first attachment).

I had to flip the sign of the loop, as the whitening in ERR1 which is used for this loop was changed from two inverting opamps (p=2.7k and 130kHz in V2) to just one inverting opamp (p=2.7k in V3) in D1600298. Our phase margin might have been this thin during O2 since the loop apparently works and the AC coupling is only necessary during power ramping.

Anyway, this is a bit too extreme, for the moment I'll decrease the gain by ~10dB (H1:PSL-ISS_SECONDLOOP_AC_COUPLING_SERVO_GAIN=-0.08 instead of -0.25, second attachment). If the IMC cannot take the power ramping with this setting we'll redesign the filters.

(Ignore 0.1Hz^2 Sallen-Key low pass in the AC coupling path on the medm screen, that filter is off, it's a logic error in the frontend model which I'll fix.)

(The measurement was done without light since we don't need it and since I didn't know the status of ISS 1st loop at the moment. According to Jason AOM isn't installed yet.)