[Jennie, Jenne]

We ran the dark offset script, since it hasn't been run since the vent.  Next up is to see how that affects the alignment work that Keita and Jennie made progress on yesterday (alog 76019).

Dark offsets set for LSC, ALS, and OMC DCPDs at GPS = 1393187280.0
TR COMM offsets at GPS = 1393187290.0

TJ, Camilla. WP 11730 Table: D1800270

TJ and I swapped the EX HWS fiber from a M92L02 200um 0.22NA multi-mode fiber to a M14L01 50um 0.22NA fiber, the same that LLO successfully uses. This gave us a focus ~150mm after the 125mm lens, where D1800125 suggests our focus should be 1.25m from the launcher (or ~100mm from L2 62398). TJ found we need to change the spacers in the D1800125 launcher, from the design 12mm closer to 11mm to get the beam focus at 1.25m. We got a beam a much more sensible size by only securing the launcher on one side and could see a return beam off ETMX, image will be commented. We plan to buy/find more spacers before continuing this work.

LLO has recently been swapping 1" optics to 2" to reduce clipping 69891. We did this in 62995 and 73878 so have M1A, M1B and M1C on EX as 2" optics but currently no picomotors in the HWS path.

Wed Feb 28 10:09:10 2024 INFO: Fill completed in 9min 7secs

Jordan confirmed a good fill curbside.

The HAM7 accelerometers were moved from temporary cabling (going through the SUS-R4 PEM patch panel) to permanent cabling. Temporary cabling removed. The HAM7 door accelerometer is not connected. Sensor needs to be glued to door.

F. Clara, R. Schofield

The temperature of Zone 5 in the LVEA was lowered by .5 degrees F. The set point is now 67 F. This is to address increased heating due to electronics in use in this zone. 

TITLE: 02/28 Day Shift: 16:00-00:00 UTC (08:00-16:00 PST), all times posted in UTC
STATE of H1: Planned Engineering
OUTGOING OPERATOR: None
CURRENT ENVIRONMENT:
    SEI_ENV state: CALM
    Wind: 54mph Gusts, 38mph 5min avg
    Primary useism: 0.11 μm/s
    Secondary useism: 0.29 μm/s
QUICK SUMMARY:
Tumbleweeds are on the move.
CDS Looks good
HAM 7 watchdog was tripped, I untripped it.

Expected work:
VAC team is going to be doing RGA scans on HAM7 and Pumping down HAM8.
SQZ team may start doing transferfunctions.

Keita, Jennie W

Keita and I were tuning the alignment of SR2, OM1, and OM3 to improve alignment into OMC after the vent and swap to the new OMC.

We started just after 22:00:00 UTC on the 27th.

Note: HEPI is still locked on HAM5 and HAM6 for vacuum work so we may have to move the mirrors again once these are unlocked.

The beam was not well aligned on the AS_C QPD.

Firstly we turned off the the offsets in H1:ASC_OMC_{A,B}_{PIT,YAW,RIN} and in H1:OMC-ASC_QPD_{A,B}_{PIT, YAW} as these are on the output of the OMC QPDs and were set for the old OMC and alignment.

Secondly:

• we turned on the matrix element (INMATRIX_P_7_27) in the ASC PITCH input matrix to route AS_C_DC input to SRC2_P.
• Then we changed the output pitch matrix element (OUTMATRIX_P_9_7) from -7.08 to 0 to ensure output from SRC2 does not got to SRM.
• We then turned on the gain in the H1:ASC_SRC2P to be 10 so we could align SR2 then get the loop to take over.
• We tweaked up the pitch alignment sliders for the optic to drive ASC-AS_C_SUM to zero until the ASC loop took over.
• We did the same for the yaw degree of freedom.

Thirdly:

• we changed the OMC_ASC_DOF2TT matric to only feed OM1 yaw to the POS X direction and OM1 pitch to the POS Y direction.
• We ran an excitation on the alignment of OM1 in pitch and yaw till we saw it on ASC-OMC_A phtodiode.
• Then we tweaked the alignment sliders till the beam was well centred in pitch and yaw.

After this we tried turning on the OMC ASC but noticed it ran away in pitch and yaw.

We went into the OM1 sus screen and noticed that there are integrators on in the OM1 M1 Locking Filters, so we should not use additional integrators in the OMC ASC filters in this case. We cleared the hostory and changed the alignment offsets to compensate and get the beam back on QPD A.

Fourthly:

• we changed the OMC_ASC_DOF2TT matric to only feed OM3 yaw to the ANG X direction and OM3 pitch to the ANG Y direction.
• We ran an excitation on the alignment of OM1 in pitch and yaw till we saw it on ASC-OMC_B photodiode.
• Then we tweaked the alignment sliders till the beam was well centred in pitch and yaw.

We set each loop up individually with gains that worked but then had to redo these once all four ASC loops were running as there is some cross-coupling.

The two ANG X and Y loops have integrators on as the OM3 suspension does not have integrators in the OM1 M1 locking filters.

The final loops gains are shown in the first screenshot.

Camilla pointed out that OM2 had high alignment offsets so we tried to slowly tune the pitch offset down while offloading this on the OM3 pitch offset.

Finished our alignment efforts at about 2:00:00 UTC on the 28th.

I have left the overall OMC ASC gains at zero and the SR2_P and SR2_Y gains at zero.

Second image is the OMC QPD SUM outputs and the YAW and PITCH outputs during the last part of the alignment.

Third screenshot is the OMC control screen before we started.

Fourth screenshot is the ASC P OUTMATRIX before we started.

Fifth screenshot is the ASC Y OUTMATRIX before we started.

Sixth screenshot is the slignment sliders before we started (except for some small changes I made to OM1 before Keita came to help). I think the original values for OM1 from yesterday were 651.6 counts in Pitch and 63.6 in yaw.

Seventh screenshot is the OMC_ASC_DOF2TT moatrix before we started.

Eighth is the OMC ASC servo filter banks before we started.

After aligning ITMY and checking HWS, I've set the ITMY ring heaters to be on at 2W/segment for the next 12 hours: ~7:30pm to 7:30am PT (03:30UTC to 15:30UTC). ITMY will then spend tomorrow cooling down. This test should help the IFO modeling team get more recent data on the effect the ring heaters have.

Using script: python3 /tcs/common/scripts/power_adj_scripts/ring_heater_schedule.py ITMY -s 'now' -d 12 -p 2.0  

Today's activities:
- HAM8 was closed, the Y+ door and the 2 pcs. of Y- ports. The pumpdown of the main volume, Annulus volume, and the RGA has been already started - see for details: https://alog.ligo-wa.caltech.edu/aLOG/index.php?callRep=76014
- HAM7 pumpdown status: ~5.5E-7 Torr
- Corner pumpdown: ~4E-7 Torr, ~171 hours after the HV pumping started
- Corner (and HAM7) further schedule: RGA scan #1: 2-27; Valve together the corner, HAM7, relay tube, X manifold (open GVs GV2, RV1, FCV1, FCV2): 2-28; Ion pumps (IP1, IP2, IP3, IP4, IP19) valve-in: 2-29; Turbo pump valve-out: 3-1; RGA scan #2: 3-4; GV5 & GV7 open: 3-4
- EX pumpdown status: 2.5E-8 Torr.
- EX further schedule: RGA scan #1: 2-29; Ion pump valve-in: 2-29; Turbo pump valve-out: 3-1; RGA scan #2: 3-4; GV20 open: 3-4
- The RGA scan #1 for the corner (see above) has been done, for details see https://alog.ligo-wa.caltech.edu/aLOG/index.php?callRep=76012

Opslogin0 (NoMachine) is down for maintenance until approximately 0400 UTC.

The readback of the LO power of the IMC REFL demodulator is indication a drop of 4-5 dB on 1/19 around 1PM. Since then it keeps decaying and now has reached ~17dBm from an initial ~23dBm.

(Randy T., Jordan V., Travis S., Janos C., Gerardo M.)

The HAM8 +Y door is on.  One known issue with this door was the inner O-ring, it had scuff marks from about 6:30 all the way up to 10 o'clock.  The O-ring was replaced with a new one.  No other issues were noted during the installation of the door.

The 2 blanks were installed on the two access ports, these conflats need to be leak tested in the coming days.

Annulus system is pumping down and will continue to pump with an aux cart until the ion pump can take over.

Roughing was started, chamber volume is currently at ~500 torr, roughing was stopped and will restart tomorrow morning.

J. Oberling, R. Crouch

Update on FARO work during the O4 commissioning break.  Previous updates at the following alogs (with associated comments): 75669, 75771.

Since the last progress update we've been testing our FARO X/Y alignment routines and attempting to re-establish Z=0 based on the door flange scribes on BSC2.  We've been navigating Laser Safe/Hazard transitions, as we can only do optical surveying (like using an autolevel for our BSC2 survey) during Laser Safe; the FARO is usable during Laser Hazard so we've been using these windows for FARO work.

FARO X/Y Alignment Testing

As a means of testing the repeatability of the FARO's X/Y alignments we have been using the brass monuments for mechanical test stand #2 (TS2) in the West Bay of the LVEA.  The FARO gives us a global X/Y coordinate for these monuments based on our alignment (which is also a local X/Y coordinate since X_G=X_L and Y_G=Y_L), which we can use to compare the Measured Local LVEA coordinates to each other and test the repeatability of different FARO alignments.  In addition, each test stand has a monument that represents the [0,0] of the test stand (monument TS2-10 for TS2). We can therefore subtract the local LVEA X/Y coordinate for the [0,0] monument from each measured test stand monument to translate from Local LVEA coordinates to Local Test Stand coordinates.  With this translation we can also compare the monument coordinates measured by the FARO to where we think they are via their as-designed coordinates (designed test stand monument coordinates taken from D1100291).

The results are shown in the attached .pdf file 'FARO_XY_Alignment_Test_TS2_Monuments.pdf'; I have also attached the reports generated from PolyWorks for each of our surveys.  To date we have done this with 3 separate alignments:

• Alignment 1: BTVE-1 as a sphere, PSI-1, PSI-2, and PSI-6 as points
  • Alignment originally created on 2/5/2024 to test differences in using sphere features vs point features; the 3" sphere fit rod was used for BTVE-1 (alog 75771, under Wednesday, 2/7/2024)
• Alignment 2: All alignment monuments as spheres
  • Alignment originally created on 1/22/2024 with a 3" sphere fit rod, to compare to the 5" rod and test the accuracy of moving the FARO (done in alog 75669)
• Alignment 3: All alignment monuments as spheres
  • Alignment originally created on 2/21/2024 as a new alignment for this test; the 3" sphere fit rod was used

Alignments 1 and 2 give us insight into how using different feature types (points vs spheres) for our alignment monuments cause variations in the alignment.  Alignment 3 was used to give some insight into the repeatability when the same alignment feature types are used with 2 different alignments (in this case 'All Spheres' vs 'All Spheres').  The first 3 pages of the results pdf file detail the measurements of the TS2 monuments, the conversion to Local Test Stand coordinates, and a comparison of the measured test stand coordinates to the as-designed ones; 1 page is used for each alignment.  The final page compares the 3 alignments to each other, both in Local LVEA coordinates and in Local Test Stand coordinates.  Some thoughts:

• Designed vs Measured Test Stand coordinates
  • All 3 alignments show significant differences in test stand monument coordinates vs our designed coordinates.  That said, as can be seen from the final page all 3 alignments match each other fairly well, with only a few deviations >0.5 mm.
  • This is somewhat baffling (and a little worrying), as this test stand, with these monuments and as-designed coordinates, were used to align the cartridge assemblies for the ITMx, ITMy, and BS optics during aLIGO install.  I don't recall seeing these large variations in optic position while doing the final alignments once these cartridges had been installed in their respective chambers (need to dig through my old notes from install).
• Alignment 1 vs Alignment 2, comparing Sphere+Points alignment to All Spheres alignment
  • The 2 alignments match pretty well in X, but are off in Y.  Alignment 2 appears to be shifted in Y w.r.t. Alignment 1 by approximately -0.5mm.
• Alignment 2 vs Alignment 3, comparing 2 All Spheres alignments
  • Some variation in X now, with a couple points getting close to 1mm; Y variation looks better, but still several points approaching 0.5mm deviation.
• Alignment 1 vs Alignment 3, comparing Sphere+Points alignment to 2nd All Spheres alignment
  • More variation in both X and Y, which isn't surprising given the variations seen when comparing the 2 All Sphere alignments to each other.  Some variations approaching 1.0mm.

We're still digesting this.  I'm intrigued by the measured test stand coordinates for the monuments in line with each other.  For example, TS2-1 is supposed to be directly in line with TS2-4, only separated along the test stand's Y axis; this is the same for the group TS2-2, TS2-10, and TS2-5, as well as the group TS2-3 and TS2-6.  All 3 alignments show these monuments being at an angle with each other, and a similar angle at that; almost like the line from TS2-2 to TS2-5 (which also intersects TS2-10) was not straight when these monuments were laid out, and that carried over in the setting of the monument groups to the sides of this line (TS2-1/TS2-4 and TS2-3/TS2-6).  I will say that I find the deviations between FARO measured and as-designed test stand monument coordinates particularly worrying; whether that's due to an error in the FARO alignment or an actual error made when these monuments were first laid out I can't yet say, some more investigation is required (could do something like use a 100' survey tape to measure distances between monuments and compare to the FARO measurements).  Also, I would like to to set up a new Sphere+Points alignment to see if using the point alignment feature improves the repeatability; as I've said a few times in the previous alogs, we suspect that the sphere fit routine and the limitations of the sphere fit rods are introducing error into the FARO alignment, and the above alignment comparisons appear to support that at first glance.  I'm interested to see if using points instead of spheres improves this, but we need a new alignment to compare to the old Sphere+Points alignment.

BSC2 Z=0 Water Level Survey

Based on the results of our FARO work detailed in alog 75771, we want attempt to re-establish Z=0.  This was originally done by averaging the 8 door flange scribes of the BSC2 chamber (1 and 3 o'clock and 1 and 9 o'clock on each of the 4 door flanges).  With all of the line of sight blockers (beam tubes, other chambers, electronics racks, cable runs, etc.) we felt the easiest way to repeat this was to use a water tube level.  To do this we used roughly 60' of flexible tubing with an 8mm OD and 6mm ID.  We filled it with water (setting up a siphon works great for keeping air bubbles out of the tube), leaving some air at each end, and set up around BSC2.  One end of the level was fixed to the unused HEPI pier for BSC8, with a scale attached nearby for measurements; the other end was placed along the door flange scribe under measurement.  We used an autolevel to set the water line on the scribe line to be measured, then used a 2nd autolevel to sight the other end of the tube and take a reading on the scale.  We ended up using several rubber bands and some tape to secure the tube to the door flange; the tape was necessary to keep the tube from sagging under the weight of the water (the BSC scribes are over 6' above the ground), while the rubber bands helped to keep it mostly secure while we were setting it on a scribe line.  The first 3 pictures show the setup, with the third one taken through an autolevel to show a close up of the water in the tube (have to sight at the bottom of the meniscus, just like with a graduated cylinder or similar measurement devices (like glass measuring cups in your kitchen)).

We did have a few issues, chief among them being that we could not get the water in the tube to stop moving at first.  We would set the water line on a door flange scribe and watch it settle, and it would keep dropping slowly over several minutes.  We noticed that regardless of where we set the water level, it would always drop to the same point; what finally clued us in to the issue was noticing that the other end of the water level was also dropping.  If the level were rebalancing we would expect one end drop while the other raised, but this was not the case.  At this point we also noticed that, even though we left about 12" of air at each end of the tube when we initially filled it, we now had almost 2' of air at each end.  The solution?  Not enough water in the tube, so add some more.  We did this and all the stability problems vanished.  We could then set the level on a scribe line, and after just a few seconds it would settle out and be very stable.  Best explanation I have is we didn't have enough water to account for the slight compression of the water column at both ends of the level, since our measurement point was over 6' off the ground.  With only a 6mm ID on the tubing, it doesn't take much to cause a big difference in how the level behaves.  By adding ~9mL of water to the tube (using a 2mL transfer pipette) all of our problems were solved.  Second issue, don't step on or touch the water level once set.  This causes the water in the tube to move, a lot.

The other big issue we had is sighting the correct scribe line on the door flanges.  Over the years since site construction several additional scribe lines have been added to many of the door flanges, all within several mm of each other.  Most have no markings on them, a few had arrows, but 1 scribe on each flange was marked with 3 punch marks; this was also true for the 3 flanges with only 1 scribe on them.  So we sighted the scribe line marked by the punches on all door flanges.  The 4th picture shows an example of these punch marks (there are 2 scribe lines in this pictures, one that is straight and one that is not; we used the one that is straight, which can be seen behind the autolevel cross hairs).

With our scribe lines chosen and other issues figured out, we set about measuring all 8 of the BSC2 door flange scribes.  The final picture is a shot of my notes from the survey.  Notice the large separation for the -X door scribes.  Mike Zucker indicated to us that he thinks the scribes were placed to within +/- 1mm of flange center (having a hard time finding documentation of this, he is currently looking for the old "end item data package" for the chambers from their initial construction in the 90s), so this 11.3mm separation in particular is puzzling (we also measured a 4.6mm separation for the +Y door, 1.3mm separation on the -Y door, and 0.5mm separation on the +X door).  One thing he suggested we can do is use a flat survey tape to check that the scribes are on a true diameter of the flange (are they 1/2 circumference apart?), which we will do once we have Laser Safe again.  Once we confirm we've used the correct scribe lines we will continue with using the average of these scribes to check the various height marks around the LVEA.  Should we find that we don't have the correct scribes then we will have to repeat the water level survey.

I. Abouelfettouh, J. Kissel, O. Patane, B. Weaver

Executive Summary: The first article BBSS transfer functions look great. Though there is some confusion about the M1 P 2 P modeled transfer functions drastically disagreeing with the measured TFs, there is a consistent story between 
    - the adjustments to the mechanics that were made during construction and 
    - deviations from the "production" model parameter set that could re-create those construction adjustments. 
Further discussion will be had with the assembly / design team as to the future course of action.
Kissel suggests that -- even as the first article stands now -- the resulting measured transfer functions with the mechanical adjustments would/should happily meet A+ O5 requirements.

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I got a debrief yesterday from Betsy, Oli, and Ibrahim of the comparison between 
    - measured transfer function results from the first article construction and
    - what had been deemed the production model parameter set for the BBSS,
i.e. what's discussed in LHO:75787.

The existing "production" model parameter set starts from Mark's update to the BBSS parameter set post-final-design after adjusting for the production wire thicknesses (see TripleLite2_mark.barton_20211212bbss_production_triplep.m, changed at rev 11625, circa Sep 2023). Oli successfully copied over to the usual matlab formating to create bbssopt.m (created at rev 11734, circa Jan 2024).

At the start of the debrief, there were (only!!) 3 outstanding issues / questions they had:
    (1) The overall magnitude scale for all DOFS for all measured transfer functions was a consistent factor of ~3.15x more than the model estimates,
    (2) After browsing through the EULER-basis drive to OSEM-basis response plots, and some of the off-diagonal EULER-basis showed little-to-no coherence, and
    (3) The measured M1 Pitch to Pitch transfer function's frequency response was significantly different than the model.

For (1), this is typically a sign of a mis-calibration of the data. We reviewed the calibration of the measured data from the processing script, plotBBSS_dtttfs_M1.m, created by Oli and Ibrahim in Nov 2023. The DTT templates that measure the transfer function use the pre-calibrated output of the sensors for response channels i.e. the channels come in units of microns and microradians, so they only need a factor of 1e-6 [meters / micrometers]. The only substantial thing that needs calibrated into physical units during post-processing is the excitation. The review of the calibration of the exciation revealed nothing suspicious in the script based on our current expected knowledge of chain
    - the test stand electronics (an 18-bit DAC = 20 / 2^18 [V/ct]), 
    - BBSS coil driver (a TTOP coil driver, coupled with a BOSEM coil = 11.9 [mA/V]), 
    - 10x10 magnet strength (1.694 [N/A]).
    - (lever arms and numbers of actuators are pre-calibrated out via the EUL2OSEM matrix, generated by make_susbbss_projections..m, and installed in EPICs)
The above factors result in an overall calibration of 1 / 1.5405 [(m/N) / (um/DAC ct) or (rad/N.m) / [urad/DAC ct]] that's displayed in the legend of each of the plots from LHO:75787.

In the end, we were more interested in understanding (2) and (3) rather than getting to the bottom of the calibration. Further, the test stand is some old, pre-aLIGO concoction whose records and modifications are unclear. So we figure we just move on, accepting that we need to fudge the data by the extra factor of 3.15x. We'll get serious about figuring it out if there's still such a discrepancy after moving the BBSS over to the production H1 system.

For (2), all concerns can we waived off with expectations. 
    (a) The first plot of concern was the P to F1F2F3 plot (page 17 of 2024-01-05_1000_X1SUSBS_M1_ALL_TFs.pdf), in that the magnitude of the F2 and F3 TFs were low and/or noisy. This is expected because F2 and F3 OSEMs are along the (center of mass / axis of pitch rotation) of the BBSS's top mass. So they see no pitch by construction (for better or worse).
    (b) The second collection of plots of concern were the off-diagonal DOFs, 
        (i) showing noise and/or 
        (ii) the opposite -- showing well-resolved cross-coupling in DOFs that we *don't* want cross-coupled. 
        We shouldn't be mad about (i) -- e.g. page 7 showing incoherence between L response to V drive and V response to L drive. What power is resolved in those transfer functions -- typically on/around resonances -- is because the TFs were taken undamped an in air. So there's just a ton of movement that an FFT might / cross-correlation might *think* is coherent with the drive, but it's really not.
        We looked closer at any of the off-diagonal TFs that *were* resolved, (ii) -- e.g. page 9 showing well-resolved cross-coupling between R response to V drive and V response to R drive. In each of these TFs, we found that the magnitude of the cross-coupling, off-diagonal TF was less, if-not-MUCH less that the on-diagonal TF, which is good. Where it was close, it sort-of "is what it is." Little attention has been typically paid to mitigating the off-diagonal transfer functions during the design phase of LIGO suspensions to-date. Further, they often are a result of the unique construction of each individual instantiation of the suspension type. There's no much we can do about it post construction, and what we *do* do if it proves problematic to the detector, is dance around the problem with fancy controls techniques if needed.

For (3), we arrive at the meat of this aLOG ::  The *model* of the M1 Pitch to Pitch transfer function looked very weird to me. Betsy mentions the during the construction of the first article they 
    (a) found a discrepancy between the fastener model vs. measured mass budget that resulted in an unclear relationship between the center of mass of each stage and their suspension points (typically called the "d" parameters)
    (b) acknowledged there would be uncertainty in the location of the suspension point for the bottom mass / dummy optic given the wire-loop + optic prism system since the final distances between masses have not been measured.

This, coupled with the fact that no *other* DOFs disagreed with the model besides P to P, led me to suspect the model parameters that only impact the pitch dynamics may be incorrect: 
    (i) each stages' separation between suspension point and center of mass, the "d" parameters, and 
    (ii) the pitch moments of inertia.

For a reminder of the physical meaning of all of the triple suspension parameters, see T040072.

As such, using the bbssopt.m "production" or "Final Design" (FD) parameter set as starting point, we tweaked these parameters by 10%-ish or factors of 2 to gather intuition of of how it would impact the response of the P to P transfer function. As a result, we have come to the conclusion that, in order to explain the data, we need to
    - increase "d1" by + 5 [mm]. This is the separation between the top (M1) mass center of mass and it's M1 to to M2 blade tip heights. In the absolute sense, this is increasing the "physical" d1 from -0.5 [mm] to +4.5 [mm], and
    - increase "d4" by 1.5 [mm]. This is the separation between the bottom (M3) mass / dummy optic center of mass and the wire/prism break-off point. In the absolute sense, this is increasing the "physical" d4 from +2.6 [mm] to +4.1 [mm].

Check out the attached plots which demonstrate this.
Citing discussion of overall scale (1) from above, all *measured* transfer functions have been scaled to the model by a factor of (1 / 3.15). This just makes comparing model to measured frequency response a lot more clear.

First attachment :: comparison between the final design model parameters and a variety of reasonable deviations of d1 between *decreased* by 2.5 [mm] and *increased by 5 [mm]. You'll notice that once d1 surpasses +1.0 [mm], the transfer function starts to look more like a standard triple suspension's transfer funtion. a d1 of FD + 5.0 [mm] lines up well with the upper two resonances of the measured data, but reduces the frequency of the lowest two L and P modes to below the data.

Second attachment :: comparison between the final design model parameters and a variety of reasonable deviations of d4. You'll notice that d4 really only have an impact on the lowest two L and P modes.

Third attachment :: comparison between the final design model parameters and a variety of reasonable deviations of the top (M1) mass' moment of inertia, the I1y parameter. 

Fourth attachment :: comparison between the final design model parameters and a variety of reasonable deviations of the middle (M2) mass' moment of inertia, the I2y parameter. 

Fifth attachment :: comparison between the final design model parameters and a variety of reasonable deviations of the bottom (M3) mass' moment of inertia, the I3y parameter. 

None of the modeled changes to the moment of inertia -- shown in the third, fourth, and fifth attachments -- show promise in reproducing the measured results.

Sixth and Final attachment :: comparison between the final design model parameters and one with only d1 increased by +5 [mm], and d4 increased by +1.5 [mm].

The modified model in this last attachment fits the data the best, so we conclude that the issues with mechanical construction (3a) and (3b) are consistent with the measured data :: the reconfigured mass budget needed from fastener issues resulted in a deviation from design value for d1, and the imprecision of the mass-to-mass distances and wire-loop / prism system resulted in a roughly ~2 [mm] slop for this assembly.

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Big Picture Systems Level Commentary by Jeff :: If these measured transfer functions end up being the reality of the final frequency response of the BBSS -- this will be totally fine. The pitch isolation one gets above the resonances (defined mostly by the moment of inertia) is the same, the lowest L and P modes are sufficiently low, and the details of where the rest of the resonances land are totally inconsequential / amenable to a damping and global control design.