Friday June 5th we completed the assembly for the CRS that is going in HAM3. 

First thing we checked the alignment of the HoQIs, Fringe visibility: SN005 86% SN007 85%

We swapped out the photodiodes and cables in SN007 due to damage in one of the cables which meant the wire was exposed risking grounding issues should this touch any of the metal parts. The fringe visibility after this was at 72%. 

The HoQIs were completed with the baffles and top cover. SN007 cover is incorrect and the correct parts were not out of C&B so the wrong cover is on it at the moment this will be switched when we have the clean part. 

They were fixed to the CRS and aligned to it. SN007 is on the right and SN005 is on the left. Fringe visibility with the CRS fixed was as follows
SN007 sin 82% cos 80% mcos 82% SN 005 sin 83% cos 75% mcos 83%
The CRS was released and the fringe visibility for both left and right was checked, they were >80%, not all PDs were recorded but eg SN007 mcos was 82% 

(Gerardo M., Brandon P. (RMC Tech), Jordan V. (buddy))

Today the second oil pump was replaced on the Kobelco unit, no issues to report.  For previous work see aLOG 90318.

The purge air was isolated at the dryer towers and at the point of delivery before starting work.  Brandon shut off the compressor and stopped the dryer system, then removed and replaced the oil pump.  Kobelco compressor was re-started, oil measured, compressor was turned off to add oil up to nominal value, then the compressor was started and load tested.  No issues noted.  Brandon handed over the system.  I started the dryer towers back up, no issues noted.  Dew point from the local reading at the towers remained below -51 ^oC since the dryer system remained with both towers "loaded", and since there was no demand both remained pressurized.  After an hour with both the compressor and the dryer towers running at nominal, I opened the purge air to the LVEA lines.  After 45 minutes I took a couple of measurements.

Measurement results:

1. Point of use, near the purge port near HAM4 yielded a dew point of -43.2 ^oC
2. Same as before, near the dryer towers in the mechanical room, the dew point was taken before the big pressure regulator, port for the TMDS air supply, both towers yielded -44.4 ^oC.

Purge air is purging into the opened volumes.  Once again, please don't waste the purge air, if you can please keep those C3 door covers closed.  Thank you!

Leaving site now.

Short story:

RM2 was nicely relieved by rotating IM1 in YAW, ASC REFL sensors were centered, RM2 only used 40% of DAC range, people==happy (ASC-REFL-centered-20260605.png).

ISS QPD was nicely centered, it was easy, people==happy (iss_CENTERED.png).

Then suddenly we found that the IFO REFL beam is clipped at the bottom of the IFO REFL baffle, ISS array was totally out of whack, forward-going beam from IM2 was not centered on the retroreflection check iris between IFI input baffle and IFI HWP baffle, people!=happy.

Turns out that something bad happened to IM1 about 10 minutes after ISS was centered (IM1_bad.png). Without any change in the drive, H1:SUS-IM1_M1_OSEMINF_LL jumped by negative 172 counts causing apparent positive 1570urad jump in DAMP_P and DAMP_Y.  UR changed by 12 counts, which is like O(100urad) type change. 

This doesn't necessarily mean that IM1 moved that much, maybe one OSEM was bumped while we're trying to put the cover on the ISS, but the thing is that this must have caused physical rotation somehow because we see the beam moved downstream of IM1.

We tried to diagnose the IM1 but found no obvious touching, transfer functions didn't look wrong, EQ stops have gaps, magnets look as if they're OK.

You don't have to read further, below is our memo to remember what was done.

Details you don't want to remember:

We've made another YAW move (of positive 300urad) for IM1, i.e. [P,Y]=[517, -687] -> [517, -387] to relieve RM2 further.

Based on calculation, also moved IM2 and IM3 to roughly align things. IM2 [P,Y]=[0, -588.7] -> [0, -19] IM3 [P,Y]=[35.3, -415] -> [35.3, -86].

Checked the IFI output baffle and noticed that it seems to have an offset in -X direction relative to DKDP baffle. The beam was well centered on DKDP baffle, but was too much in +X direction on the IFI output baffle.

Moving IM2 further, [P,Y]=[35.3,  381] to split the difference.  Distance between the forward-going beam and the parking beamdump pickoff was ok.

This resulted in large IM3 Yaw step (IM3 [P, Y] = [35, 654]) to bring the beam close to the nominal position in front of PRM.  Horizontally, the beam was off by 0.5+-1mm in negative Y direction. Nominal beam height at the measurement point for PRM 159.05mm. Measured 157.1+-1mm.

IM4 Yaw moved by positive 350 ([IM4 [P,Y]=[761, -205]) to set the beam position in front of PR2. Horizontally, the beam was off by 0.5+-2mm in positive Y direction. Nominal beam height at the measurement point for PR2 168.56mm, measured 169.8+-2mm.

Centered the retro-reflection check iris to the forward-going beam between IFI input and IFI HWP baffle. Aligned PRM to center back-propagation on that iris. 

PRM [P, Y] = [-1185, -270].

Centered refl asc sensors. This worked really well, RM2 DAC was only using 40% of its range after centering ASC REFL sensors. Even though this was one attempt, I'm comfortable enough to say that we'll be fine.

Moved on to center ISS array. We removed the ISS array cover from the assembly to use the input aperture tool to make things easier. Large hole aperture as well as medium was used, didn't bother to use the small aperture. We used two pico mirrors (had to use hand because no driver is connected until the IOT2L is back). Iteration between two mirrors was easy and the centering was done by 22:37 UTC.

RM1 P, Y = -196, 209 (13% of DAC range)

RM2 P, Y = 609, -602 (40% of DAC range)

Happily we put the ISS array cover back on. Unknown to us, something happened to IM1 at around 22:46 UTC.  
 

Remaining task:

solve IM1 mystery?

shoot the pic of pico mirrors to see how much range we have.

center IM4_TRANS

take aperture photo for IFI inpjt and IM4 baffle, recenter IM4 baffle if necessary.

ground check.

Robert, Ibrahim, Anamaria

We finished alignment of the CP cage baffle on ITMX, which was the last item to do regarding cage baffles. The panels are locked so they can now be removed and replaced while retaining the alignment. 

Both ITMs have the (new) face shield on. ITMY suspension is unlocked and ITMX is locked. 

Begum, Ryosuke, Camilla, Madi

Summary: Were able to retroflect the beam off FC1 (when placed at O4 sliders) with ZM2 and ZM3 but the beam height is incorrect, as expected, VOPO optics will need to be moved to fix this. 

With the new cage baffles installed on ITMY, Anamaria and I went in to hammer the cage. The suspension was unlocked and the ISI locked. The accelerometer was mounted on the cross member between the L3 and L2 stages (photo) with the accelerometer axes X,Y,Z = IFO -Z,Y,X. All plots have the accelerometer axes plotted. Measurement numbers are just for my own reference. LLO's results for their ITMs can be referenced in LLOalog81300.

Longitudinal first peak 57.5Hz

Transverse first peak 64Hz, with a second nearby at 67.5Hz

TITLE: 06/05 Day Shift: 1430-2330 UTC (0730-1630 PST), all times posted in UTC
STATE of H1: Planned Engineering
INCOMING OPERATOR: None
SHIFT SUMMARY:

ISS PD Array alignment was most of the day and there was several activities in BSC1+2+3, as well as work in the CRS lab, @HAM7, and brief SPI feedthru work at HAM3
LOG:

• 1415-1501 Moving ISC Table & garb room @HAM3 (randy)
• 1442-1758 technical cleaning/organizing in lvea (kim)
  • Came out for lunch break
  • 1646-1758 Back for another hr to finish lvea
• 1525 Power cycle purge air (dew point is high) [gerardo]
• 1539-1543 Look at HAM2 & opening light pipe (keita)
• 1614-1753 HAM7 in-chamber (camilla, begum)
  • 1653 out for break
  • 1829-1957 Back to HAM7
  • 1957 lunch
• 1615 B&K organizing (tj)
  • 1641 TJ out briefly to chat with ibrahim but back in shortly after
  • 1642 Annamaria working with TJ
  • 1642 Ibrahim working with TJ + First Contact staging + ITMx baffle prep
  • 1816 robert joining
• 1620 ISS PD Array alignment (rahul.dishal.keita)
  • 1913 out for lunch
• 1705-1710 Megacleanroom visit (mitch)
• 1722-1752 Dewpoint measurements in the lvea (gerardo)
• 1736-2003 Cyllindrical Rotation Sensor (CRS) team at the CRS Room (jim.shoshanna.alexandra)
  • 1838 Arnaud joining
  • 2003 out for lunch.
• 1838-1902 Optic Lab tour (jason.visiting postdoc)
• 1843 Betsy heading out to beer garden 
• 1845 BSC3 ITMx measaurements (robert.anamaria)
• 1905 ETMx transfer functions (oli)
• during my lunch:
  • 1927-2037 SPI feedthru swap (jeff.jennie.jim)
  • 1938 tour (maggie)
  • 1946-2133 Boxing up old BS suspension (randy.betsy)
    • 2131 Betsy out.  BS in box, but needs brackets to put on lid.
• 2020-2030 Running tools out to SPI
• 2025-2032 Tried to got to SUSR2 rack (josh)
• 2038 CRS work continues (alexandra, shoshanna)
  • 2200 joining CRS team (arnaud.jim)
• 2058 ISS team returning for alignment work (keita.rahul.disha)
  • 2224 Jennie tagging rahul out (he left at 2258)
• 2115-ish - 2313 SQZ team at HAM7 (camilla, begum, ryosuke)
  • 2201-2313 Maddi joining SQZ team, laptop in-hand
• 2123 Cameras for BS went down briefly
• 2203-2222 FARO set-up for Mon (jason)
• 2231 staging parts in beer garden (anamaria, ibrahim)
• 2235-2238 Ran cable out to SQZ (corey)

J. Kissel, J. Warner, J. Wright

Jim, Jennie, and I started working on converting the HAM3 D5 feedthru from D1002874-v9 to D1002874-v10. This feedthru (in -v9) serves "corner 2" of the HAM2 ISI. 

We didn't finish, but here's how far we got (all in the -X / -Y / -Z corner of the chamber where the D5 flange is):
    - Powered OFF all HAM3 ISI sensor (via ISI interface) and actuators (via coil drive) in the SEI-C2 rack.
    - Disconnected the in-air connections (2x 3W3 actuator cables, 2x BNC CPS cables, 1x GS13 D25 cable) to the feedthru. (easy, by hand / thumbscrew)
    - Removed all strain-relief cylinders (easy, simply hose clamped)
    - Disconnected as many in-vac connections as was easy (Horz CPS BNC, Both 3W3 actuator cables)
    - Removed the dual D4-2B 4.5" flange, wrapped and tagged it with the copper gasket on to protect the knife edge
    - Covered the exterior / air-side in foil, mindful of the knife edge.
    - Restored C3 parachute 

I could not easily thumb-and-finger off the Vert CPS BNC (Jim says they're notoriously difficult on the clean side).
I could not find the right allen key size to unscrew the GS13 D25 connector.

We'll pick it up on Monday next week (2026-06-08).

Pictures attached:
BEFORE AIR SIDE
BEFORE VAC SIDE

END-OF-DAY STATUS

J. Kissel, S. Koehlenbeck, J. Wright
D2400144
D2400145

For those who haven't been following closely, the SPI pathfinder was chasing what we *thought* were alignment issues -- during which we pointed our finger at the (Newport) pico-motor actuated (Siskiyou) IXM100 1" mount assemblies (both IXM100.C2 right-handed and IXM100.C2L left-handed). While this ended up *not* being the problem, we still took action on these mounts. This is the summary of that activity -- we figure worth a separate aLOG because LIGO has been using these pico-motor actuated IXM100 assemblies for a long time, and we're surprised that no one has raised this as an issue before. 

The issue: (repeat of what's burried in LHO:90382) 
   - "Out of the box" the Siskiyou IXM100 mount has two carbide plates that kinematically receive the manual alignment pushers on the moveable front-plate (item 1) of the mount. See item 7 in D1100362. 
   - Notice that v-grooved carbide plate, item 7, is called out twice, one for pitch and one for yaw, just installed in a different orientation such that the v faces the pitch drive and the flat faces the yaw drive.
   - This means the pitch and yaw manual drive pusher screws must be at different depths to achieve the "nominal" position of movable mounting plate. Not my favorite, but it's a non-issue with system "out-of-the-box" because the outer-diameter of 1/4-100 alignment screws clears the inner diameter of the housing carved out for the carbide plate in the movable mount plate.
   - HOWEVER this discrepant drive depth matters when we instead drive the plate with the Newport 830X-UHV, because its stopper nut (see it called out in 830X-UHV_RC_revA.pdf from E1000197) *sometimes* does NOT clear the moveable plate. And this *only* happens in the pitch drive, because Siskiyou didn't need to worry about the different depth of drive.

So, you get varying results of how close the stopper nut is to the movable plate depending on how it was fixed by the Newport manufacturer, potentially shorting the design-intended, kinematic, ball-to-v-groove connection. If it's the stopper nut making contact then you get circular-plate-to-flat-plate drive which can rock-around and is poorly-constrained enough to cause some unwanted cross-coupling to yaw drive.

See pictures of "BEFORE" of the two examples we were suspicious of:
    2026-05-29_M_B4_PicoPitch_BEFORE.jpg
    2026-05-29_M_B4_PicoPitch_BEFORE_02.jpg
    2026-05-20_M_M2_PicoPitch_BEFORE.jpg
    2026-05-20_M_M2_PicoPitch_BEFORE_02.jpg

It was recommended to us to "just move the stopper nut." Like I mentioned in LHO:90382, this was *not* a thing meant to be done often. With an allen key holding the pusher screw in place from the back, it required a great deal of clamping and twisting with pliers to get the nut to budge. Plus, given the 100 TPI thread, you gotta turn it a lot in order to get the nut to back off even 1/4 [in] (a few [mm]).

See pictures of "cranking":
    2026-06-02_PicomotorScience_StopperNut_Cranking_01.jpg
    2026-06-02_PicomotorScience_StopperNut_Cranking_02.jpg
    2026-06-02_PicomotorScience_StopperNut_Cranking_03.jpg
    2026-06-02_PicomotorScience_StopperNut_Cranking_AFTER.jpg

Once done, you can see an obvious and clear gap, see pictures of "AFTER:"
    2026-06-02_ISIKTransceiver_M_M1_PicoPitch_AFTER.jpg
    2026-06-02_ISIKTransceiver_M_B4_PicoPitch_AFTER.jpg
    2026-06-02_ISIKTransceiver_M_M2_PicoPitch_AFTER.jpg

J. Freed, S. Koehlenbeck, J. Kissel,

(Belated post)

Wednesday we continued from 90352, We installed the full rack for SPI in SUS R2 and made all connection between different SUS R2 racks as well as connections from TIA to DAC. (Front:IMG_6377.jpeg)(Back:IMG_6388.jpeg). As well as, hooked up to all the required power supplies. Still missing some connections namely SUS R2 to chamber connections 

The RF power measurement going out from the 2W amp to the SPI prep was measured to be 31.6 dBm for both outputs of the 2W amp (after the amp warmed up). 

This was measured by using the on site power mon w/ small power probe (Exact make and model will be added later). The site has a large power probe (the HP 8484A) but from 89523 it was found to have some strange nonlinearities. As such, the small probe was prefered for this test. Since the probe only has a 20dBm max power limit, 2x 10dBm attenuators (UNAT-10a) were used to lower the power to be able to be measured. Of course the UNATs have a 33dBm limit so special care was used to make sure that limit was not reached. This set up was calibrated by using a 0dBm signal which the probe measured as 0.1dBm. attaching the 2 attenuators the same signal read -20.0dBm (a 20.1dB difference). Then I measured the 2 ports which read 11.8dBm for the Meas output and 11.7dBm for the Ref output. Once the amp warmed up (~20 minutes of waiting with it on) these values both fell to 11.5dBm. Adding back that 20.1dB, this leaves both port with the expected output of 31.6dBm. With an error of +/-0.1dBm.

This value is 0.4dBm lower than the optics lab measurement with the larger probe but is a more precise measurement with less error.

The SPI wiring diagram D2400111 is labeled wrong. Namely it says Output 2 of 2W amp connects to the meas port of SPI prep. And Out 1 of of 2W amp connects to the ref port of SPI prep. This is wrong.  Out 1 of of 2W amp connects to the meas port of SPI prep (I physically labeled SPI_RFDist_008) and Out 2 of of 2W amp connects to the ref port of SPI prep (I physically labeled SPI_RFDist_009)

We also installed 3 BNC cables for the monitor channels of SPI Prep. These are not labeled in the wiring diagram but Jeff physically labeled them as SPI_LPMON_M1_PD, SPI_LPMON_M2_RFMEAS, and SPI_LPMON_M3_RFREF. Which monitor: the PD inside SPI prep, the RF power for the 80 MHz Meas signal, and the  RF power for the 80- MHz Ref signal respectively.

J. Kissel
(Belated aLOG)

After the great success on Tuesday 2026-06-02 solving the mystery with the MEAS IFO's efficiency drift (LHO:90455), we declared victory and spent the morning packing up all of the clean assembly to be ready for install.

We 
   - Weighed the mass of what components will be suspended on Stage 1 of the HAM3 ISI (see LHO:90504)
   - Packed up the optical fiber feedthrus (see Attached Pic #1)
   - Packed up the ISIK Transceiver and its in-vac cabling on a roll-able clean cart quadruple wrapped in foil (see Attached Pic #1)
   - Packed up the two D25 to D25 pico-motor and PD cables that span from Cable-Table-Bracket #3 to the HAM3-D6 feedthrough.
   - Powered down, disconnected, and packed up 
       . the laser prep chassis (see bottom of Attached Pic #3)
       . the transimpedance amplifiers (no pic)
       . the commercial RF source Keysight 33600A (no pic).
       . all in-air cabling, including mock-feedthrough

The fiber-coupled NPRO is still set up, but the laser is shuttered and key'ed off.

Thank you optics lab, and good luck trail-blazing to you, SPI pathfinder!
   

J. Kissel, S. Koehlenbeck

Here's the mass of the SPI pathfinder's ISIK transceiver that will be suspended on HAM3.

10.958 [kg] :: Fully assembled ISIK Transceiver (D2400107)
    - no installation handles
    - no baffle bracketry
    - sensor/actuator cabling draped to the side

0.898 [kg] :: Fully assembled portion of in-vac cabling that will be suspended on Stage 1 of HAM3 ISI
    - includes cable table brackets
    - excludes QTY 2x D25 to D25 signal bus cables that span ST1 to ST0 to Feedthru.

0.703 [kg] :: Fully assembled portion of in-vac fiber optic cabling that will be suspended on Stage 1 of HAM3 ISI
    - 0.570 [kg] :: QTY 2x will be suspended so expect 2x0.285 = 0.570 [kg]
       . 0.285 [kg] :: QTY 1x Fiber spool (what was actually measured, rather than two)
       . Most of the optical fiber, plus wont-be-there fiber mating sleeve and cap
    - 0.133 [kg] :: QTY 2x Fiber spool assembly feet and installation hardware: 

12.599 [kg] :: Total suspended mass.

All this mass will be on the -X side wall of the ISI, below the optical table, as shown in D2400103.

We have not measured the mass of any component of the ISIK shroud assembly D2400106.

J. Kissel, T. Shaffer, M. Robinson

While in HAM2 doing BnK hammering (LHO:90493), Mitch asked us to note the serial numbers of all parts of HAM2 ISI Table Baffle Assembly (D1700335).

  Location           Drawing        Serial Number
  +Y Assembly         
       Baffle      D1700263-v1    004
       +Y Bracket  D1700264-v2    020
       -Y Bracket  D1700264-v2    014

  Middle Assembly
       Baffle      D2600042-v1    001
       +Y Bracket  D1700264-v2    013
       -Y Bracket  D1700264-v2    015

  -Y Assembly
       Baffle      D1700263-v1    006
       +Y Bracket  D1700264-v2    022
       -Y Bracket  D1700264-v2    017

Happy ICS'ing!
(Will post picture proof in due time.)

When I hammered the below table "beard" baffle on May 26 (alog90335), something wasn't correct with the way I saved the data or my configuration or something else. Either way, Jeff and went in today and hit that as well as the SPI ISIJ assembly with and without the cylindrical shroud (D2500030). Preliminary results are looking much better, so I'll clean those up and post them soon with a whole writeup.

(Jordan V., Gerardo M.)

We have noticed that the dew point at the dryer towers has been increasing since opening to HAM7 and or related to power glitch last Saturday, we are still looking into it.

We have had a constant dew point at the dryer towers of -72 ^oC, and now we are at -57 ^oC, because of those numbers we ran a short test, once work was finished for the day in-chamber, we stopped the purge air to the LVEA to see what the effect was on the drying tower's dew point, it changed but very slow, we started at -57 ^oC and after 20 minutes we ended up at -59 ^oC, and we noted that the right tower may be in trouble since the dew point did not improved while this tower "works", the dew point instead went up by the one degree, erasing the improvement of the left tower.  We will look more into it.

To complete the test we took a dew point measurement at the point of use in the LVEA, and we got a -40.7^oC, getting close to a bad dew point, and remember this is while no in-chamber work is going on, so please mind those C3 soft covers on chamber doors, thank you.

Today started with removal of the HAM7 +X and -Y doors, WP 13283.  These were already on 4 bolts/piece, so removal was relatively quick.  Nothing to note other than the usual O-ring sticking business. The 2x 12" CF blanks on the +Y door were also removed for ISI locker access.  Since the final cleaning happened ~1 hour from door removal, particle counts were very minimally elevated, 40 @ 0.3um and 20 @ 0.5um (see pic).

We then moved on to the X-beamtube manifold vent work, WP 13284.  The X-manifold turbo pump was valved out.  We then cycled GV7 open, then hard closed again.  The GV opened at 10 psi, and hard closed at 20 psi, so we believe the piston O-ring replacement is working well.  Then, the X-manifold vent was started.  After burping the system a few times and confirming there was no communication past GV7, we started venting the manifold section.  We noted that the Kobelco's duty cycle seemed to increase audibly as the pressure in-vacuum rose, which isn't surprising given that we have 7 doors and 2x 12" ports open on the corner volume, so we elected to keep the vent slow, maybe 5 torr/minute.  We stopped the vent for the night at around 400 torr.  Tomorrow, we'll finish venting and open GV2.  

J. Kissel, S. Koehlenbeck [remote], J. Wright, M. Simmonds 

Here, we post an aLOG to cover the whole week's slog, picking up where we we left off at that end of last week (LHO:90332). 

%%%%%%%%%%%%%
Executive Summary: the remaining outstanding issue to solve before instal for the SPI pathfinder is the irregular and intermittent drift in heterodyne efficiency in the SPI's MEAS IFO (item (3) from the list of issues after finishing re-assembly up to D2400107-v5 after ECR E2600106). Importantly, we've seen this issue in the MEAS IFO even with the previous version of the assembly back in Mar 2026.

After a week of hard work, we've ruled out just about everything that it could be *except* for flaws in the alignment system of the picomotor-actuated IXM100 mirror mounts which steer the MEAS beam to the MEAS IFO (see D2400107 for the annotated layout).
%%%%%%%%%%%%%

I spend the rest of the aLOG summarizing what we've ruled out and how. Remember -- thru out all of these studies, the REF IFO remains a rock-solid high efficiency, and the power monitor PDs so the input power on the MEAS and REF is quite stable.
And a reminder for those not-so-versed in heterodyne interferometers, the primary metric is the efficiency, defined as 
    eta = [peak-to-peak beat note amplitude] / (2 * beat note mean)
(nominally 100% if the mode overlap between two interfered beams is perfect).
"Drift" of this efficiency means that -- without 

Electronics Issues with the IFO MEAS PDs.
    - Suspected cause: "maybe the bias voltage is drifting?" "Something's going on with the electronic ground because we're only looking at the positive leg?"
    - RULED OUT: we swapped the REF B PD (which had been measuring a rock-solid, high efficiency from the REF IFO) into the MEAS B PD position, and had it measure the MEAS IFO beam. The drift was measured by the REF B PD as well. This convinces us that "it's on the incoming interfered beam.
    - PLUS: The bias voltage is supplied by a single circuit for all PDs in the ISIK transceiver system, so the bias-related issues would be common to REF or MEAS PDs.

Spurious drive on the the picomotors
    - Suspected cause: "maybe the ambient electric field in the room is driving the floating pins of the picomotors, pushing the mirrors around?"
    - RULED OUT: At the start of the week, the picomotor signal chain was hooked up as designed from motor might-mouse to quadrupus to mock feedthru and out a ~6 ft D25 in-air cable that had been used for testing. 
       (a) Just like we'd already done with the PDs, we hooked up ever single picomotor pin to rack power supply ground via a breakout board and clip leads at the end of the in-air cable. Still see drifts. 
       (b) We also tried simply disconnecting all picomotors at their might-mouse connector. Still see drifts.

Clipping along the MEAS path into the MEAS IFO
    - Suspected cause: "If there's some sort of clipping anywhere, you lose efficiency. If it's just barely clipping, the clipper might be moving / breathing with the environment."
    - RULED OUT: 
       (a)Checked centering on M_M1, M_B4, M_M2, M_B3, and the two MEAS IFO PDs, and can confirm they're either
           :: On Optics -- offset in yaw by design (but still several beam diameters away from the optic barrel in REFL or TRANS), or 
           :: Well-centered.
       (b) We'd started the week with the M_M4 and M_M5 mirror mounts and adapters in place (with no dump in either). The beam comfortably goes through the transmission ports of both adapters. But, over the course of the week, as we ruled out "everything else" out, we removed them so now they're off the board. We still see drift.
       (c) Confirmed that "tight-squeezes" between beam dumps still allow for at least ~1x beam diameter of clearance (on an IR card).

Optics' Quality, dust and schmutz
    - Suspected cause: "If there's a scratch or schmutz on the optic, then maybe as the beam alignment drifts, the optical quality of the reflected beam is intermittently spoiled."
    - RULED OUT: We replaced all optics in the MEAS path with just-as-good, same production/coating run, equally class-A cleaned, high quality mirrors. Still see drifts.

Beam quality and Optical Mode-shape
    - Suspected cause: "We haven't found anything, so let's look at the beam shape on a profiler. This should be the definitive answer."
    - RULED OUT: We removed IFO_MEAS_B, and projected the beam on to a WinCam beam profiler head, and read it's data out in the WinCam software. We looked at the beam both with a very slow heterodyne beat-note of 250 mHz, and the nominal 4096 Hz (adjusting the modulation frequency at our commercial RF source's digital interface).
        (a) With the MEAS IFO's alignment just tuned up, Compared mode shape and alignment during episodes of high and low efficiency, and we see nothing but excellent beam shape. (In the slow-beat-note-configuration) the only difference in profile results is the expected lower intensity when the efficiency was low.
        (b) We deliberately misaligned the REF and MEAS beams going into the MEAS IFO such that both beams still visible on the WinCam profile, but not interfering. No movement of beams, no mode shape change, no beam diameter change, nothing that would cause the efficiency drift.

Polarization
    - Suspected cause: "The last thing that it could be if it's not mode shape / quality / alignment is polarization drift..."
    - RULED OUT: With the same IFO_MEAS_B beam, we inserted a polarizing beam splitter and monitored the s-pol power in the reflected port.  S-pol (max) power (at peaks of beat note) is stable at ~10-15 [microWatt] during periods high and low efficiency. The designed p-pol light at IFO is at the 5-10 [milliWatt] level, so the extinction ratio is awesome. And we still see drifts. This also rules out, e.g. the nightmare scenario that the optics' mount PEEK set screw -- only on the MEAS path optics -- are cranked on the optic so hard that is causes birefringence.

    - PLUS: if there was a board-level polarization drift, the REF IFO would also be seeing this same drift.
 
Optical Power fluctuations (Sanity Check)
    - Suspected cause: "I know you don't see any drift in the REF IFO, but ... is the input power drifting?"
    - RULED OUT: 
        (a) The REF IFO beam samples the same beams as the MEAS IFO, and
        (b) Yes, we've had the power monitor PDs up on the o-cope for days, and we see no obvious or reproducible correlation.

RF Modulation Modulation power fluctuations (Sanity Check)
    - Suspected cause: "I know you don't see any drift in the REF IFO, but ... did you check the RF electronics?" 
    - RULED OUT: Yup, we've had the RF power monitor outputs of the laser prep chassis on the scope most of the time, and these are rock-solid and at the expected level.

Environment (Insanity Check)
    - Suspected Cause: "The only thing that intermittently drifts on the minutes time-scales you're seeing is things like air currents and thermal effects."
    - RULED OUT: We tried SO MANY different combinations of 
        (a) Clean room Lights ON/OFF
        (b) Room Lights ON/OFF
        (c) Clean room Fans ON/OFF
        (d) People in/out of clean room
        (e) People in different positions in the clean room
      and none of these showed obvious or reproducible correlation.

Conclusion -- It's gotta be the mechanical assembly of the picomotors in the IXM100 mounts.
The picomotor-actuated assembly process was documented in LHO:87497 following the assembly procedure E2500163.
 
[1] The most accurate mechanical drawing we have the IXM mount itself D1100362
[2] The vendor drawings of the picomotor E1000197
[3] The most accurate assembly of a pico-actuated IXM mounts D1500494 and D2100433
[4] The SPI's drawings for its IXM mounts D2400144 and D2400145

Looking at the assemblies:
    - We're worried that the pico-motor "stopper nut" is pushing the movable-mounting plate (item 1 in [1]), rather than the pico-motor's ball bearing. And somehow this poor kinematic connection is drifting and slipping with the environment or something.
    - The v-groove of the item 7 in [1] carbide plate faces toward the pitch actuator, and away from the yaw actuator. While this is the right thing to do to prevent over constraint -- because the same part is used in both actuator interfaces, the ball of the pitch actuator (be it manual- or pico- driven) will sit "deeper" in longitudinal than the yaw actuator. The yaw actuators' stopper nuts all easily clear the movable mounting plate, but the pitch actuators do not.
   - The manually driven IXM100s show there's no issue, given that the cupped 8-100 alignment screw clears the carbide plate holes in the moveable optic-mounting plate.

ACTIONS FOR NEXT WEEK
We're at our wits end, and considering just ax'ing the picomotor actuation on M_B4 and M_M2. In fact, we've already replaced the mount of M_M2 with a manually driven IXM100, and may do so for M_B4 if we find enough clean IXM100 mounts.

However, we've got some ideas to fix the problem and still have remote actuation:
    - Place a washer between the carbide plate and the front plate.
    - Remove the stopper nut.
    - Mill away parts of the front plate.
    - Get a thicker carbide plate.
    - Use only one pico-motor per holder.

Pictures and further commentary to come in due time.

Mitchell and I placed the new central beard baffle with a cutout for SPI on HAM2. It went on without issue after Mitch reminded me how to adjust the panels.

I then attached the accelerometer to the L bracket and tried hammering in various places. There wasn't much real estate, so I'm not sure how this will turn out. Data still needs to be analyzed, and I'll post pictures of hit locations then as well.