I was running the TFs for SRM to confirm that the suspension is good before we close HAM5, but the transfer function for L was a noticable amount lower in magnitude as compared to the measurements taken on July 17th right before we vented HAM5(pdf). We reran L but this time with the OPTICALIGN offsets turned ON, and the results matched our pre-vent measurements, which seemed strange and prompted looking into whether the RT OSEM has a problem. I've looked back at the state of SRM when the July 17th transfer functions were run, and T, V, R, P, and Y were run in DAMPED and OPTICALIGN offsets were OFF(T&R, V&Y, P). HOWEVER, when the L transfer function was run, SRM was ALIGNED and OPTICALIGN offsets were ON (P: 4254, Y: -9231)(L pre-vent). So that's why today's regular (offset OFF) L didn't match the pre-vent L, and why my second measurement of L with the offset on did match.
Yaw is the only other dof that was affected by the offsets being ON vs OFF, but the pre-vent Y measurements were taken with the offsets OFF, and these match with today's offset OFF measurements so we're all good there.
I'm not sure if there are still concerns about the RT OSEM on SRM M1, now that we know why L matches the pre-vent L, so I'm not sure if these results clear us for closing HAM5 or not.
Main set of measurement results (2024-08-01_2000) - SRM DAMPED, OPTICALIGN offset OFF:
$(sussvn)/HSTS/H1/SRM/SAGM1/Results/2024-08-01_2000_H1SUSSRM_M1_ALL_TFs.pdf
Second set of measurement results (2024-08-01_2200) - SRM DAMPED, OPTICALIGN offset ON:
$(sussvn)/HSTS/H1/SRM/SAGM1/Results/2024-08-01_2200_H1SUSSRM_M1_ALL_TFs.pdf
Full comparison of Pre-Vent, Post-Vent offset OFF (main) set, and Post-Vent offset ON set
$(sussvn)/HSTS/Common/Data/allhstss_2024_July17vAug01_H1SUSSRM_M1_PreVsPostOFIVent_ALL_TFs.pdf
I agree with Oli that SRM will be "as good as it has been between 2021 and 2024" if we leave it as is, but for different, more expanded reasons covered below.
To expand on Oli's "RT OSEM has a problem" -- the issue is that with the alignment offsets OFF, the RT (right) OSEM sensor / flag has expanded enough that the flag is only barely occulting the LED / PD beam, close to "open light." Thus the drop in magnitude response for Length and Yaw, the only two DOFs that use the RT OSEM. In fact, the M2 UL and M3 UL OSEMs, i.e. the lower stage left OSEMs are further *engaged*, close to "closed light." This is consistent with an overall positive yaw (+RZ) of the suspension w.r.t. the cage.
The *current* SRM alignment offsets are
(P: 1663, Y: -3095 [urad]) 2024-08-01, post OFI recovery and aux laser alignment, in air, HEPI locked
This digital request of *negative* ~3 millirads of yaw *centers* the OSEMs, implying that the OSEMs have been centered to the *alignment offsets ON* position. With the OSEMs centered, the magnitude of the response increases and looks cleaner because both LF and RT are contributing to measurement of the L and Y DOFs.
This checks out with the last time that we moved the OSEMs w.r.t. to the flags -- in 2021 -- see LHO:60576 where our goal was to keep the physical alignment of the optic the same, even though moving the OSEMs changes the digital request to get there. In that case, we kept the physical alignment and centered the top mass OSEMs to the alignment offsets ON, preserved physical alignment.
Importantly -- the result of the 2021 adventure of reproducing in-vac alignment while the IFO was in air was to leave the alignment offsets at
(P: 1738.28, Y: -3296 [urad]) 2021-11-08 (LHO:60578)
which is within 200 [urad] of the current slider values, and where the OSEMs are reasonably centered and responding well now.
These are also consistent with pre-vent in 2024 with OFI KTP damage in place, where with HAM5/6 HEPIs locked, a single-bounce IFO beam was used to find a good alignment
(P: 2192, Y: -3166 [urad]) 2024-07-17 (LHO:79246)
The offsets that Ibrahim used only on the L DOF during the July 17th health check, as Oli quotes, are:
(P: 4254, Y: -9231 [urad]) 2024-07-17 (LHO:79202)
which I believe were some very-lost, very-brief, temporary "I didn't realize HEPI was locked" alignment state used just prior to the official single bounce restoration later that day, LHO:79246, so the values of these should be ignored.
Finally, I attach a trend of the SRM alignment offsets over the last 4 months of sordid history with the OFI's KTP damage, covering both in-air and in-vac, HEPI unlocked and unlocked.
This also confirms that SRM's alignment, in all of these scenarios, is never needed to be outside of the
(P: 1600-2400 [urad], Y: 3000-3500 [urad])
range, i.e. less than 1 [mrad] from the 2021 positions (and the I suspect the large low pitch request after venting is the need to account for bouyancy of the optic, so really the alignment should stay within 0.5 pmrad] = 500 [urad] of those 2021 values.)
Note that this requested alignment range consumes a little less than half of the DAC range at ~50000 [ct] for the pitch OSEMs (T2 and T3) and yaw OSEMs (LF and RT).
Thus, if we just leave SRM alone, I suspect we'll be just as happy the SRM alignment and OSEM performance as we were between Nov 2021 until Jul 2024. This was the same decision we made in 2021. We'll just have to somehow remember that, for SRM, in order to get centered OSEMs to provide a OK performance on the top-mass "health check" transfer functions, we need the alignment offsets ON and in within 500 [urad] of (P: 2200, Y: -3300 [urad]). We can test where the boundaries of the alignment slider requests vs. RT OSEM performance decay during pump-down.
That being said, if we really want to mechanically relieve the yaw offset and restore centering of M1 RT (and M2 & M3 UL) we can...
Opened IIET Ticket 31769, to record that we've thought carefully about this decision to "let the 2021 OSEM position and SRM mechanical alignment ride," but that we reserve the right to try to mechanically offload *next* time. Hopefully the ticket and this aLOG series provides enough breadcrumbs to remember that we should relieve the alignment when we have the AUX laser set up already.