It was not advertised well but the HEPI isolation loop actually caused the alignment jump of [+25, -8] urad according to IPS in HEPI when it was turned on Friday evening (attached). We've never had a good alignment after that but this might be a coincidence. This is not a huge deal but annoying nevertheless.
When I started to restore the known good alignment of HEPI by giving new setpoints to HEPI it tripped the watchdog repeatedly because IPS completely railed, and later I was told that that was the reason why SEI gave up on restoring the original alignment.
Anyway, I was sufficiently annoyed and relieved the HEPI offset by pushing TMSX as far as we can in a hope that this is the last time I'm annoyed by the difference between offset-only and isolated (because we'll always isolate from here on).
- Get back to offset-only for HEPI. This didn't exactly give me the old no-isolation alignment because the output filters (FM1 in H1:HPI-ETMX_OUTF) were changed, but was close enough.
- Turn ISI isolation on.
- Adjust TMS bias to hit one of PDs on ITM baffle and confirm that the offset is not that much different from before.
- Put more TMS PIT bias and reduce the HEPI offset to get the beam back on the baffle PD. I reduced the offset such that, in the end, TMSX has 20urad headroom when the beam is on PD4. This means that even if there's a PIT drift of +20urad we can still perform full initial alignment.
- Read the IPS location monitors for PIT and YAW (H1:HPI-ETMX_IPS_RY_LOCATIONMON and RZ, these are calibrated by nrad). These were (-610000, 107900) before I made adjustment, and (-369800, 111650) after.
- Disable the offset, turn on the HEPI isolation.
- Put the new setpoints (H1:HPI-ETMX_IPS_RY_SETPOINT_NOW, RZ)=(-369800, 111650).
- Fine adjust the HEPI setpoints until we fully recover the beam on PD.
In a retrospect I didn't have to go back to offset-only, I could have just changed the setpoints, but that's what I did due to various reasons. Anyway, In the end, I obtained this: (H1:HPI-ETMX_IPS_RY_SETPOINT_NOW, RZ)=(-389800, 111650).
I also copied these numbers and put them in H1:HPI-ETMX_IPS_RY_TARGET and RZ.
IPS are still yellow (about 20000 cts on V1 and V2) but not railing.
Just so that it's clear:
- HEPI EX is isolated.
- If you know you caused alignment change that is not negligible, note it in the alog with a supporting plot or numbers.
- If you know that what you did is totally negligible, note it in the alog with a supporting plot or numbers or observations (e.g. "ETMX oplev didn't change after our work so YAW is good, H1:SUS-ETMX_M0_DAMP_P_INMON didn't change so PIT is also good").