Since we have relocked with a cold OM2, our range has dropped to 123 Mpc. Some of this range loss can be explained by the increase of the 120 Hz jitter peak, but there is also a loss of low frequency range (see first plot). This loss of sensitivity is likely to due an increase in MICH and PRCL coherence (see second plot). Gabriele and I performed an iterative retuning of MICH feedforward during last week's commissioning time. We used MICH feedforward injection data from when OM2 was hot. Now that OM2 is cold, it appears this new iterative feedforward is no longer optimal. Luckily, the previous MICH feedforward was tuned on June 22 after powering down to 60W and with a cold OM2. If we revert to this MICH FF, this should take care of some (perhaps all) of the low frequency sensitivity loss. We can investigate later if further iterative tuning is needed.
What to change: MICHFF filter FM3 to FM5. This should be changed in the LOWNOISE_LENGTH_CONTROL guardian state (ISC_LOCK.py line 5431, change "FM3" to "FM5"). It can also be changed in lock: ramp the MICH FF gain to zero, switch FM3 to FM5, and ramp the gain back to 1. This will also need to be SDFed in Observe. This is a very quick fix; I recommend that if we go out of observe or lose lock it should be quickly changed.
There is also increased PRCL coherence, but that is likely to change once the MICH FF is fixed. It doesn't appear that the SRCL FF is affected by the OM2 change.
Edit: I updated the ISC_LOCK code. It will need to be reloaded whenever we next lose lock. Tagging opsinfo. Also expect an SDF diff for the LSC model.
The filter has been changed back to FM5. I think this bought us a few more Mpc, previous lock was around 125ish and we're at 130 Mpc now. The rest of the limit to the range appears to be from jitter peaks such as the 120 Hz and the 52 Hz and other 13 Hz harmonics.
There is still some MICH coherence present (see the attachment) which indicates to me we could get a bit more range around 40 Hz if we did another iterative tuning. I don't know what our plan for the OM2 TSAMS is- we should hold off on the iterative tuning until we are happy with the settings there. Once the OM2 and OMC alignment fiddling is complete, dedicating maybe 20 minutes of future commissioning time to this would be beneficial.
This aLOG implicitly marks the start of a brief, 6-day period when we operated with the OM2 TSAMS system "cold" again, after originally turning it on Jun 27 2023 12:04:02 UTC (05:04:02 PDT -- see LHO:70849) The last observation ready segment that the OM2 TSAMS heater was ON and "HOT" ended on 2023-07-12 14:48:47 UTC (07:38 PDT). The first observation ready segment with the OM2 TSAMS heater OFF and "COLD" of this 6-day period started 2023-07-13 01:04:41 UTC (2023-07-12 18:04:41 PDT) (It gets turned on again, "permanently," on 2023-07-19 with the first observation ready segment with the TSAMS back ON starting 2023-07-19 00:00:37 UTC -- see LHO:71484) ON and "HOT" = means = - 4.6 V requested of the heater (H1:AWC-OM2_TSAMS_POWER_SET) - its temperature steady at 33.02 and 56.60 [deg C] on Thermistor 1 and 2, respectively (H1:AWC-OM2_TSAMS_THERMISTOR_[1/2]_TEMPERATURE).