aLIGO LHO Logbook

H1 PSL (INS, IOO, ISC, PSL)

keita.kawabe@LIGO.ORG - posted 17:41, Thursday 14 May 2026 - last comment - 20:58, Saturday 16 May 2026(90251)

Done with ISS array alignment, not yet ready to go laser safe due to IFO alignment mystery (Rahul, Elenna, Keita)

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.

Comments related to this report

keita.kawabe@LIGO.ORG - 20:58, Saturday 16 May 2026 (90257)

Link

For location of things, refer to HAM2layout_annotated.png. Blue line = IMC reflection. Red line = IMC transmission. Orange line = IFO REFL rejected by the IFI. Green things are baffles (two-hole baffle and the last IFO REFL baffle are circled in green).

The alignment status at the start of Thursday morning:

The beam was very high on the two-hole baffles but was OK on the input hole of the ISS array. See alog 90237, see this picture as well as this, and this video. This just meant that we were shooting down the beam from the 1" lens toward the center of the array QPD.

Work done on Thursday:

We moved the beam down using the two pico mirrors such that ultimately the beam goes through the center of the input hole of the array and reasonably centered on QPD.

Detailed procedure was:

Use the first pico mirror until the flash is just seen in segment 3 and 4 of the array QPD (i.e. PIT=-1 if this were a stable DC light) and then use the second pico mirror to bring up the beam until the QPD was roughly centered. Measure the beam position on the baffle holes as well as the ISS input. Repeated until the beam comes close to 1/8" or so of the center of the input hole of the array.
Install the biggest array alignment aperture tool (D1300986, -03 variant with 2.4mm diameter hole) in the hole of the array. Use pico mirrors until no clipping at the alignment aperture tool is visible using IR viewer and the QPD is roughly centered at the same time.
Repeat with the middle sized alignment aperture (-02 variant with a 1.6mm hole). That is enough as the final adjustment before pumping down.
- We didn't bother to use the smallest alignment aperture (-01 variant with 0.79mm hole). Refining the alignment at that level for in-air initial alignment is pointless, the beam WILL move once things are in vacuum and the IMC is properly aligned and IMs adjusted. What ultimately matters at this point is to make sure that, once that in-vac adjustment for IMC/IFO is done, the beam could be found on the QPD. From there on we need to fine-tune to find a good ISS array alignment and minimize the jitter coupling.

Status after Thursday's work:

right_baffle_hole_spot.png (seen from +Y), 2nd_pico_spot.png (from -Y), 1inch_lens_spot.png (from -Y) and 2inch_lens_spot.png (from -Y) show the beam spot on various positions in the order of the distance from the array in the ascending order. You can see that the height on the right baffle hole as well as the second pico mirror are pretty good. Pictures of the lenses have significant parralax because of the distance between the card and the lenses and because of the camera height, but the beam doesn't seem to be centered on the lenses. I don't see that as a problem, it's fine as far as the beam is far from clipping.

We have found no unexpected behavior here, I was surprised that the process was easy and things made sense given the difficulty people had in the past to improve alignment of the array in vacuum with the old unit. That's probably because the beam was already clipping back then.

The only thing was that the YAW actuator of the second pico mirror didn't have much range to start with even before we moved anything. At some point it didn't hit its end of the range but was close (2nd_pico_position_before.jpg). Since we need a healthy headroom for adjustment both ways, I relieved the pico by mechanically rotating the pico mirror assy (2nd_pico_position_after.jpg).

IFO REFL beam was clipping on the baffle:

See IFO_REFL_baffle.jpg

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