Tagging EPO for photos.

Note that the same glitches we had in the original installation (alog 8934) were still there. Quoting my alog from 2013,

it was still difficult to obtain good data because of some kind of glitches. It's not clear if it was due to NanoScan or the beam, the beam was well damped and was not moving on the viewer card, there was no noticable intensity glitch either. But the symptom was that the statistics window shows nice steady data for anywhere from one second to 30 seconds, then there's some kind of glitch and the scan/fit image looked noticably different (not necessarily ugly), the diameter mean becomes larger and the stddev jumps to a big number (like 10% or more of the mean, VS up to a couple % when it's behaving nice), and the goodness of fit also becomes large. Somehow no glitch made the beam diameter number smaller. I just kept waiting for a good period and cherry-picked.

We measured the beam radius using NanoScan at four points around the WFS sled (roughly WFSA position, roughly WFSB position, far field 1, far field 2). We used D4sigma numbers instead of 1/e**2 numbers. NanoScan outputs diameter not the radius, and the table below shows the raw number.

We assumed that the WFS position would be ~0.5" from the +Y edge of the WFS sled for both A and B. Distances were measured using stainless steel rulers and are relative to the 50:50 splitter on the WFS sled that also acts as the steering mirror for WFSa.

In all of the above measurements, "Profile averages" was 10, "Rolling profile Averages" was 3.

We also measured between M5 and 50:50 splitter for ASC-LSC split as well as between M2 and RM1. Numbers will be added to this alog.

We'll also measure the beam size at LSC REFL_B location on Monday before proceeding to POP path.

Here are some comments about the measurement process:

The beam profiler is difficult to use because the profiler head easily swivels once it is place. The swivel seems to be driven by the fact that the cable is very stiff and made stiffer by the addition of the foil so it is cleanroom safe. Several times today, I would pick up or set down the profiler and the head would swivel. I tried tightening the screw holding the post to the base, and I tried tightening the screw that holds the post to the head, but it is not tight enough to prevent swiveling. I found the best method was to line up the profiler in the designed location, and hold the head and cable in place while someone else ran the measurement. That makes this a minimum two person job, but there was enough juggling that having a third person was sometimes helpful.

When we went in around 3 pm to do the final measurement of the day I measured the particle count: 0.3u was 10 and 0.5u was 0. I used the standing particle counter on the +Y side of the HAM1 chamber- briefly unplugged to carry it over to the -Y side for the measurement. I didn't measure when closing up because Keita is heading out to do a few more tasks on HAM1. The handhelp particle counter isn't working, so we have to carry this large one on a stand around to use.

WFS sled is still excellent, 84 to 85 deg Gouy phase separation.

In the attached, four measurement points have error bars both in the position and the beam size but it looks negligible. There's no concern for WFS, it's good to go as is. 

However, just for the record, the astigmatism is bigger now (which is inconsequential in that ASC DOF separation is determined by the Gouy phase even if there's an astigmatism). The waist location difference is ~49mm now VS ~14mm or so before (just eyeballing the old plot from alog 8932) for a beam with the Rayleigh range of ~200mm. Not sure if this is the result of the AOI change or beam position change on curved mirrors and lenses, but I won't fix/correct this.

This morning we entered to do one more beam profile measurement. First Jennie and I refoiled the cable of the nanoscan profiler, since it was very stiff from multiple layers of foil. Then, before opening the cover to the table, I measured the dust counts by carrying the stand particle counter over to our working side like I did on Friday. The read was 0 and 0 for 0.3um and 0.5 um particles. I know it was working however, because as I carried the counter over outside the cleanroom it counted 19 each of 0.3 and 0.5 um particles.

Then, Jennie and I took one more beam profile measurement, this time on the LSC REFL path, after the final beamsplitter (M18). LSC REFL A (on transmission of M18) is placed on the table as in the drawing, but the LSC REFL B sensor (reflection of M18) was further away relative to the splitter. My quick rough measurement showed that LSC REFL B was about 160 mm away from M18.

I measured the distance of LSC REFL A to the front surface of M18 to be 128 mm. Then, I set LSC REFL B off to the side, and placed the profiler about 128 mm away from M18 on reflection of the splitter. We measured the beam profile, and then I re-placed LSC REFL B, this time at a distance of 128 mm to M18.

I have attached a very rough drawing of the REFL path and the locations where we made beam profile measurements. Each X on this drawing marks a beam profile measurement location. I also marked the Xs with letters A-G.

The measurements Keita reports above correspond the measurements C, D, E and F on this drawing. The difference between E and F, which is not depicted in my drawing, is a different placement of the temporary steering mirror relative to the sled.

 We still need to report details on the measurements for locations A, B, and G.

Beam size upstream of the WFS sled

Unfortunately this is preliminary.

We measured the beam size at 4 different location upstream of the WFS sled marked as A, B, C and D. D data cannot be used as there's no data/picture of D locaiton but that's fine as far as position A data is good. Unfortunately, though, the position A horizontal width looks narrower than it really is (2nd attachment). The beam might be clipping in the nanoscan aperture or there might be a ghost beam or bright background light in the Region Of Interest (ROI), or ROI is defined poorly, effectively clipping the beam. Must remeasure.

LSC REFL_B (and therefore REFL_A) beam radius is ~0.1mm, which is tinier than my preference, the diode is 3mm (in diameter) so the beam could be larger.  The diodes are placed close to the focus of the lens upstream (number 18 in a circle in the first attachment) so the beam won't move when the beam position moves on that lens. Moving away from that position will be fine as far as the deviation is much smaller than the focal length (~200mm). Rayleigh range is like 3cm or maybe smaller (0.1mm waist -> RR=10*pi mm),  it should be easy to double the beam size by moving the sensors away from the lens by a couple inches. We'll do this after POP alignment.

After everything is done we'll make a good measurement of distances between everything by either using a long/short ruler (preferred) or counting bolt holes or both.

Yesterday Betsy and I measured the distances between these optics:

• 856mm from RM1 to RM2, measured 827mm between the front D1000767 plate structure of each SUS and Don and Rahul measured on D1001396 the horizontal distance between plate and optic is 14.3mm for each RM1 and RM2.
• 895mm from RM2 to M5: Front of RM2 mirror to HR of M5 mirror
• 758mm from M5 to M6 (BS): HR of M5 mirror to front HR of M6 BS (-X side of M6)
• 468mm from M6 (BS) to 2" Lens on SLED: front HR of M6 BS (-X side of M6) to front side of 2" lens (0.5" thick lens holder, from siskiyou FOH 2", so add 0.25" if you want center of lens).

Camilla and I went back out today to redo the measurements at the locations labeled "A" and "D" in Keita's diagram. This table reports the D4sigma values, like Keita's tables above.

We forgot that we had left ITMX aligned, so the original measurements in this alog are no good. Keita and I remeasured these again today (May 6) and I am updating the table below with the new data. We also got two more measurements in new locations that are not indicated in Keita's diagram.

Leaving this older comment: It is difficult to measure these distances well with the ruler, so I would guesstimate error bars of a few mm on each distance measurement reported here.

Some new notes: when we reduce teh purge air flow, the measurements become much more stable and there is no need to "cherry pick" data as Keita discussed in earlier comments. Also, I think we have finally managed to tighten the screws on the nanoscan posts enough that it doesn't slide around anymore.

beam height measurements.

Swapped the coax cables on POP-X and it is working as expected now.

We did flash light tests on all detectors and were able to observe DC shifts on all detectors. LSC REFL_A and POP_A were swapped. This is a wiring diagram error and the in-air d-subs were swapped at the vacuum feedthrough.

We measured the current draw of ASC REFL_A and REFL_B, they are very similar to each other and close to the values measured in the test procedure.

Still no RF transfer functions for REFL_B.

Dust monitor 5 could use a power cycle, same with the diode rooms dust monitor. The counts are stuck, I restarted the diode room dust monitor.

Attaching a bunch of photos of the current alignment:

(I don't have the names of the optics on the SLED so I'm calling them by the order that the beam goes through them)

- Alignment from RM2 onto M5 (attachment1, attachment2)

- Coming from M5, through M6, and onto first lens on SLED (attachment3)

- Coming through first SLED lens onto second SLED lens (attachment4)

- Through SLED lenses and onto first steering mirror (attachment5, attachment6)

- Alignment onto SLED pico mirror (attachment7)

- Top view of pico showing that there is available range in both pitch and yaw (attachment8)

- Distance between L1 and front of LSC REFL A (L1 focal length is 222mm) (attachment9, attachment10)

- Distance from LSC REFL A to M18 (attachment11)

- Distance from LSC REFL B to M18 - needs to be moved closer (attachment12)

About REFL WFS sled:

• When the beam was centered by eyeballing on both the 2" lens and the 1" lens, the beam missed the last steering mirror (circled in red) but all other mirrors were without clipping.
• When the beam was centered on the 2" lens but was off-centered by about 1 mm in +Y direction on the 1" lens (again eye balling), no clipping anywhere but the beam was still closer to the edge of the last steering mirror than I liked.
• We used the 2nd corner mirror (circled in blue) to steer the beam back to the center of the last steering mirror.
• Moved the steering mirrors for the WFS heads to place beam dumps at convenient locations.

Tagging for EPO

Since after the July-Aug break, the low frequency response got closer to unity due to other mixed effects, we only apply the TF correction in uncertainty calculation before the break: 

From 1396796418 = Wed Apr 10 15:00:00 UTC 2024 to 1404864018 = Sat Jul 13 00:00:00 UTC 2024

After the break, we leave it to GPflow to take care of the unmodeled residual using monitoring data.

Although the low-frequency response got closer to unity at later times, we found that GPflow still could not sufficiently capture the unmodeled residual.

We now determine that the above TF correction should be applied thoughout O4b at LHO.

I've finished the set of test measurements for this latest set of filter files (where we now have the calibration filters in)
These tests were done with HAM5 in ISOLATED

Test 1: Baseline; classic damping w/ gain of Y to -0.1(I took this measurement after the other two tests)
start: 04/29/2025 19:22:05 UTC
end: 04/29/2025 20:31:00 UTC

Test 2: Classic damping w/ gain of Y to -0.1, OSEM Damp Y -0.4
start: 04/29/2025 17:16:00 UTC
end: 04/29/2025 18:18:00 UTC

Test 3: Classic damping w/ gain of Y to -0.1, EST Damp Y -0.4
start: 04/29/2025 18:18:05 UTC
end: 04/29/2025 19:22:00 UTC

Now that we have the calibration in, it looks like there is a decrease in the noise seen between damping with the osems vs using the estimator.

In the plot I've attached, the first half shows Test 2 and the second half shows Test 3

I analyzed the output of the tests for us to compare.

1) First attachment shows the damping of the Yaw modes as seen by the optical lever in SR3. We can see that the estimator is reducing the motion of the 2 Hz and 3 Hz frequency modes. This is most easily seen by flicking through pages 8-10 of the .pdf attached. The first mode's Q factor is higher than OSEM only damping at -0.5 gain, but it is lower than if we kept a -0.1 gain.

2) The second attachment shows that we get this by adding less noise at higher frequencies. From 5 Hz onwards, we have less drive going to the M1 Yaw actuators, which is a good sign. There is a weird bump around 5 Hz that I cannot explain. It could be an artifact of the complementary filters that I'm not understanding, or it could be an artifact of using a 16Hz channel to observe these transfer functions.

Considering that the fits were made on Friday while the chamber was being evacuated and that the suspension had not thermalized, I think this is a success. The Optical lever is seeing less motion in the 1-5 Hz band consistent with expectations (see, for example some of the error plots in 84004), with the exception of the 1Hz resonance. We expect this error to be mitigated by performing a fit with the suspension thermalized.

Some things of note:

- We could perform an "active" measurement of the estimator's performance by driving the ISI during the next round of measurements. We don't even have to use it in loop, just observe M1_YAW_EST_DAMP_IN1_DQ, and compare it with M1_DAMP_IN1_DQ.
The benefit would be to get a measurement of the 'goodness of fit' that we can use as part of a noise budget.

- We should investigate the 5 Hz 'bump' in the drive. While the total drive does not exceed the value for OSEM-only damping, I want to rule out the presence of any weird poles or zeros that could interact negatively with other loops.

Attached you can see a comparison between predicted and measured drives for two of the conditions of this test. The theoretical predictions are entirely made using the MATLAB model for the suspension and assume that the OSEM noise is the main contributor to the drive spectrum. Therefore, they are hand-fit to the correct scale, and they might miss effects related to the gain miscalibration of the SR3 OSEMs shown in the fit in 84041 [note that the gain of the ISI to M1 transfer function asymptotes to 0.75 OSEM m/ GS13 m, as opposed to 1 m/m].

In the figure we can see that the theoretical prediction for the OSEM-only damping (with a gain of -0.5) is fairly accurate at predicting the observed drive for this condition. The observed feature at 5 Hz is related to the shape of the controller, which is well captured by our model for the normal M1 damping loops (classic loop).

In the same figure, we can see that the expected estimator drive is similarly well captured (at least in shape) by the theoretical prediction. Unfortunately, we predict the controller-related peaking to be at 4 Hz instead of the observed 5 Hz. Brian and I are wary that it could mean we are sensitive to small changes in the plant. The leading hypothesis right now is that it is related to the phase loss we have in the M1 to M1 transfer function that is not captured by the model.

The next step is to test this hypothesis by using a semi-empirical model instead of a fully theoretical one.

We were able to explain the drive observed in the tests after accounting for two differences not included in the modelling:

1) The gain of the damping loop loaded into Foton is different from the most recent ones documented in the sus SVN:
sus/trunk/HLTS/Common/FilterDesign/MatFiles/dampingfilters_HLTS_H1SR3_20bitDACs_H1HAM5ISI_nosqrtLever_2022-10-31.mat
They differ by a factor of 28 or so, which does not seem consistent with a calibration error of any sort. But since it is not documented into the .mat files makes it difficult to analyze without ourtright having the filters currently in foton.

2) There was spurious factor of 12.3 on the measured M1 to M1 transfer function due to gains in the SR3_M1_TEST filter bank ( documented in 84259 ). This factor means that our SR3 M1 to M1 fit was wrong by the same factor, the real transfer function is 12 times smaller than the measured one, and in turn, than our fit.

After we account for those two erroneous factors, our expected drive matches the observed drive [see attached figure]. The low frequency discrepancy is entirely because we overestimate the OSEM sensor noise at low frequencies [see G2002065 for an HSTS example of the same thing]. Therefore, we have succeeded at modelling the observed drives, and can move on to trying the estimator for real.

_____

Next steps:
- Recalibrate the SR3 OSEMs (remembering to compensate the gain of the M1_DAMP and the estimator damping loops)
- Remeasure the ISI and M1 Yaw to M1 Yaw transfer functions
- Fit and try the estimator for real