The B4 phase camera has been misleading, indicating lower powers on B4 for both the 6MHz and 56MHz than in March.

Figure 1 shows the power in the last two days, with 0.05 on PC B4 56MHz, 0.075 on PC B4 6MHz, up to 0.024 on B4 112MHz mag and between 0.03 and 0.04 for B4 12MHz mag.

Figure 2 back in March the power on B4 112MHz mag was 0.026 and on B4 12MHz mag was 0.036, so similar to the last two days, but the PC powers were 50% higher, with 0.08 on PC B4 56MHz and 0.11 on PC B4 6MHz .

Figure 3. looking at the long term trend of the phase camera, all of the powers including carrier power decreased, the ratio is about a factor 1.4. So that would explain why the sidebands as measured by the phase camera decrease. For example the phase camera alignment changed, on the reference beam power changed, and it shows value that are 40% lower now compared to two months ago.

This morning the NNN NCal (Box 10 rotor R4-10) has been replaced by the NCal (Box 06 Rotor R4-17) which was installed on the NWN slot.

This NCal swap required the removing of the box top plate for the metrology target holder (in order to fit in the vertical support), as well as the swap of the bottom spacer( to provide the right twist angle).

During this change, it was noticed that the power supply channels for the NNN NCal motor (32 V channel 1) and North setup position sensors (5 V, channel 2) was broken. More specifically, it could not be turned off by the “output” button, and once the main power (red) button was cycled off/on, these two channels could not anymore supply any current. They have probably been damaged during the fast stop of the rotor that might have send back more current than the shunt regulator could stand. Therefore, the position sensor cable was connected together with the 5V photodiode cable, and the NNN motor power cable moved to the channel 1 of the NCal East power supply. However, the channels 3 and 4 of the North power unit are still working properly.

The four nominal NE NCals have been restarted with their usual configuration at 5:58 UTC.

This morning I have checked a possible workaround to the increase in B1s power during SR misalignment: https://logbook.virgo-gw.eu/virgo/?r=66910

Figure 1. During the lock acquisition of the OMC a check on B1s power is a critical trigger to close the safety shutters for unlocks during that stage of the lock acquisiton

Figure 2. However once B1 is used for DARM control then the trigger on LSC_DARM and the trigger on B1 DC power are much more sensitive than B1s. So the trigger on B1s becomes un-necessary

I have added two lines in DET_MAIN  (line 486 and 662) to change the weight of what is in the trigger between the case when the shutter is open and closed, and tested that it works:

2025-06-06-05h46m34-UTC>INFO...-AcSumChSet> fast_shutter_trigger - rampTime 1s, nStep 7, nb 6/7:  1 B1_PD1_DC_saturation, 1 B1_PD2_DC_saturation, 1 B1_PD3_DC_saturation, 1 B1p_PD1_DC_saturation, 1 B1p_PD2_DC_saturation, 0 B1s_PD1_DC_saturation, 1 fast_shutter_manual_close,
2025-06-06-05h47m43-UTC>INFO...-AcSumChSet> fast_shutter_trigger - rampTime 1s, nStep 7, nb 7/7:  1 B1_PD1_DC_saturation, 1 B1_PD2_DC_saturation, 1 B1_PD3_DC_saturation, 1 B1p_PD1_DC_saturation, 1 B1p_PD2_DC_saturation, 1 B1s_PD1_DC_saturation, 1 fast_shutter_manual_close,
 

I have commented it back out, as it is the wrong place to do it. We don't want to turn off this trigger as soon as we open the shutter, but only once B1 is used for DARM control. If we need again the work around for the B1s increase when SR TY is misaligned, then these lines should be added in ITF_LOCK, with the reactiviation of the B1s check in DOWN state, and the deactivation of the B1s check for example when we arrive in LN1.

This morning, the automation was stuck in ‘locking arms on IR’, even though the arms were already locked on IR. To recover the lock acquisition, I put ‘down’ and restarted the lock acquisition.

The sidebands in CITF were quite low (B4 12 MHz ~0.27 mW), so at 05:54 UTC, I undid yesterday’s last power increase on the WI outer ring. After a few attempts, the ITF successfully re-locked in CARM null 1f at 03.42 UTC.

I performed two different steps on the NI DAS:

1) NI IN power increase:  +400 steps (03.53 UTC)

-pick off: 0.76 → 0.774

2) NI OUT power decrease:  -200 steps (04.17 UTC)

-pick off: 0.774 → 0.76

The ITF reached LN2 at 4.51 UTC.

The ITF unlocked during ‘acquire LN3’ at 05:32 UTC, but not because of excessive power on B1s (see figure below).

Then, Francesco told me that work had started at the NE, so the ITF likely unlocked because of that (o_o).

Both ITMs should have an additional ZnSe viewport (DN63 - 57791 and 57772) with a much better viewing angle—similar to the one used for the photon calibrators on the ETMs. However, a taller tripod will be required for the thermal camera.

The shift was dedicated to the continuation of the ITF recovery and CH / DAS tuning (see report #66906). Activity concluded at 17:50 UTC, further attempts were performed by Was from remote (see comment #66910).
From around 19:07 UTC, after an unlock at ACQUIRE_LOW_NOISE_3, it was not possible to reach CARM_NULL_1F again.
ITF left relocking with AUTORELOCK_FAILSAFE Engaged.

DAQ
From around 18:15 UTC the cameras B1p, B1, B4_Cam1 and B1s2 became unavailable. Experts informed of the issue.

About 1.5 hours after the last DAS step I have tried to lock in LN3, with a MICH offset to make the sideband balanced, and with the gain of the SR TY loop to reduce the DCP divided by a factor 2 to have a slowe motion.

Figure 1 shows that the reduced SR TY gain did reduce the speed of the SR misalignment, but B1s still increased and crossed the 42mW threshold, it only reduced the speed of the increase. This would disprove the theory that the issue is a gain of a loop being too low, the issue is somewhere else.

Figure 2. Comparing to last night, when the power on B1s was slightly smaller during the peak, the peak occurs at the same relative SR misalignment of slightly less than 1urad. 

Figure 3 shows the B1s image during the peak, Figure 4 a few minutes after (with SR more misaligned) and Figure 5 a few minutes before. The SR misalignment amplifies some spurious beam on the right side of the main one. That spurious beam is odd, and maybe teaching us something important about the interferometer.

We could remove B1s from the trigger of the fast shutter once we are in LN1. As in that case the B1 photodiodes are a better sensor of unlocks, and so is DARM, and the trigger on B1s is no longer critical as it is during the lock acqusition of the OMC itself.

Didier’s observation of the bump in the gated strain channel is very important. It left me puzzled and I spent a bit of time trying to understand what was going on (and ended up rediscovering something pretty basic in signal processing...). I’m sharing some notes here, especially for anyone working on Continuous Wave searches who uses the gated strain channel or applies gating.

This bump is an artefact of the gating process, and doesn't exist in the original data.

Explanation: When we gate a time series x(t) we actually perform a multiplication with a time domain gate window w(t) to zero out certain parts of it. This can introduce sharp edges if we do it "the hard way", that is, with no tapering to smooth the edges: w(t) = 1 - rect(t), with rect(t) the rectangular window centered around each piece of data to be gated. These sudden changes in the time domain lead to broadband energy in the frequency domain: x_gated(t) = x(t) · w(t) -> X_gated (f) = X(f) * W(f) (convolution theorem). That energy can cause a sharp spectral line to spread into nearby frequencies. For example, for a rectangular window w(t), W(f) = delta(f) - sinc(f). So, if X(f) is a spectral line at frequency f0, X(f) = delta(f - f0), and X_gated(f) = delta(f - f0) * [delta(f) - sinc(f)] = delta(f - f0) - sinc(f - f0).  That means, instead of a sharp peak at f0, we get a main lobe and side lobes. With many lines this spreads into the observed "bump": figure 1. 

Using tapered gates (like by means of a Tukey window) helps smooth out the edges and reduce this leakage, but it doesn’t completely solve the problem.

I’ve attached a plot from a simple simulation with three sinusoids (at 38, 40, and 42 Hz) in white noise. The blue line shows the Amplitude Spectral DEnsity of the original data. The orange shows what happens when I randomly cut out sections using the "hard gating" described above. There, the bump is very clear. The green line shows the same with tapered gating using a Tukey window: better, but the bump is still there, just smaller.

I’ve also attached the script I used to make the plot: feel free to play with the number and size of gated parts and the window.

EDIT: fixed an error in the formula for the window function after feedback by Samuel.

MORNING/AFTERNOON up to 19.35 LT

To verify the lock acquisition in the cold state of the ITF, we kept the interferometer unlocked for one hour.
Julia performed the unlock at 07:35 UTC.
The sideband behavior was monitored for one hour by repeatedly locking and unlocking the CITF. It was observed that the sideband level gradually decreased as the ITF cooled down (see figure 1).
After several steps increasing both WI and NI CH powers, we were able to recover some sideband power and proceed with the lock acquisition (see fig.2).

The new powers compared to the standard ones (pre WE mirror replacement) are summarized in the  table below .

The ITF relocked at CARM NULL 1F at 10:24 UTC. However, the interferometer experienced multiple unlocks at different stages: one occurred at CARM NULL 1F (at 11.06 UTC) — shortly after I opened and closed the flip mirrors of the CO2 actuators to check the power individually — and two others took place during the LN2 acquisition phase (at 10.51 and 11.44 UTC).

With Michal, we understood that many unlocks during the night and throughout the day were caused by the saturation of B1s during the acquire Low Noise 3.
We hypothesized that the issue could be related to the last step of the NI outer ring, performed yesterday at 17:00 UTC.
As a result, this step was reverted in the CARM NULL 1F configuration at 13:19 UTC.

However, although the fringe became brighter, we tried  to reach LN3 but the ITF unlocked due to a glitch in the BS.

The ITF relocked in CARM NULL 1F at 14:57 UTC, and at 14:59 UTC I increased the NI outer ring by 5%.

At 15:33 UTC, the WI outer ring was increased by 14% (from 0.36 W to 0.41 W on the pick-off).

Following this step, we observed an improvement in the 6 MHz. We then attempted to reach LN3, but the ITF unlocked during the acquire LN3 phase due to B1s exceeding the threshold of 42 (again).

Then we relocked at CARM NULL 1F and I increased the WI outer ring by 12%  (from 0.41 W to 0.46 W on the pick-off) at 17.21 UTC.

The main signals to date are shown in fig.3.

The sign of the NCal lines subtraction for the lines recently added at 36.52 and 36.56 Hz have been flipped at 17:14 UTC. NNF frequency which was left by mistake at 18Hz instead of 18.04 Hz, leaving lines in h(t) at 36.00 and 36.04 Hz, was fixed at 17:26 UTC. (Thanks to Samuel Salvador for reporting these issues)

On June 4, between 20:00 and 21:00 UTC, a set of "checkhrec" injections has been done. A summary of the analysis of those injections is available in the VIM pages
Plot1: https://vim-online.virgo-gw.eu/resources_archive/2025-06-04/resources/20250604_cal_daily_checkhrec1.png
Plot2: https://vim-online.virgo-gw.eu/resources_archive/2025-06-04/resources/20250604_cal_daily_checkhrecunbias1.png

Plot1: We can see that the injections done on WE give hrec/hinj modulus values about 4% larger than those on NE.
We can suspect that this overestimation on the reconstructed h(t) comes from an underestimation of the gain in the WE actuator's model used currently in Hrec and that this model needs to be updated. I have done a Hrec reprocessing that will confirm this or not.

Plot2: We can see also that, after the online unbiasing, the hrec/hinj modulus for NE is about 2% lower than what we had for instance for the 15 mars injections:
https://vim-online.virgo-gw.eu/resources_archive/2025-03-15/resources/20250315_cal_daily_checkhrec1.png
https://vim-online.virgo-gw.eu/resources_archive/2025-03-15/resources/20250315_cal_daily_checkhrecunbias1.png
Here, the problem may come from a wrong model of the ITF optical response in Hrec. We should check the ITF condition (TCS status and SR alignment for instance)
during those injections. And, whenever possible, we will do optical response measurements for WE and NE and do again checkhrec injections.

Here are the images of the WI mirror surface (the one facing the WE payload) done with the Optris 640i thermocamera:

- Fig. 1 shows the WI mirror surface with the ITF unlocked;

- Fig. 2 shows the mirror with the ITF locked in CARM_NULL;

- Fig. 3 shows the temperature measurements of three diffrerent areas of the mirror (in CARM_NULL): the highest part of the mirror ("area 1") is hotter and has a temperature of about 20.3°C. In the middle, there are two cold spots and they measure around 19.3°C ("area 2"). The lowest part of the mirror has a temperature of about 19.8°C. I couldn't find any point absorbers present on the mirror. However, this seems strange compared with the last measurements done with the HWS in May 2023 (#60377).

- Fig. 4 shows a slightly different color bar range that gives a better visual perspective (CARM_NULL_1F). The temperature in the right upper side of the figure is the background mean temperature of the mirror surface and the temperature written in the middle of the mirror corresponds to the cold spot value. 

ITF found in DQSTUDIES with autorelock enabled.

At 7:35 UTC the commissioning crew started to work on ITF recovery and TCS tuning.

ISYS

The DMS was reporting a red flag for ML- PSL_ML_AC_FS. I informed Matthieu; he decided to temporarily shelve the flag.

SUSP

Two red flags on DMS:

• WE_Electr - SAT..WE_Sc_DSP_STATUS. The DspServerWE is reporting the following message: ERROR..-Tower WE - wrong filter in DSP #43157.  Configuration filter: /virgoDev/Sa/DSPCode_Adv/WE/LC/WE_PSDm.map. Running: /virgoDev/Sa/DSPCode_Adv/WE/LC/WE_PSDf.map
• WE_IP - Sa_WE_F0_COIL_H1 coil close to saturation

Experts have been informed by mail.

PAY

Temporary installation of thermocamera at WI

the etalon down condition for the NI and WI have been set back

I've hacked a Virgo DQR script to zoom onto those glitches over a few minutes -- see plot in attachment

• Top: the Omicron triggers (before time clustering)
• Middle: the standard Omicron clusters (after time clustering; what we usually call Omicron "triggers")
• Bottom: an attempt to make 2D (time-frequency) clusters

=> Most of the strong glitches (dark brown) are short in time and very wide in frequency.

After discussion with Benoit Mours, these new lines (36.52 and 36.56 Hz) are related to 2 new NCals started during the break and that are not well subtracted yet. NCal group is onto it at the moment.

Following the recovery of Low Noise 3 last night, a pair of loud spectral lines appeared around 36.5 Hz. These are in addition to the glitches and range instability already documented in entry  #66895.

Figure 1 compares the reference sensitivity curve of O4b (blue), the range observed at the end of O4c before Spring Break, the final range before the West End (WE) mirror replacement, and the most recent range from last night (purple).

Figure 2 provides a zoomed view of the 35-37 Hz region, where the new peaks at 36.0, 36.08 and 36.52 and 36.56 Hz are clearly visible in the pink trace.

This link provides the result of a brute-force coherence analysis performed using BruCo. The analysis reveals a clear correlation between the new 36.5 Hz structure and the Newtonian Calibrator (NCal) at the West End. See the corresponding figure for details.

We can suspect that these are additional lines from WE NCal that Hrec is not yet subtracting properly. If you take the channel Hrec_hoft_raw instead of Hrec_hoft_16384Hz, you can see that the amplitude of the NCal lines is similar for 31 March and 05 June, except the ones around 36.5 Hz (see attached plot1). Plot2 is same as plot1 but with higher resolution, where we can see the two lines not subtracted in Hrec. Plot3 shows that there is also a bump in this frequency band 35-38 Hz.

This could be a déjà vu of the line that appeared one year ago, in late May 2024:  #64376.

Today I dismounted the Optris 640i thermocamera on the east ZnSe WE viewport of the HR mirror surface (#66865) and installed it on the same viewport of WI tower (ZnSE). I contacted Federico who kindly came to WI base tower to verify the position of the magnetometer after the intervention. 

FWIW, online Bruco wasn't able to run overnight as it didn't find any long enough segment w/o strong glitch.

Daily UPV finds some SDB1 witness channels but they only account for a few percents of those glitches.

Figure 1. There is steady high rate of glitches in the mid-frequency band. I do not remember seeing such an issue in the past few years. It is possible that will resolve itself as the adjustment of the interferometer thermal compensation and controls are in progress, but it is not likely. Starting investigating what are the properties of these glitches should start in parallel of the on-going interferometer tuning.

Figure 1. The balancing of the sidebands has greatly improved, and even crossed zero, with the MICH INPUT needed to bring the sideband to be balanced and the PSTAB coupling in LN3 to be low crossing zero, and remainining close to zero (+/-10). However, the 6MHz sideband gain has decreased, as visible on the B4 phase camera, and on B4 12MHz mag (which measure the product of the upper and lowe sideband amplitudes, times the overlap between the two sidebands). It is not simple to see how the decrease is related to each step of the DAS, as the 6MHz sideband gain is also affected by the balancing, if there is a large misbalance not corrected by a MICH offset, then there are losses for the 6MHz sideband towards B1p which reduce the 6MHz recycling gian. 

The starting point for the 6MHz and 56MHz sideband gain was already low, and it has decreased further. Both sidebands have no 40% less power than a few weeks ago, as measured by the phase camera. 

Figure 2 shows the sideband powers on the phase camera yesterday, with the first half of the day in CARM NULL 1F, and the second half in LN3, and different state of MICH offsets in CARM NULL 1F affecting the power as seen on B1p.

FIgure 3 shows the same a few weeks ago during a recovery, with again the first half of the day in CARM NULL 1F and the second in LN3. I also checked that back in March the sideband powers on B4 were similar.

There are several possible direction to explain and solve this issue

• We have made a good tuning for one combination of the DAS but not the other. For example the differential tuning between the DAS is good, but the common tuning has become worse. This would mean contrary to expectation both DAS corrections need to be lowered.
• We have tuned the outer DAS but not the inner DAS powers. And the inner DAS powers need to be increased too, to compensate the higher YAG power.
• We need to tune the PR CHROCC, in principle the power absorbed by the POP has also increased, which would mean that the CHROCC correction needs to be reduced to keep the same lens in the POP. Or it need to be adjusted, because the PR CHROCC was turned off by mistake for a few hours last week https://logbook.virgo-gw.eu/virgo/?r=66864

Figure 4 looking at the temperature of the PR CHROCC it has returned to the same value. Which would mean the lens created in the POP by the PR CHROCC has not changed, but I do not know if that is reliable enough to be certain about that conclusion.

ITF found in CARM_NULL_1F, in UPGRADING mode.
COMMISSIONING mode set. ITF back in LN3 in stable way at 15:18 UTC.

At 18:21 UTC it unlocked (TBC). Relocked at 2nd attempt.
Under request of M. Was and according with the crew we decided to perform the usual calibrations with ITF locked.
19:52 UTC - CALIBRATION mode set
19:52 UTC - CALIBRATED_DF_SENSITIVITY started. Unfortunately ITF unlocked so the calibrations failed.

ITF left in AUTORESCIENCE_ON (upon reached LN3 It goes in DQSTUDIES).

A DAS tuning was attempted in CARM. NULL 1F with the goal of reducing the power on B1p, in order to continue with the lock acquisition in LN3.

STEP 1:

* The WI outer ring has been increased by 7 %

** In calculating the NI outer ring power increase, we took into account that 30% of the total applied power is required to compensate for the cold lens.

After this step, a slight improvement was observed in the sideband power and imbalance, but no significant change was seen in the B1p signal.

The ISC team attempted to reach LN3 by applying a MICH offset to reduce the B1p power to the level required for continuing the lock acquisition. However, a large offset (around 80) was needed, and the ITF did not remain locked (12.36 UTC).

Subsequently, the ITF relocked at CARM null without requiring any further tuning.

Two tuning steps were applied to the WI DAS: the WI outer ring power was increased in two steps, reaching 1.9 W. The last step  was completed at 13:55 UTC.

Approximately one hour after this last step, the fringe level decreased to about 0.025 mW (from an initial level of 0.045–0.05 mW). At that point, the ISC team managed to reduce the B1p power to a suitable level by applying a MICH offset of approximately 30, allowing the lock acquisition to proceed. The ITF successfully locked in LN3 at around 15:03 UTC.

At 17:00 UTC, while the ITF was still in LN3, I applied an additional step to assess the effect of increasing the NI outer ring power. The increment was kept small to minimize the risk of an unlock in case the tuning direction was wrong.

The final DAS power values injected into the ITF are reported in the table below:

The effect of the last step on the NI outer ring still needs to be evaluated.

The lock acquisition still needs to be verified. 

The mail signals during the tuning are shown in the attached figure.