After in-chamber crews physically moved the BS, I was able to restore both the IMC flashes and the sqz beam reflected by ITMX as well as ITMY in AS as well as ISCT1 REFL camera by just using the BS sliders. sqzbeam_as_isct1.png and the video PXL_20260625_204834048TS.mp4

show the sqz beam in the AS (left) and ISCT1 (right) camera when IMC was misaligned, while PXL_20260625_204905669TS~2.mp4 is the video of the IMC flashes when SQZ beam diverter was closed.

BS top suspension offsets were  H1:SUS-BS_M1_OPTICALIGN_P_OFFSET = 107.00 and H1:SUS-BS_M1_OPTICALIGN_Y_OFFSET = 441.00.

Right now F2 coil is using ~10% of the DAC range (~13.3 million counts out of 135 million), F2 ~ 6%, and others are small. See BS_DAC_range.png. I don't think further trying to relieve this will be fruitful or useful.

Adjusted the dark offset of OMC DCPDs in A0 and B0 filters and zero-ed A and B offsets (and disabled them).

I don't know exactly why people split these offsets in two different places, maybe that's because dark offset scripts was written when 512k ADC didn't exist?

Let's use A0 and B0 offsets for ADC rather than putting ridiculously small numbers in A and B.

We tried to diagnose the scattering or clipping or whatever that is visible inside the IFI output baffle aperture (alog 90536, especiall this video from that alog). It's not subtle, is always there even when the beam is blocked between PRM and IM4, and it's not just once in a while, it looks to be as frequent as strong flashes from IMC.

This is not the clipping of the forward propagating beam on the baffle as the beam height is pretty good (beamheight_dkdp_baffle.mp4) and YAW is also OK on dkdp baffle as well as IFI output baffle (dkdp_baffle_yaw.mp4, IFI_output_yaw.mp4).

We took a video of the back (i.e. -Y) side of the IFI output baffle through the output CWP by inserting a big dentist mirror between DKDP baffle and CWP and shooting from the +Y side. Video will be posted later (the raw video from IR sensitive camera exceeds 15Mb limit of alog and I don't have a good editor on my laptop).

Anyway, it seems like something is maybe hitting the bottom edge of the baffle from the back, Disha and Rahul think that there's something at the top too but I'm not sure.

We don't know what it is but we've done everything that could be done in situ without resorting to drastic measures (like temporarily removing all suspension baffles that block our view, which takes time despite that we don't know if that helps or not, or moving IFI to the lab which I won't do at this point in time). Since I'm pretty sure that this is NOT the clipping of the main beam as was noted above, my recommendation is to give it up at this point and move on, knowing that this thing does exist.

Together with alog 90536, we're done with HAM1/2/3 alignment today. Let's hope that the IM1 mystery motion won't return.

Fil already started ground check of suspensions, we'll do the ISS unit and the QPDs in HAM2/3 tomorrow.

I wrap/bagged/tagged all of the stuff still out in the West Bay Cartridge cleanroom and we turned it off.  I also unplugged the garb room.

No more need to stock that area.  

We will move the bags and finish cleanup after the vent.

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.

Belated, as usual.

Things make more sense, no railing of RM2

We continued HAM2/3/1 alignment. Things are making more sense now after realizing that RM1 and RM2 slider polarity was wrong but we're still finding things.

1st attempt: We rotated IM1 to relieve RM2 YAW (IM1 y -987->-687 i.e. +300 from Tuesday value), aligned PRM so the beam retroreflects (but didn't touch up other IMs in the interest of time). After some confusion it worked and we were able to center REFL ASC sensors without railing RM2. Happy. (See ASCREFL_centered_RMs_dont_rail.png. If you do the math using flash peaks, REFL_A PIT=0.034, YAW=0.013, REFL_B PIT=-0.0097, YAW= -0.017, so it was very good.)

2nd attempt: Encouraged, we proceeded to center the beam on the IFI output baffle using IM2, steer the beam to the nominal beam position in front of PRM using IM3, steer the beam to the nominal position in front of PR2 using IM4 (which moves the beam position on PRM by a small amount but we didn't bother to iterate), and finally aligned the PRM so the retroreflection is restored. We were able to center REFL ASC sensors without railing RM2. Happy again.  (eod.png, REFL_A P= 0.034, Y=-0.009, REFL_B P=-0.023, Y=-0.022.)

We looked at the beam position on IFO REFL baffle and it was off in YAW,  so we changed the baffle position slightly in +Y direction.

Large difference in RM2 DAC output for two attempts, 80% or DAC range used in the 2nd attempt

We didn't change IM1-IM2 line, which means that the alignment into HAM1 should not have changed assuming that the PRM was retroreflecting, but the YAW offset necessary for RM2 to center the ASC sensors in the 1st and 2nd attempt were very much different.

In the 2nd attempt, RM2 Y offset changed by negative 500urad and the DAC output of two of the RM2 coils reached ~107 million, which is about 80% of the DAC range. That's closer than I'd be comfortable with before closing down the chamber.

That's consistent with the retroreflection error 

It turns out that this is consistent with our retroreflection accuracy which I claimed to be "like +-50urad" in PRM rotation (not the beam rotation) in alog 90451.

Equivalent of positive PRM rotation of 50urad in YAW is replicated by the combination of physical negative 55urad for RM1 and physical negative 380urad for RM2 (note that the sliders for RMs as of now has the opposite sign as the physical rotation). 

If retroreflection is off in terms of PRM YAW rotation by +-50urad, when the REFL ASC sensors are centered, RM2 YAW would be off by +-380urad from what would be required to center the beam that is truly retroreflecting. See attached script. 500urad difference between two attempts is consistent with that.

On top of that, it's possible that +-50urad error estimate is too optimistic. We set the PRM angle by centering an iris (placed between IFI input and IFI HWP) to the forward going beam and centering the back propagating beam on the back of the iris. The basis for the accuracy is that we could start seeing how the circumference of the back of the iris is unevenly illuminated by the back-propagating beam when we gave the PRM 50urad offset, but the beam is always moving in YAW and sometimes rather slowly, so we have to eyeball the average beam position.

We don't know if the 2nd attempt was closer to the true retroreflection or, for that matter, if the true retroreflection is "on the other side of" the 2nd attempt relative to the 1st.

As such, it's prudent to relieve RM2 YAW offset further.

Tomorrow

Relieve RM2 further by giving IM1 a positive 300urad YAW rotation. IM2, IM3, IM4 and PRM should be readjusted accordingly.

IM1 YAW offset hasn't changed much in the past 2 years except for this week, and we'll go back to that neighborhood. Even though this means that the beam on IFI input baffle will be off-centered (probably it's been like that for years),  I'm absolutely sure that the forward-going beam won't be clipped, I'm also quite sure that the clipping-like thing of the IFO REFL beam on the IFI input baffle won't come back as far as PRM is retro-reflecting.

We might have to move the IFO refl baffle again.

We WILL move the IM4 baffle because whenever the beam is aligned to the nominal position in front of PRM the beam is too close to the +X edge of the baffle.

Good thing:

In the morning, we placed an iris placed between the IFI input baffle and the IFI HWP baffle (irisposition.png). We used an IR sensitive DSLR as a viewer to center the iris for the forward-going beam, and observed the back side of the iris using the same camera to see the PRM reflection.

This allowed us to set the PRM angle in much better accuracy than using a card with a bigger-than-needed hole handheld at an inconvenient location and observe with naked eyes through the laser glasses. We had to close down HAM3 cover as well as one side of HAM2 so PRM quiets down, but the retro-reflection accuracy was like +-50 urad (rather than +-500urad of yesterday).

With this new PRM alignment ([P, Y]=[-1165, +750]), I was able to center REFL sensors despite that RM2 was so close to railing, RM2 YAW offset was -1800urad. (ASC_REFL_balanced_20260602_morning.png and ASC_REFL_balanced_but_RM2_close_to_railing_20260602_morning.png.)

Nice thing was, at this point the beam was much closer to the center of IFI input baffle, IFO REFL beam clipping-like thing observed on the IFI input baffle was completely gone as well as the clipping on IM4 baffle. So things looked better in general than they were yesterday.

Bad thing:

RM2 was so uncomfortably close to railing nobody should leave it like that before pumping down, so I decided to relieve RM2 by tilting IM1. (Calculation is in alog 90404.)

I thought RM2 should be rotated counter-clockwise seen from the top because that would mean the positive YAW offset, we added negative 100urad to IM1 YAW offset (i.e. from -887.1 to -987.1 urad) based on my calculation, expecting to shave off 300 or 400 urad from the RM2 YAW. We reset the iris position for the forward-going beam, then reset the PRM angle, fully opened the iris and started centering the REFL sensors. Disappointingly, we couldn't center them, RM2 railed.

What was wrong:

We wasted some time trying to somehow find a magic slider values for RMs and going through how the angle of IM1 propagates to RMs, but it turns out that RM alignment sliders seem to have the opposite sign than all other suspensions. As far as we believe that the OSEM sensor (not actuator) produces positive DAC output that is proportional to the number of photons, the conclusion is that the sign of the sliders is wrong for both RM1 and RM2.

This is clearly shown in wrongsign.png where slider offsets of RMs were zero-ed one by one (top row) while the OSEM sensors in Euler bassis (bottom) were observed. Ignore the cross coupling from P actuation to Y sensing. In a suspension that follows the LIGO sign convention, the slider and the sensors move in the same direction roughly by the same amount. RM is the opposite of that, sensors move in the opposite direction of the sliders. We have done the same test for RMs, PM1 (see PM_is_good.png), OM1 (because they're all tip-tilts) and IM1 (because we touched it today) and confirmed that RMs are an exception.

If you're not sure about the rotation convention of LIGO optics, positive YAW rotation means counter-clockwise seen from the top. This will increase the light power on RIGHT OSEM sensors and decrease it on the LEFT. Given that the ADC reports positive voltage (i.e. positive counts), and given that usually OSEM YAW signal is proportional to voltage(RIGHT)-voltage(LEFT), positive physical YAW rotation will produce positive OSEM YAW change.  See my cartoon (LIGO_YAW_sign_convention.jpg).

Now, RM1, RM2 and PM1 sensing look identical to me (osemsensors_look_good.png).

All OSEMs produce positive numbers for all three suspensions, there's no sign flip in the filters, filter output sign is only determined by whether or not the input is bigger or smaller than the mid-point offset, they all use identical OSEM2EULER matrix as well as SENSALIGN matrix, and the sign of DAMP_P as well as DAMP_Y are all according to the filter outputs and these matrices. So the sensors for RM1 and RM2 are working in the same way as PM1, they're fine.

The conclusion to me is that somehow the actuation sign is wrong for RM1 and RM2.

Rahul told me that the magnet polarity was wrong for RM1 or RM2 and people spent some time to address that issue. But that sounds like the actuation side issue which should NOT affect the validity of the sensors. Unless people tell me that somehow the physical UL/UR/LL/LR sensors are wired to the LR/LL/UR/Ul channel or something, we just believe the OSEM sensors and assume that the sign of sliders is flipped.

Tomorrow:

Regardless of the reason of the sign flip, we'll proceed to move IM1 in the opposite direction, i.e. somewhere between this morning and Friday. 

FYI below is the history of IM1 YAW offset in the past week. The change made on Monday was motivated by the "need" to relieve RM2, but now that we know that the PRM retroreflection accuracy matters, I'm not sure if it would have been impossible to center REFL ASC sensors on Friday if we used the iris technique to align PRM. 

Belated alog from Friday.

Summary:

After realigning IMC and IM2 on Thursday, we continued aligning IM3 to put the beam in front of the PRM to the nominal position, IM4 to set the beam position in front of PR2, and PRM so the beam retroreflects. IM1 was left untouched, so the only change in IM1-IM2 line is from the change in the IMC alignment.

Clipping on IM4 baffle (HA12) disappeared completely. Beam position on IFI output baffle looked OK. Beam position on IFI input baffle didn't change (as expected) and still looked OK. There are some mystery 

We saw flashes on ASC REFL sensors but they were weak. We tried to center them using RMs but RM2 seems to rail in YAW.

What to do:

Scan PRM to make sure that we're NOT looking at some kind of ghost beam on ASC REFL sensors.

Assuming that we're looking at the real beam on REFL sensors, we can do either (or some of) these:

• Relieve RM2 by rotating RH9 (the fixed IFO refl pick off mirror in HAM2) by positive 500urad or so in YAW. This is the equivalent of the combination of negative 0.53 mrad for RM1 and negative 2mrad for RM2 in YAW. After doing this we can add 530urad to RM1 and 2000urad and continue centering from there. See attached cartoon (doesn't scale but the sign of things is correct) and the script.
• IM1 is almost degenerate as RH9 as far as relieving RM1 and RM2 is concerned, so we can rotate IM1 by negative 500urad or so in YAW. This requires that we tweak IM3, IM4 and PRM again but has an added benefit of moving the beam spot on the IFI input baffle closer to the center. See IM1rotation.png.
  • If you aren't sure how IM1 rotation is translated to the IFO refl beam,  IM1rotation_straightened.png is a simplified and unfolded version. The beam going out of IM1 goes through CWP at the IFI input (deflected but it's not written in the cartoon for simplicity), is retroreflected and comes back to CWP and defflected, the deflection for the REFL beam is depicted here. Without IM1 rotation the beam goes along the black path. With IM1 rotation (red path), the red refl beam converges with the black refl beam where the distance from the conversion point to the CWP is equal to that from the IM1 to CWP. The angle change of REFL is the same with the input and the lateral displacement of red refl relative to black is ~(distance(IM1_CWP)-distance(RH9_CWP))*angle_change.
• Make the OSEM coil driver output impedance smaller for RMs to gain more range. 1.2k to 500 Ohm should be safe. 

IM4 (and IM4 baffle) beam position

This is a belated alog from Tuesday.

Starting point: Same alignment for MC123, IM123 as last Friday, i.e. the IM4 baffle NOT centered nor unclipped.

Without touching IMC and IM1/2/3, we rotated IM4 to steer the beam close enough to the nominal beam position in front of PR2, and measured the beam position horizontally and vertically relative to the known screw hole in front of PR2 and PRM.

The distance between IM4-PRM beam line as of now relative to the nominal PRM-PR2 center line was calculated to be 3.9+-1.1mm in -Y direction and too low by 2.9+-1.1mm. See measurement_cartoon.jpg for the horizontal position case. Height is derived in the same way. Since the beam position in front of PR2 is not grossly off, and since IM4 is much, much closer to PRM than PR2, you can use these numbers as the spot position on PRM, too.

From IM4baffle_20260526.jpg, centering the beam on PRM will correct the height on IM4 baffle (HA12). Horizontally it will within a couple mm or so from the center.

What we should do

As we're running out of time, give up the idea to understand what happened to the alignment during the vent before closing HAM2. We should recenter things whenever possible (but only when the IFO REFL path and POP sled path are not disturbed to the point we clip or lose the beam).

For IMC, 

• Since MC3 beam spot is great but MC2 spot is off by 3.6mm in -Y direction, center the beam spot on MC2 (i.e. move it by 3.6mm in +Y direction using JM3) but don't move the beam spot on MC3.
• This is a scaled-down version of the yellow-orange trace in the plot ("Rotation around MC3" case in cartoon_IMC_alignment_to_unclip.png)  from alog 90295, causing about 220urad angle change for the beam coming out of MC3 and will shift the beam by 3*3.6/11~1mm on PRM in positive Y direction.
• This is done by:
  • Measure the beam spot of MC1 transmission in front of MC2. Rotate JM3 until the beam comes to the expected position.
  • Measure the beam spot on MC2 reflection in front of MC3. Rotate MC2 until the beam comes to the expected position.
  • Use MC3 to roughly make the 1 round-trip beam (MC3 reflection reflected by MC1) go on top of the input beam near MC1.
  • Use MC1 to roughly make the 1-round trip beam go on top of the input beam near MC2.
  • Only use MC3 and MC1 to refine.

For IMs,

following is the table of slider offsets as of now.

Even though the physical PIT angle of the optics relative to the local vertical axis is arbitrary, it seems that IM1 is bringing the beam down on IM2, IM2 is bringing down the beam further on IM3, and IM3 is bringing up the beam on IM4 and PRM. But IM2 is twise as efficient as IM3 for changing the beam position on PRM. Besides, the beam is already coming down from MC2 to MC3 (about 10mm height difference over 16m, or about 220urad) and I don't know if it makes sense to use IM2 to bring the beam further down. It's worth redistributing PIT as well as YAW offsets to relieve big offsets.

However, note that ultimately IM1-IM2 line defines the IFO REFL path when PRM retroreflects. (Even if you rotate IM2, as far as IM1-IM2 line doesn't change the IFO refl beam won't move.) I won't touch IM1 as moving the beam on e.g. IM2 even just a few mm using IM1 (i.e. a few mm over ~1.8m leverarm) will result in a much bigger change for REFL path in HAM1, potentially risking yet another clipping or maybe the loss of the REFL beam. IMC alignment noted above will change the IM1-IM2 line, but that's basically the angle change of ~3.6mm/16m (i.e. an order of magnitude smaller than when moving IM1 to steer the beam on IM2 in a meangful amplitude). That's small enough it's hard to imagine that the beam will be clipped by IFO refl baffle nor the downstream optics.

So,

• Don't touch IM1 as the lever arm for IM1 to anything in HAM2 is much shorter than MC2-MC3 length and it will affect the IFO refl path in a meaningful manner. Don't try to center the beam on IFI input baffle using IM1, it's not too bad (see this picture from alog 90295). It's not ideal but we can live with that, IFI aperture is 20mm and the IFI input baffle aperture is 16mm.
• Before starting to move IM2/3, make sure that PRM is retroreflecting. Adjust PRM if not.
• Look at IFO refl on IFO refl baffle. If it's clipped, something is wrong, come out of the chamber and talk to others before proceeding.
• If IFO refl baffle looks fine, confirm that the flashing is visible in HAM1 LSC and ISC sensors.
• Look at the beam spot on IFI output baffle, if possible at all. Depending on how it looks, we could try something like below.

 Of course this is assuming that the slider calibration is correct, so take this as a qualitative reference to get the sense of sign of angle changes. Anyway, when this is done, the DAC counts for IM2 and IM3 will be smaller while the beam height on PR2 will be fine. 

After doing the above,

If centering HA12 (IM4) baffle is important, relocate HA12. I'll ask Rodica.

Make sure that PRM retroreflects. Readjust if not. Check IFO REFL beam on IFO REFL baffle, LSC REFL and ASC REFL sensors. 

Recenter IM4_TRANCE by pico. 

Realign ISS array (simply because it's easier to do it in air than in vacuum).

Homework

Think about POP sled path. Is it conceivable that we'll somehow miss the beam there because we change the beam spot position on PRM?

(I learned from T0900486 "IO Stray Light Analysis and Baffle Design" that the IFI input baffle is called HA3, IFI output baffle is HA6, the baffle right in front of IM4 is actually supposed to be a pair of HA12-a and HA12-b but there's only one baffle which I suppose is HA12-a, two-hole baffle for ISS array is HA11, and the last IFO REFL before the beam leaves HAM2 is HA13.)

Clipping on the IFI input baffle (HA3) might not be real

The beam spot on this baffle was OK before we did anything to IM1 on Tuesday (IFIinput_before.jpg). It's low and toward +X, but nowhere near clipping.

This baffle is right in front of the calcite wedge that deflects the IFO REFL beam away from the incoming beam path from IM2 (HA3_calcite_wedge.png). The lever arm from the wedge to the baffle looks to be an inch or so at most. Hard to imagine that the REFL is clipped while forward going beam is not, but the scattering goes away when I block the beam between PRM and IM4.

The reported "IFO REFL beam clipping" on this baffle is either because the PRM is not retroreflecting, or maybe it's some kind of ghost beam produced from the PRM reflection somewhere.

If we establish that the main IFO refl is NOT clipped when PRM retroreflects, we don't have to worry about this baffle too much (though ghost beam is still a problem).

We will have to bring a card with a hole to make sure that the beam is retroreflected as good as we can.

FYI, IFIinput_aftercentering.jpg shows the same baffle after we made a huge change in IM1.

We THINK that the beam on the IFI output baffle (HA6) is OK too but we need to check

We don't have any good view of that baffle so it's hard to assess, and we forgot to check it before making changes to IM123.

However, given how small the change was on IFI input baffle, we don't expect that it was very bad before. We'll have to revisit and confirm.

IMC beam spots

As of now, the measured beam position in front of MC mirrors are as follows this. For measurement points, see mc_beampos_measurement_cartoon.jpg. The height is pretty good for all. MC3 is great horizontally too. Beam spot on MC2 and MC1 are both shifted in -Y direction.  MC2 by 3.6+-1mm, MC1 by a couple +-1mm.

Horizontal positions were determined by covering half of the beam with a vertical hard edge (ruler etc.) and then measuring the position of the edge relative to the neighborhood screw holes using a small ruler, and then using the drawings (D0901088, D901089, D0901099) as well as other IO documents (e.g. T0900486) to figure out the nominal beam location. As an example of tedious work done, see ham2mc1.png. Due to the way it was done, we cannot determine the horizontal position of the beam much better than maybe 1/2 of the beam radius. I just put +-1mm error for all measurements. Height numbers were measured off of a ruler, the error bar (if any) is the difference between Rahul's reading and mine divided by two.

What if we move MC2 or MC3 beam spots (or both) to unclip IM4 baffle (HA12)

To get more sense of magnitude of IMC motion relative to the beam motion on IM4, I calculated how much the IMC alignment should be changed to move the beam on IM4 by 3mm in -Y direction (comfortably far from clipping but not enough to center) without moving IMs.

There are many linear combinations of the MC3 spot position and the angle of the beam coming through MC3 that will move the beam on IM4 by 3mm, so I just chose "parallel transport of MC2-MC3 line" (i.e. no angle change of the angle of the beam coming out of MC3), "rotate MC2-MC3 line around MC3" (i.e. no beam displacement on MC3) and something in-between ("rotate around MC2").

See cartoon_IMC_alignment_to_unclip.png (not to scale but the sign of displacement/rotation is correct along the entire path) and IMC_to_unclip_HA12.png (actual calculation). IMC is not the only thing that moves, we can also move IM2, but anyway. In the "parallel transport" case the beam will be move further away from the center of MC2 (remember it was already 3.6+-1mm in -Y direction to start with so the end result will be 6.8+-1mm in -Y direction). OTOH in the "rotation around MC3" case, the beam on MC2 will move by 11mm in +Y direction so the end result will be 11-3.6+-1=7.4+-1mm in +Y direction.

In all cases the beam will likely still hit the IM4_TRANS because the QPD (Excelitas C30845) has a huge 8mm active diameter, but it will likely be completely in one quadrant. So all of these will be bad solution if we believe that the IM4_TRANS position should be close enough.

Note that the "rotation around MC3" case will result in about 1mrad beam angle change on IM4. This needs to be absorbed by IM4 rotation by about 500urad to send the beam to PR2. 

It's also worth noting that IM4-PRM HR distance is almost the same as IM4-IM4_TRANS distance.

What if we fix the beam on IM4_TRANS?

Instead of IMC alignment, now let's think about the beam positions from the end point (IM4_TRANS).

Again, assume that we want to keep the IM4 TRANS beam position. We tried two different IMC alignment, and the beam was clipped on IM4 baffle (HA12) after bringing the beam back to the target IM4 TRANS position.

Moving the beam position on HA12 by 3mm in -X direction without changing the IM4_TRANS position means that we shift the beam position on IM3 by about 8mm. IM3-IM4 path beam angle changes by 4.8mrad counter-clockwise. This is an absolutely huge change. 

PRM should be moved by 2.4mrad, and 8mm on IM3 is already the radius of IFI output baffle (HA6)  so we'll be worrying about clipping there. There seems to be no solution where the beam is far enough from the IM4 baffle (HA12) edge AND the beam is on the same position on IM4_TRANS as in vacuum. 

As far as we assume that IM4_TRANS is trustworthy, it's very likely that the beam was clipping or at least very close to clipping on HA12 in O4.

However, if IM4_TRANS path moved after HAM2 was opened (i.e. somebody bumped something), IM4_TRANS position as of now doesn't mean anything. We have to at least grab and wiggle the steering mirror as well as the QPD for that path to make sure that nothing is loose. (I already did that test for MC2 TRANS, and they didn't move.)

[Keita, Rahul, Elenna]

Today I moved IM2 and IM3 to bring the beam back to our reference position on IM4 trans QPD and center it on ISS QPD after Keita's move of the mode cleaner mirrors in 90259. This requires some iteration back and forth on both suspensions.

Our desired positon on IM4 trans is P = 0.22 and Y = -0.06. On ISS QPD, it is centered, so P=0 and Y=0.

We are driving MC2 in length, so the mode cleaner is flashing, and there are bright flashes on the QPDs. I am pausing ndscope on a flash and measuring the height of the peak in the fast channel and calculating the pit and yaw position from each QDP segment.

Unfortunately, this still results in clipping on the baffles Keita notes above, so we will keep going.

At the nominal IM4 trans position, yaw is pretty well centered. I then moved IM2 to both edges of IM4 trans QPD.

I changed the IM2 yaw slider to -250.7, which brought the yaw position on IM4 trans to 0.85. This made the clipping worse.

I changed the IM2 yaw slider to 90.3, which brought the yaw position on IM4 trans to -0.88. This was still not good enough to fix the clipping on the baffle.

By making a very large move to IM2 yaw slider value of 710.3, this centered the beam in the IM4 baffle. This is a ~800 urad move according to the osems and slider. The IFO REFL beam is still clipped.

I undid the 800 urad move, so the IM2 yaw slider is back to -88.7 for now.

Keita and Rahul went out to measure the position of the beam on MCs 1,2 and 3. We think that we need to make a move of these three mirrors to see if we can unclip the beam on these baffles that way.

We want to note that the positive yaw move of IM2 corresponds to unclipping on the IM4 baffle, this is consistent with the beam motion observed in chamber in the -X direction. However, this is contradictory to the sign on IM4 trans QPD, which was moving to negative yaw when we did this move. We suspect that the segment defintion must be wrong somewhere.

Keita will say more later once we have a chance to analyze the positions in chamber.

To clarify, the slider values of IM2 and IM3 are left at the "end" positions on the table above.

One problem solved, two problems found.

Clipping on IFO REFL baffle is gone.

The problem of IFO REFL beam clipping by the last IFO REFL baffle in HAM2 (see alog 90251, especially this picture) seems to have been caused by a huge PRM change in the PRM alignment sliders made on Monday May/11/2026 past 1700 local time (that none of us knew/remembered/understood).

Once Sheila reverted PRM back to March/2026 alignment, changing the IFO REFL beam path (but not the alignment into ISS path), IFO REFL was not clipped any more even though it looked low on the baffle (IFOREFL_baffle.mp4). LSC and ASC REFL sensors on HAM1 ISI saw the flashes right away without any adjustment of RMs, and the flash peaks were already reasonable. According to Sheila,  compaing to a time when DRMI was acquiring in November:

• 2W into IMC vs 0.18W into JAC now (9%, the power into IMC is less than that due to extra losses expected for JAC)
• REFL A nsum 4000 counts then, 218 now (5%)
• REFL B nsum 3400 then, 204 now (6%)

All of these were a good sign.

Forward-going beam into the IM4 is clipped by the baffle in front of IM4, and the IFO REFL beam is clipped by the baffle at the -Y edge of the IFI.

Unfortunately I've found that there were two other clipping points that I must have missed yesterday using IR viewer. See clipping.png, sorry for a blurry through-IR-viewer picture shot from the distance. Go back to HAM2layout_annotated2.png to figure out which baffle is what.

There's a baffle in front of IM4 between IM4 and IM3, and the forward-going beam is slightly clipped at the +X edge of that baffle.

Also the IFO REFL beam is clipped by the baffle installed at the -Y edge of the IFI. I know this is IFO REFL because the bright spot goes away when I block the beam on PRM by a sheet of aluminum. I cannot quite tell from the picture which edge of the baffle is clipping, but I think it's also +X edge.

In the past, when PRM spot position was measured with full IFO, it was like a mm off according to Sheila, so it's hard to imagine that the beam was clipping on the baffle in front of IM4.

Changing IMC alignment (trial 1, didn't fix clipping).

We decided to back off the changes we've made  for MC1, 2 and 3 since Monday and revert back to in-vac March alignment. If we have to make a huge adjustment to JM3 to follow the IMC, maybe the beam coming from JM3 is suspect (which means that the MC2 trans path is somehow suspect too).

Before I did anything, the starting point was captured in alignment_2026-05-15_16-14-21.png.

MC1/2/3 was reverted just based on slider values and not using OSEM readings as per alignment_2026-03-18.png.

Then JM3 was adjusted in PIT and YAW to give the maximum transmission measured in IM4_TRANS_NSUM. Actually I didn't move JM3 as much as I expected.

That alone couldn't recover a good 00 mode flashing so I moved MC2 and MC3 to follow the input beam.  MC2 moved back closer to the position this morning.

After that I felt as if it was somehow easier to optimize further using MC1 as well as MC3, thinking that it would be a minor adjustment but somehow ended up moving MC1 by a large amount after iterations.

My end point is captured in alignment_202605151726_sliders.png.

Without IM3 and IM2, the beam was still on IM4_TRANS as well as ISS QPD, the flashes were good, so I looked at the clipping on the baffle in front of IM4 as well as the baffle at the -Y edge of the IFI, and they were still there. IFO REFL baffle was still not clipping.

Will have to think about what these all mean.

Good news is that we're done with the alignment of the ISS path. Pictures and details are to follow.

Then I reinstalled the last IFO REFL baffle right in front of the HAM2-HAM1 septum window. (The baffle was removed after marking the exact position using three temporary dog clamps on the ISI on day 1 because it was in the way, I was supposed to record that in alog 90158 but forgot. I'm absolutely sure that the position of the baffle was restored within 0.1mm of the original position.)

Bad news is the IFO REFL was clipped on that baffle. 00 mode flash isn't clipped that much but horizontal modes certainly are. That was not THAT surprising because the IMC alignment was/is not great (see my comment 90224). This looks to me that the uncontrolled degree of freedom of the IMC is wrong regardless of the reason why a huge alignment change had to be made to center the MC2 TRANS QPD.

Good news is the ISS path alignment is not impacted by that, at least greatly, because we made sure that the beam hit the right location of IM4 TRANS as well as ISS array QPD before we did anything in the ISS path. Note that the baffle clipping the beam is not in the ISS path.

In other words,

• the beam position on MC3 might have been poor but we used IM3 and IM2 to steer the beam INTO the ISS path such that the alignment there is reasonably close to in-vac alignment,
• when the beam coming out of MC3 is brought back to the good position/angle, we can bring the beam back onto IM4 trans using IM3 and IM2,
• if the beam is not found on the ISS QPD at that point, we can scan IM3 (and IM2 if necessary) further to find the beam,
• and use the first pico for the ISS array path to keep the beam on the array QPD while slowly bringing IM3 (and IM2 if that's touched) back to the good angle.

However, since we don't want to revisit HAM2 once we close it, I'd like to understand what's going on for the IMC/IFO alignment. 

Following yesterday's alog 90219:

• IM2 and IM3 were reverted back to Monday evening position just using the slider values.
• Using flashes, confirmed that the IM4 trans as well as ISS QPD were roughly at the same position as the Monday evening position.
• Elenna confirmed that ISS OUTER SUM flash peak minus dark offset was 0.031 with 200mW input. (INNER SUM was much smaller.) Back in March it was 0.062 with 2 W input. So somehow the alignment into ISS array used to be REALLY poor.

At this point I and Rahul checked the beam positions in HAM2. Some things to note:

• The beam was uncomfortably close to the holes in the last baffle before the ISS array. It might have been clipped back in March. See ISS_two_hole_baffle.png showing the flash on the right hole. That hole is the "exit" hole for the reflection of the last pico mirror. Left hole is for the incoming beam from the 1" lens (which is right behind the baffle in the picture) but we couldn't take a good photo of the flash in the left hole at the time.
• Once the beam clears the last baffle, the beam is not clipped by the array cover nor the input hole of the array elevator plate.
• IM4 trans beam goes to the first pico mirror cleanly without getting clipped, and it's not clipped at all on the 2" lens.
• IMC trans beam from MC3 hit the IM1 cleanly, clears the baffle after that (IMC_REFL_peri.jpg), and appeared to go to IM2 without getting clipped. Couldn't check IM2->IFI->IM3 but no reason to get suspicious either. 
• IM1 transmission was clearly visible using a card and the IR viewer, it was not clipped by the suspension cage with considerable marging and was hitting the steering mirror behind IM1. 
• IMC REFL comes out of the last (second) IMC REFL periscope cleanly, the beam is well centered on the baffle hole. See IMC_trans_after_IM1.mp4.

We proceeded to swap the ISS array unit.

• Rahul went inside HAM2 and disconnected the QPD and array PDs after powering down the transimpedance amplifiers. Note that ISS QPD TIA shares the chassis with the IM4 trans, so IM4 trans went dark for some time too. 
• The old array assy was separated from the ISS array spacer. The latter remained in the chamber, only the top assy was swapped, as was explained in alog 90158.
• The new array assy SN1202965 (S1202965) was put on top of the array spacer. We took time to double check that the bias voltage for QPD comes to the expected pin on the QPD connector before making connection because the connector could be rotated by 90, 180, or 270 degrees and we can ruin some quadrants. Array PD cables were straightforward, it went as planned in alog 90158.
• TIAs were turned on.

At this point we saw flashes on QPDs as well as array PDs right away. INNER as well as OUTER SUM flashes were both about 0.06 (in the old unit it was 0.03 for OUTER but INNER was much smaller).

We started trying to center the QPD using the first pico mirror. Since pico driver is temporary unavailable (IOT2L is moved away) Rahul turned the pico manually while Elena looked at the laptop screen to monitor flashes in the individual segments. We managed an OK job (arrayqpd_centered.jpg) and checked the beam spots again.

• The beam was very high on the left hole of the two-hole baffle (Rahul has a good pic), high on the right hole (right_hole_after_new_array_QPD_centered.mp4), and OK on the input hold of the array (array_input_after_new_array_QPD_centered.mp4).

The baffle height might not be the same as the lens height and/or the ISS array input hole height, but otherwise it seems that we're shooting down the beam from the 1" lens to the ISS array. We'll check if the lens, baffle and the array are all at the same height or not, and decide how to proceed.

Restored the alignment sliders for IM2 and IM3 back to Monday values ([IM2P, Y] =[765, -187.7], [IM3P, Y]=[-560.7, 320]).

Measured the flashes. Chose the flash that gave the MC2 trans sum maximum power, which in general agreed with IM4 trans sum max but didn't with ISS QPD nor array PD inner sum nor outer sum. 

Anyway, 

• MC2 trans: SUM=21; seg1=0.0027; seg2=0.0029; seg3=0.0024; seg4=0.0022
  • i.e. PIT=0.098, YAW=-0.039
• IM4 trans: SUM=0.0276; seg1=0.00876; seg2= 0.00760; seg3=0.00456; seg4=0.00526
  • i.e. PIT=0.25, YAW=0.07
• ISS QPD: SUM 362; seg1=67; seg2=13; seg3=13; seg4=271
  • i.e. PIT= -0.56, YAW=0.86
• ISS Inner: SUM 0.023
• ISS Outer: SUM 0.104

PIT and YAW numbers were after normalization divided by (seg1+2+3+4), not by SUM, so there's a small difference but that doesn't matter at this point.

IM4 TRANS and ISS QPD PIT and YAW are broadly in agreement with Jenne's alog 90206 i.e.

• IM4 Trans Pit 0.2 Yaw 0.05 (Monday)
• ISS QPD Pit -0.53 YAW 0.85 (Monday)

I haven't compared the ISS Outer SUM with March 2026 when IMC was locked with 2W. Somebody check please. We'll go into chamber and start checking the beam path.

I went to HAM3 to see that the MC2 beam position wasn't crazy. See the first photo, the beam is to the left (+Y direction) relative to the baffle on the HR side but the baffle itself is offset in -Y direction relative to the cage.  Green lines are extension of the EQ stop screws to guide your eyes.

I also opened the ISCT1 and moved the mirror in front of the REFL BBPD out of the way and directed the beam to IFO REFL camera (because I couldn't see the beam at all without moting the mirror).

While manually aligning the IMC, we found that somehow things are in such a bad state that the MC2 TRANS SUM decreases when IM4 TRANS SUM increases, and vice versa. Improving the IMC alignment using IM4 TRANS as well as IFO REFL camera made the flashes stronger to the point that we can see 00 mode once in a while. I was also able to see the beam in the ISS path. But MC2 TRANS was nowhere near centered. Attempts to resolve this by incremental changes failed.

We wondered if something behind MC2 was bumped and changed the alignment into MC2 trans. I looked at the path and didn't see anything obvious. The beam transmitted through MC2 was visible using a card and a viewer, it was not clipped by the baffle behind MC2 nor the BS for the beam dump. I could not see the transmission of the BS, though, it was too weak, so I cannot confirm if the steering mirror in front of the QPD was bumped or not.

Elenna started making big changes for JM3 in PIT (to make big YAW changes in the beam injected into IMC, remember that YAW and PIT are flipped between JAC and IMC, IMC WFS takes care of this but that won't help when you're manually aligning JM3) and moving MC2 and MC1 so that the IMC follows the input beam. Repeating this in YAW anShe successfully centered the beam on MC2 trans. 

At this point I looked at the MC2 beam position again, see the second picture. Apparently the beam moved in YAW by a few mm to the right (i.e. -Y direction).

Elenna will post which optic was moved by how much in which direction.

Jenne is now trying to put IM4_TRANS and ISS QPD beam position back where they used to be using IM2 and 3.

PSL power into JAC 200mW. Jennie was able to lock JAC without much problem.

Jenne put MC2 back to march 2026 (in-lock) alignment in PIT and YAW using osem (not the slider) taking into account ISI (HAM3) Rz. 

To see the fringes, we injected 600 counts 0.2Hz into H1:SUS-MC2_M1_DRIVEALIGN_L2L_EXC. IMC flashed right away but the transmisison fringe looked like a weak horizontal line. Moving MC2 alone did not make the fringe converge.

Jenne moved MC3 and MC2 (and MC3). Improved some but not drastic. Tried JM3, again improved, but not drastic. Jenne was able to see the MC2 TRANS SUM but it was awkward and slow. At this point, at least the beam was not a straight horizontal line any more, we saw HOMs both in PIT and YAW, sometimes even 10 and 01 and what might have been a dim 00 spot. According to Jenne, the maximum MC2_TRANS number observed (about 3 after subtracting DC offset with 200mW input) is about 10% of what we expect with a good alignment (300 counts with nominal 2W).

We need a better alignment to be able to see the beam in the ISS path, though, I couldn't see anything.

I wanted to see the PRM reflection on the ISCT1 REFL camera as it will make the alignment easier (because we can see the IMC transmission beam shape).

Inside ISCT1, the beam was making it to the BS for the 1F diode but was hitting the mirror holder of the mirror for the BBPD. I touched up the common steering mirror for 1F and BB (just downstream of the shutter) so the beam goes to the direction of BBPD (and of course 1F PD) but could not see the transmission of the mirror for the BBPD using a card and a viewer, probably because the power was too small. (We're injecting 200mW, MC2 transmission is a small fraction of that as of now, and the REFL path is supposed to handle 60W.) No beam was seen on the camera either.

On Monday we might temporarilly move the BS for the BBPD out of the way so we can see the beam on the camera.

Summary:

The problem of "Sense" pin of OMCA QPD1 short-circuited to the BHDS structure (alog 90029 from yesterday) was tracked down to the free "sense" wire inside the QPD enclosure touching the aluminum part inside the enclosure. We solved the problem by trimming the wire short.

Merely opening the QPD enclosure broke the short circuit temporarily:

We lifted the OMCA QPD1 from the BHDS while QPD2 is still attached to the BHDS and confirmed that the sense pin for the QPD1 is conductive to the QPD1 enclosure but not to the BHDS. As soon as we opened the back of the QPD1 enclosure, the short-circuit to the enclosure was broken. 

It seemed that the free part of the sense wire was a bit too long and bowed in the enclosure, allowing the tip of that wire to touch the inside wall of the enclosure itself (see Elenna's first image in her comments).

Cutting the sense wire short broke the short circuit permanently:

I cut the wire short such that it cannot touch the enclosure (Elenna's second picture), reassembled the enclosure and confirmed that there's no short circuit between the sense pin and anything else. 

Repeating the tests:

We put the QPD1 back on the BHDS and connected the cable to the transimpedance amplifier. On the oscilloscope, it was immediately apparent that the terrible 60Hz noise (which was 6.5V p-p) was gone (Elenna's 3rd picture).

We repeated the flashlight test, all segments responded with O(1)mV negative voltage (i.e. positive output minus negative output from the TIA amplifier was negative few mV maximum) while the dark offset was O(0.1mV).

We also measured the dark noise. It seems that all measurements for both QPD1 and QPD2 were limited by the noise of the TIA. Low frequency noise especially 60Hz and its harmonics varied from channel to channel, but in all cases it seemed that there's very little difference between the noise measured with QPD connected VS the noise without QPD. As such, I'll just show here a few examples. First attachment is the QPD1 segment 1 noise with the QPD attached, the second is the same thing but without QPD connected to the front panel of the TIA. They look identical. The third one is the QPD1 segment 4 (with QPD attached, not much different without QPD). The fourth one is the QPD2 segment 3 (with QPD attached, not much different without QPD).

There's no reason to suspect that QPD1 for OMCA is broken (nor QPD2).

Problem

We started checking the health of QPDs and were puzzled that all four quadrants of the OMCA QPDA are much, much noisier than OMCA QPDB.

Note, there's a naming inconsistency about the QPD numbering between D2200276 wiring diagram and D0981811 (see "cartoon version"). 

Cause of the problem

Anyway, the problem was tracked down to short-circuit of common mode noise sensing line for OMCA-QPD1 (pin 11 of DB25 inside the vacuum, pin 3 outside of the vacuum, see attached) to the metal part of the suspension structure (i.e. ISI surface and the chamber) which in turn is grounded to the lab ground.

OMCA QPD-2 as well as both of the QPDs for OMCB are fine.

All QPD segments (incl. OMCA QPD1) responded to the flashlight.

Dark noise test: OMCB QPDs looks OK, not sure about OMCA QPDs

We connected a dual-QPD trans impedance chassis (S1102832, which is the original version of D1002481) to the OMCB QPDs like this:

QPDs-DB25 assy - in-vac cable (DB25F-DB25F)  - feedthrough simulator - DB25F-to-DB25M-cable - S1102832 front panel.

We looked at the output of the TIA with SR785 when there's only ambient light on QPDs (not much), segment by segment. No segment was extra noisy or anything.

OMCA QPDs are a different story because of the 60Hz problem. 

We'll see if the problem could be solved by somehow reassembling the QPD or cutting the noise line. 

[D. Kapasi, S. Dwyer]

We can calculate the SR3 hot radius of curvature (sr3_roc_hot) from the HWS measurements [1] and readback heater power for SR3 [2].

• The average wavefront change, delta_W = (5.9 µD/W + 5.4µD/W)/2 * 1.9W. This is also called as the spherical power (delta_S since it is a differential measurement). delta_W = delta_S = 1.0735e-05 D.
• The spherical power is the inverse of ROC. Therefore, delta_S = 1/delta_ROC where delta_ROC = sr3_roc_hot - sr3_roc_cold (this is a positive quantity since we know that heating an optic (assuming uniformity) will increase the ROC).
• In diopter units, for a curved mirror we can write : D = 2/ROC. Therefore, delta_D = D_cold - D_hot (since ROC_hot > ROC_cold).
• Hence, 2/delta_ROC = 2/sr3_roc_cold - 2/sr3_roc_hot -> 2(delta_S) = 2/sr3_roc_cold - 2/sr3_roc_hot. Solving this gives sr3_roc_hot. 
• The factor of 2 highlighted above is independent of the factor of 2 used for double pass on SR3.

In our case - 
sr3_roc_cold = 36.013; % radius of curvature in m [3].
Therefore, 2*(1.0735e-05) = 2/36.013 - 2/sr3_roc_hot -> sr3_roc_hot = 36.027 m.

Sources
[1] alog 88413 - this give HWS values in µD/W.
[2] alog 88155 - gives the reported readback power and requested power for SR3 heaters.
[3] git issue #33.