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-MC2 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.

We started with yesterday's alignment. I checked flashes in ISS array PDs and didn't like that the balances between them were very much different from how they were when IMC was locked back in March.

Then we started changing IM2 and IM3 to see if we can get back, but I was appalled that the flashes became stronger and stronger on array PDs as we aligned, which probably means that the beam was almost falling off of the PDs when we started. 

That shouldn't have stopped us because we don't know where the beam was on the array PDs when MC was locked (people gave up optimizing that path because things didn't make sense), it could have been falling off of the diode (because each diode is 2mm,  QPD is 3mm, the beam size is ~250um or so, and the beam was almost in one quadrant of the QPD). But I started thinking about many what-ifs and wasted time.

After all, since IM4 TRANS and ISS array QPD are both reasonably close to March 2026 position, we know that the beam position as well as angle of the beam injected into ISS path from IM4 are already close to those when IMC was good in vacuum.

What we should do is this:

• Revert back to Monday IM2/IM3 alignment. Confirm that the IM4 trans position as well as ISS array QPD position are OK.
• Check flashes in 2nd loop inloop and out-of-loop sum. Recheck if at least one of them is comparable to when IMC was locked back in March, taking into account the power difference, even though Sheila checked it yesterday. At this point, we'll have established again that the position and the angle of the beam coming into the ISS path are reasonable.
• Check that the beam in ISS path is nowhere near being clipped by baffles and ISS array cover and the elevator aperture of the ISS array. Don't fix clipping right away, do it after swapping the array because we'll have to move pico mirrors anyway.
• Swap the array. There's no assurance that the beam will be on the QPD right away but that's OK. We can move the unit on the spacer if that helps. Go as close as you can, then start using the two pico steering mirrors. Use the array input aperture to aid the process.
• Once you see the beam on the QPD, continue to center the QPD, fix clipping, not necessarily in this order.
• Check the flash height of array PDs as well as QPD, see if things makes sense.

Other things to note:

This is not required for the alignment but it's helpful to know how much we can move the beam on QPD. In an unlikely event where we somehow lose the beam in vacuum on the array QPD after IMC is locked, we might be able to scan IM3/IM2 to regain the beam on QPD, and slowly move IM3/IM2 back to the nominal position while pico-ing so the beam isn't lost on the QPD. If the beam is not clipped, the beam displacement on the QPD is ~3mm per 1mrad of IM3 rotation using parameters collected from various documents, see attached script.

The same script shows you that the Gouy phase separation between two pico mirrors is 10 degrees (if we use the beam parameter in Matt Heintze's alog 12537, which was probably obtained by the measurement of the bypassed beam) or about 20 degrees (if we use IMC eigenmode parameter propagated to the ISS path using D1200693).

Another thing is, if we believe the design eigenmode number rather than the measured bypassed beam in Matt's alog, the beam waist will be smaller and at about 20cm upstream of the PDs. The beam size on the PDs is about twice as large as it should be (470um rather than 250um). I cannot measure the beamsize of the flashes so we won't do anything, but that's something to remember. If IM3 has enough actuation range, we can later lock the IMC, scan the beam across the PD until the beam falls off, see how sharp the fall-off is, and use the diameter of the PD (3mm) to determine the size of the beam. Maybe a good project for a fellow.