T Vo, Sheila
We measured a transfer function from PCAL X to DARM this afternoon. The attached screenshot shows a comparison of driving the ETMX ESD in the low noise state (with the settings that allowed us to transition to it from ETMY) and a drive to PCALX_DARM with the filters that Thomas copied from the ESD filter banks. The red trace (ref 14+15) were taken with a gain of -150 in the PCAL filter bank.
Based on this measurement, Thomas and I estimated this morning that we would need a gain of about -3000 in the PCAL X DARM bank to transition to PCAL X from the ESD, but that this would result in about 50 V rms on the OFS PD. (There is about -82 dB of gain between the OFS PD (in Volts) and the output of the PCALX DARM bank. This means that we would not have enough range on PCAL to simply replace the ESD with PCAL, so we have not tried the transition or locking ALS on ETMY.
If we aren't able to disconnect the ESD cables while in lock, we could look at using one of the ITM ESDs to transition from high voltage to low voltage on one ESD with the other disconnected. We also talked with Daniel, RIchard and Fil who had some ideas about different ways to try disconnecting the cables while in lock. We were about to try these when the EQ hit us.