J. Kissel

Sina has posted some of the first SPI L results in LHO:91794 and subsequent comments in LHO:91861. They conclude, via passive ASD measurement and minimal unit conversion, that "the visual agreement is remarkably good."

Here I post some retrospective results from the driven transfer function set I took on 2026-09-03 (LHO:91798) to get a more quantitative comparison.
Using all channel conversions described in LHO:91809, which are, in summary "just" calibrating the front-end channels into the SI order of magnitude units (i.e. diplacement channels from [nm] to [m], and inertial sensor channels "inertial sensor response asymptoting to 1 [nm/s] at high-frequency" to [m]), I compare the ASDs and transfer functions during the reference time and HAM2 injection times, 
    Reference Time  2026-09-03 17:09:49 UTC 
    HAM2 excitation 2026-09-03 18:34:14 UTC - 18:58:09 UTC

Note -- while I had thought the CRS as blended into the HAM3 sensor array during this time, Jim confirms that it was NOT in play in the HAM3 RY loop (LHO:91866).
Also -- there were questions why "I didn't just use the same excitation that Jim did for the optical lever signals;" the real answer is that I hadn't had the chance to talk to him, couldn't find it easily on my own, and assumed it was some matlab infrastructure that I wouldn't know how to use. I now have talked to him, and he's pointed me to LHO:91754, which points to LHO:91607, which points to the actual path to the file in LHO:91179. But, even if I did find that text file to drive awggui with, it has a low-frequency-focused tilt de-coupling color too it, which is different from what I ended up concocting in DTT. C'est la vie, I think both sets of TFs will be interesting. Certainly this one was.

Attachment 1
Building up an understanding of the front-end-computed HAM3-HAM2 super sensor signal, H1:ISI-DIFF_H23_SS_X_OUT_DQ.

This ASD collection compares the CPS and GS13s of HAM2 and HAM3 against the differential super sensor during the HAM2 excitation (only). Since HAM3 was NOT being driven, we can treat this like the "reference" performance for the HAM2 ISI -- because the HAM2 and HAM3 ISI typically perform similarly in the longitudinal, or ISI and IFO X direction (when the CRS is not engaged into the HAM3 blend). 

- Compare BLUE, DARK GREEN, and HOT PINK traces. We see that the drive on causes displacement on HAM2 is factors of 2x to 50x above the reference level
- Compare THIN DARK PURPLE against CYAN and BRIGHT GREEN traces. We see that the blended input to the super sensor for HAM3 has -- for some reason -- a lot more motion than the "raw" blended CPS and GS13s input. 
- COMPARE BLUE, DARK GREEN, HOT PINK, and BLACK traces. Where the CPS and GS13s are not noise limited (i.e. where're signals are used in the blend to form the super sensor sum), All of the HAM2 and the differential HAM3-HAM2 signal agree, showing the excitation on HAM2.

Conclusion: the front-end computed measure of the differential motion between HAM3 - HAM2 using the onboard CPS and GS13s is functional, comes with a calibration that makes sense, and is measuring the right thing.

Attachment 2
Building up more trust in the HAM3-HAM2 super sensor signal, H1:ISI-DIFF_H23_SS_X_OUT_DQ as a faithful signal for comparison with the SPI L signal H1:SPI-H23_DIFFDISP_MAIN_OUT_DQ.

This ASD collection compares the CPS- and GS13-computed differential X motion (BLACK) and SPI L measured differential X motion (RED) during both the reference quiescient time (DASHED traces) and during the HAM2 excitation (SOLID traces). The excitation is also shown (H1:ISI-HAM2-ISO_X_EXC, calibrated into displacement units [m]).

- During the reference time (DASHED), the SPI L and the on-board sensors only agree above 5 [Hz]. *very interesting* We know the low-frequency-end -- below ~2 [Hz] -- is dominated by the SPI seed laser's frequency noise -- which is dominated by the IMC's displacement. Remember, at the time of measurement, only the IMC is locked (but now with the JAC locked between the PSL and IMC). It looks like -- at least during this measurement -- that's larger or different or incoherent with the HAM3-HAM2 motion. So... maybe this is all suspension noise (or ISI tilt)? Needs more study / noise budgeting.

- During the excitation time (SOLID), the SPI L and the on-board sensors agree across a much broader frequency-band, only disagreeing between 1.5 and 10 [Hz]. *very interesting*. *great* that the excitation i.e. this amount of differential motion *makes* the SPI L and on-board sensors agree for the most part. I have even less of a guess at an explanation for the frequency region where they're not, tho.

Conclusion: Under large differential displacement, the SPI L agrees with the on-board sensors over a very broad range of frequencies, from 0.005 - 2 [Hz] and 10 - 60 [Hz] Lots still to investigate, tho.

Attachment 3
Linear transfer function between the HAM3-HAM2 super sensor signal, H1:ISI-DIFF_H23_SS_X_OUT_DQ and the SPI L signal H1:SPI-H23_DIFFDISP_MAIN_OUT_DQ, as well as the length/frequency control channel for the input mode cleaner (H1:IMC-F_OUT_DQ. 

I show the driven transfer function magnitude for these signals on a log-log plot. Coherence shown separately below.

- Where the SPI noise didn't match the ISI-DIFF noise between ~2 - 10 [Hz], the TF is incoherent (coherence shown separately below), so ignore that.
- At other frequencies, where the ASD "visually agree" the transfer function is NOT exactly 1.0 -- see more discussion of the values of the TF magnitude in the semilogx version of the plot below. *very interesting* 
- I don't understand the magnitude of the IMC-F transfer function at all below 1 [Hz]. The next plot shows it's coherent... *very interesting* or *I'm missing something obvious*

The IMC-F channel has the following calibration into [m]:
    Gain: 1.1683e-06
    Poles: 0
    Zeros: (none)
I don't remember how I calculated this, or where I got it from. To be (re)investigated... but I would have guessed -- since IMC-F_OUT_DQ is already calibrated into [kHz], that it *should* be something like a factor of (2 * L_IMC * lambda / c) = 1.1724e-13 [m/Hz] or 1.1724e-13 [m/kHz] with no poles at 0 [Hz], from df / f0 = dL / L_rt math, but that doesn't seem to be it at all. I probably just need to go back to the code used to produce plots in the SPI final design doc ... just haven't had time.

Also -- side note -- looked into the IMC_X calibration infrastructure that uses the length drive to MC2 to calibrate the control signal into displacement units and that doesn't work.

Attachment 4

Coherence between SPI L and the ISI-DIFF channel and the IMC-F channel for the two TFs shown above.
I also show the coherence between the SPI L channel and the individual ISI CPS and GS13s to understand from where the coherence comes (expecting more from HAM2 since this is during the HAM2 drive.)
I also show the coherence between the ISI-DIFF channel and the IMC just to see how that's different.

Conclusion: Lots to see here, but I haven't really digested it or tried to make sense of it.

Attachment 5
Same transfer function as in Attachment 3, just shown in semi-log x so we can read off what the magnitude of the TF is at coherent frequencies.

- The TF is indeed almost a flat 1.0 [m/m] above 10 Hz; but not quite. *very interesting*
- Between 0.1 and 1 [Hz], where the SPI and ISI-DIFF are *definitely* coherent, the TF magnitude is *not* 1.0, not is it flat. It's got bumps and wiggles between 1.0 and 0.7. Best first guess it that this has something to do with gain peaking in the blend filters. *very interesting*

Conclusion: this TF is going to be very interesting, and we won't be able to "just" blend the SPI "right in" with a simple filter. And, I need to try to get more coherence below 0.1 [Hz].

Very interesting! 

FAMIS81871, last checked in alog91654.

For measurements below, measuring from the "top" of the red float ball.

• TCSx Chiller:  Had level of 28.3 cm.  Added 770mL to reach 30.5cm.
  • This was almost at the level noted in the procedure of < 28.0cm, but it has been almost a month since the last time these were topped off.
• TCSy Chiller:  Had level of 9.3 cm.  Added 400mL to reach 10.7cm.
  • When I first started to add water, the level quickly dropped by ~half a cm, I had checked beforehand to make sure the filter was off level so that there was no seal. After a few hundred mL it shot back up by that half a cm, which led me to overfill by 0.2 cm.  
• There was no water in the small cup under the slow leak, just some spots of leftover glycol.

I monitored the new OLDAMP matrix values on the BBSS (these are not guardianized).

TITLE: 09/21 Day Shift: 1430-2330 UTC (0730-1630 PST), all times posted in UTC
STATE of H1: Corrective Maintenance
OUTGOING OPERATOR: None
CURRENT ENVIRONMENT:
    SEI_ENV state: CALM
    Wind: 7mph Gusts, 4mph 3min avg
    Primary useism: 0.02 μm/s
    Secondary useism: 0.17 μm/s 
QUICK SUMMARY:

Slow controls and Beckhoff are currently Down. -Fil is currently troubleshooting this.
EX has been pumping down all weekend and now down to 3.68e-7 Torr according to H0:VAC-EX_X4_PT525_MOD1_PRESS_TORR. 
Gate valve 20 Is still closed.

The EY Slow controls issue escalated overnight, DEV5 now has no communication with EY.

The link deteriorated over the course of 5 hours, starting at 23:24 Sat 19sep2026 and was completely lost by 04:23 Sun 20sep2026.

Trending the slave_count_actual for DEV5 shows it erratically drops from 127 to 1 over this time period.

I turned off the JAC heater servo at 23:45:00 UTC today to do a longer step response test over the weekend.

TITLE: 09/18 Day Shift: 1430-2330 UTC (0730-1630 PST), all times posted in UTC
STATE of H1: Corrective Maintenance
INCOMING OPERATOR: Corey
SHIFT SUMMARY: Keita worked more on PRMI and ASC, we now have working ASC MICH loops. We also locked DRMI around 23:00 UTC at ~68 and ~12. The pump down continues over the weekend on both ETMX and the X/Y BMs.
LOG:                                                                                                                                                                                                                                                                                

ASC-MICH loop works after ASC-AS_B rephasing (it looked nonsensical before).  I have no idea why they changed. AS_A phase was OK though not great, no adjustment was made.

Before making any adjustment:

I refined PRMI alignment manually, locked PRMI and measured the response of ASC-AS_B_RF45 used for MICH. I also measured AS_A just because.

In the first attachment (AS_B_RF45_VS_AS_A_before_phase_adjust.png), I'm misaligning BS in PIT both ways without enabling MICH nor PRC1 ASC feedback (but did enable DC3 and DC4 to center the beam on AS_A and AS_B) to test how AS_A and AS_B RF45 output respond.

MICH ASC error signal (AS_B_RF45_Q_PIT, yellow) is useless as it has a very small response to BS misalignment and it never crosses zero. This is what Elenna, Louis and I observed yesterday, too. In comparison AS_A_RF45_Q_PIT looks more promising (but should not).

In ASC-AS_B_RF45_phase.png, I'm injecting PRCL (not angle) dither at 80Hz to PRM M3 stage TEST_L and see the line in on each quadrant of each WFS. The idea is:

1. PRCL signal must appear in I phase (dashed lines) not Q (solid) for AS_A and AS_B quadrants (left panel, top is AS_B, bottom is AS_A) as well as the LSC sensor (H1:LSC-REFLAIR_A_RF45, the right panel). 
2. For each WFS, all quadrant signals in I phase MUST have the equal phase relative to the PRCL error signal at the dither frequency (middle panel).

As you can see, AS_B doesn't make sense. For all AS_B quadrants, PRCL line shows stronger in Q than in I, and the phase of the third quadrant is flipped.

ASC-MICH works after rephasing.

I made the following changes to roughly minimize PRCL line in Q AND to make the phase relation of all I signals the same relative to PRCL error:

The phase numbers themselves look more similar to each other now, PRCL line in Q phase is much smaller in all quadrants (ASC-AS_B_RF45_phase_after_adjustment.png), and most importantly, ASC-AS_B_RF45_Q_ERR responds to the MICH misalignment sensibly with good amplitude and it crosses zero (AS_B_RF45_VS_AS_A_after_B_phase_adjust.png). 

Dark offset adjustment might be needed, though.

Anyway, after this adjustment ASC MICH loop worked fine with ASC-MICH_P_GAIN and Y_GAIN of 0.5. See ASC-MICH_works.png.

Demod phase values were accepted in sdf.

Ryan C, Dave:

The Beckhoff CS-AUX-DEV5 slave_count_actual dropped from 127 to 0 five times between 08:01 and 08:07 this morning. As far as we know there is no Beckhoff work ongoing at this time. 

TITLE: 09/17 Day Shift: 1430-2330 UTC (0730-1630 PST), all times posted in UTC
STATE of H1: Corrective Maintenance
INCOMING OPERATOR: Corey
SHIFT SUMMARY: *I forgot to click post for this last night*

The LVEA was transitioned to SAFE, the CHETA viewports were installed, EndX pump down continues, and the HAM2 CPS cables were rerouted. Commissioners spent the afternoon touching up some corner alignment then they worked on PRMI and some phasing. A few optics lab and PSL anteroom dust alarms in the late afternoon, corresponding roughly with wind increase but the wind increase 3 hours previously did not increase the dust counts.

LOG:                              

• 14:30 UTC A 6.5 from the Aletians hit us
• 15:30 UTC LVEA to SAFE
• 18:30 UTC HAM2 cabling finished and corner alignment start, JAC work/alignment had been going on beforehand

TITLE: 09/18 Day Shift: 1430-2330 UTC (0730-1630 PST), all times posted in UTC
STATE of H1: Corrective Maintenance
OUTGOING OPERATOR: None
CURRENT ENVIRONMENT:
    SEI_ENV state: MAINTENANCE
    Wind: 5mph Gusts, 2mph 3min avg
    Primary useism: 0.01 μm/s
    Secondary useism: 0.07 μm/s 
QUICK SUMMARY:

• Overnight Optics lab and PSL laser room dust alarms, 10k and 500 cts respectively
• Temps, and CDS look good
• EX pump down continues, the corner station X/Y BM pump downs will be resumed this morning

(Travis S., Jordan V., Richard M., Gerardo M.)
After the installation of CHeTA viewports, we connected an ISP-1000 scroll pump and a SS-500 cart to each of the main turbo pumps at the X beam manifold and Y beam manifold.  After getting power sorted out to the ISP-1000s, thanks Richard, we started the pumpdown for both of the manifolds.
Something that we did noted was the new gauge for PT-180 shows almost 1000 Torr for pressure at atmosphere, this is not correct, then the other gauge for PT-170 shows 750 Torr for pressure at atmosphere, also not right.
Pumpdown has been stopped at the end of the day, and will be restarted tomorrow morning by Jordan.

Attached is a plot of both manifolds initial pumpdown.

After closing up BSC9 yesterday we started pumpdown of the EX volume this morning using the two mobile ISP1000 scroll pumps.

There was minimal overpressure in the system, so I could not get a good dewpoint measurement of the blow down air.

It took ~6 hours to rough the system down from atmosphere to ~500 mtorr, where we start the turbopump.

The cooling water was adjusted on the turbopump stand so that the pump and controller pressures were <= 40C. Once the turbo was at full speed and the inlet pressure ~5e-5 Torr, I switched the backing pump to the dedicated ISP250 on the turbo stand.

Setpoints were adjusted to 5E-2 Torr on Channel 1 (Foreline) and 5E-5 Torr on Channel 2 (Turbo inlet).

We will need to leak check the TMDS port gate valve since it was removed to verify the gate o-ring was in place.

Also, the EX cleanroom was powered off at ~3:10 pm local time.