ITF Found in CALIBRATION Mode and LOW_NOISE_3_ALIGNED State.
All times are UTC.
Hrec calibration activity ongoing (Verkindt). Concluded at 15:30. Hrec process updated and restarted on VPM at 18:02 (Verkindt, see #68509).
From 18:48 to 19:03 LSC noise injections (Inject_lsc.py) as requested by Pinto.
ITF left in COMMISSIONING Mode and LOW_NOISE_3_ALIGNED State.

19:03 UTC (20min) OMC EDB stable locked on order 2 mode. The goal was to check that there is no saturation of the photodiode when locked on a HOM due to some dither line. There was no saturation so this should not be a problem for the shift of this topic next week Friday afternoon. 

After that I have unlocked the OMC and restarted the slow scans.

Today, around 18h02 UTC, I have restarted Hrec, after updating the bias file from /virgoData/Hrec/Hrec_bias_202510_with50.txt
to /virgoData/Hrec/Hrec_bias_202601_with50.txt
This update was needed to take into account that SR is aligned, while the previous bias file, used in the last months of O4c, was done with SR not aligned
and introduced a wrong online unbiasing in Hrec (as can be seen on the attached plot).
and is based on checkhrec injections done between 15h00 UTC and 16h00 UTC.

I added a path to work with the squeezer in stand alone. I.e. with the main PLL not locked and only the CC PLL locked

This allow us to measure FIS with the HD, either reflecting the squeezing with the EQB1 delay line or with the SQB1 retroreflector. This state does not allow to use the FC. 

LOCKED_PLL_CC_STANDALONE locks only CC PLL, main and SC are not checked

SQZ_LOCED_NO_FC_STANDALONE the CC PLL is locked and also the squeezer. 

I checked a couple of time and I did not find any major issue

N.B as an additional safety, if it is expected that INJ unlcks often probably it is worth to open by hand the fast loop of the main PLL. 

Remember to restore before going in the standard path with also main PLL

The etalon loops have done several fringes since the restart of the loop. It is visible that:

- there is a MICH set and B1p increasing trend not correlated to etalon.

see plot 3, MICH set and B1p in time for locks >LN2. for MICH SET and B1p seems that there is an other very low frequency modulation. to be understood

- the best finesse asymmetry that can be reached is about 2.3e-3 (2*(B7-B8)/(B7+B8))

At 08:35 UTC, in the CITF configuration, we performed a differential step on the WI and NI CH powers.

The WI CH power was increased by 20%, while the NI CH power was decreased by 20% relative to their initial values.

The table below summarizes the current values.

ITF found in LN2.

The activity of the shift was: Improving the LN3 aligned configuration.

At 14:30 UTC Didier started to perform injections to check Hrec.

- Starting from LN2 this morning we went in LN3 aligned to try the new 200s timer added to smooth the transition while changing the filters. It worked smoothly.

- Then in CITF the Central Heating has been tuned with a differential step (more details in TCS entry) improving both the B1p DC and sidebands (plot 1). We relocked in Carm null 1f and no bursts were present

- Finally, we locked  in the new LN3 aligned configuration and increased the SR TY gain from 6 to 10 (saved in the ini file) in order to increase the accuracy of the SR alignment control (plot 2). The improvement has to be better assessed during long locks

- From around 11h to 12h Manuel started to inject noise (see dedicated entry)

I'm trying, as usual, to do a personal decomposition of the noise curve. I start putting some known components, which should be not far from the ones used in Michal's budget. I also put a manual copy of many large structures visible in the noise above the floor. Then I add a few parametric curves and I run an automatic search of the best parameters allowing a good fit of the sensitivity with those curves.

I did an attempt on the best data found yesterday (gps 1452475518, dur 800 s, 37.5 Mpc, DCP 430 Hz). The additional curves are:

- a constant in DARM, multiplied by a zero at DCP, to match the calibrated shape of Hrec - this is indicated as 'readout noise'

- a curve with a constant slope in DARM, multiplied by a zero at DCP - this is indicated as 'mystery noise'.

The slope is used as a parameter of the search together with the amplitude of the two curves.

The result, shown in fig 1, gives an astimation of the slope -0.679, very close to first estimation of 2/3. Unfortunately the fit clearly overestimates the noise around 90 Hz and 45 Hz, saying that the slope could be lower. In that case, the quality of the fit become worse at higher frequency, but there is no reason to rely too much on the method and conclude that no constant slope can reproduce the mistery noise.

Anyway, I tryed a different assumption: a constant slope in Hrec. I know that this is a strange assumption, because an optical noise should have a simple shape in DARM and the zero due to DARM response should be applied in any case. My assumption is done for its simplicity and correspond to the case of a noise with constant slope at low frequency and some cut-off in the frequency region of the DCP.

The result is shown in fig 2: it gives an estimation of slope -0.425 and it is very satisfying everywhere. It is so much satisfying also because I built the bumps and lines starting from this fit, so I arbitrarily covered some discrepancies, but I did not make any relevant change of the floor. The parameter search has been done keeping in the game also a mystery noise component with fixed slope 2/3: the result for its best amplitude has been 0. In the plot, the new curve is called 'stray' just to distinguish it from 'mystery'; the name is not a hint concerning its possible nature.

The second fit forces the readout noise to be a bit larger that in the first fit. If there is a precise indipendent estimation of the readout noise, which is lower than the sensitivity curve, the second model of excess noise cannot fill the gap and should be discarded. But there is also the possibility that the gap is due to another mystery noise, coming from a different source.

I tried to use the low-passed model of mystery noise also for different data. In particular, I was very curious about the very bad data collected before the TCS adjustment which removed the big problem on B1s. In fig 3, blue curves, some characteristic figures of merit of that working point are shown, compared to the ones obtained two hours later by a translation of WI DAS. The differences are a much darker B1s, with no change of B1p, and a higher range. The increase of the range looks like a big reduction of mystery noise (fig 4). The two curves are different also regarding the amplitude of some well kown structures normally associate to stray light. This is not surprising: always happened that more light coming out from the dark port creates problems of stray light and problems of sensitivity.

The application of the automatic fit to the bad data (before TCS tuning) gives an acceptable result (fig 5) and an estimation of excess (mystery?) noise 50 % larger than the one obtained for the very good data analysed in the first plots.

I will draw some conclusions from this analysis:

- The sensitivity we can observe with SR aligned is largely dependent on the working point tuning. The effect on the sensitivity of the diafragm installed on SR can be somehow covered by other changes and any attempt to make pre-post comparisons faces that difficulty.

- We should not exclude the presence of different sources of noise affecting the sensitivity in this complicate state of the ITF. If we want to evaluate the amplitude, the shape and the variability of the specific noise we use to call 'mystery' noise, it should be better to minimize the noises that can have more normal explanation, like the stray light. As the mystery noise, the stray light depends on HOM content in the dark fringe and can put some bias in our attempt to have a precise correlation between HOM and some 1/sqrt(Hz) shaped noise of different origin.

The 2026-01-15-21h13m56-UTC,  there was an issue in the 100MHz timing distribution used by the following Daq boxes

• SQB2 : SQB2_DBOX_down, SQB2_DBOX_up, SQB2_DBOX_LC and SQB2_DBOX_SBE 
• EQB2: EQB2_DBOX_01, EQB2_DBOX_02  and EQB2_DBOX_03

They have been reconfigured to restore the standard running conditions: 

• operations performed between 2026-01-16-11h01m05-UTC and 2026-01-16-11h06m45-UTC
• EQB2_QD, SQB2_LC and SQB2_SBE stopped at the beginning and restarted at the end
• As the ITF was in  going in ACQUIRED_LOW_NOISE_3_SR state, the IRIG-B  was OFF in the 100MHz-IRIGB distribution : it has been enabled  at the beginning and  disabled at the end
  • 2026-01-16-11h02m12-UTC    info     Command '100MHz-IRIGB:irig on' executed
  • 2026-01-16-11h06m19-UTC    info     Command '100MHz-IRIGB:irig off' executed

The attached plot show the time plot of  the timimg_error channel for the related Daq boxes 

The etalon fringes as the loop is turned and sweep through several fringes of the etalon effect in the input mirrors is modulating the BNS range.

Figure 1. Corresponds to the case of a high BNS range.

Figure 2. Corresponds to the case of a low BNS range.

The frequency noise projection is not playing a significant role. The DARM gain is different by 3% between the two cases so it is also not a dominant contribution. What varies to explain the sensitivity curve is the level of mystery noise in mW/rtHz units, they are different by ~20%, which explains most of the BNS range changes that are of 15% between the two cases. The BNS range varies less, because in the sensitivity curve there are also other noises than just the mystery noise, and these do not vary.

/users/mwas/detchar/toySensitivity_20260116/toySensitivity.m

The scans have continued during yesterday, but looking at them directly becomes intractable because there is too many of them. Instead I have made a simple analysis of each of the HOM temperature regions based on the limit of the figures in the previous logbook entry. That is for each 5mK region around the HOM take the peak value of the EDB OMC transmission, and the integral of the transmission over that 5mK region. The motivation for having an integral is that the HOM of higher orders become more spread, and the integral should be an approximation of the total power of a given order N from TEM_N0 to TEM_0N.

Figure 1 shows the time series of the mode peaks, the etalon fringes are clearly visible, especially for the order 2 and order 3 modes. But all of the modes are not synchronized as noticed previously. The order 2 mode maximum is slightly after the order 3 mode, while the order 9 mode is almost exactly in opposite phase as mentioned previously. The drop on Jan 14 at 22:00 is because the OMC scan was disutrbed and the HOM peaks were not inside the preselected temperature regions. 

Figure 2 shows the time series of the mode integrals. To compare it with B1s there is also the sum of the first 10 modes. The sum of the modes follow relatively well what is seen directly on B1s, but with more dynamic as the sum changes by up to a factor 1.6 while B1s changes only by up to a factor 1.35. This could be explained by the fact that B1s contains more than just the first 10 modes, and the modes from 10 and above are not affected by etalon fringes anymore. Some of the HOM change by much larger factors, for example the order 3 mode change by a factor 4.

The B1s etalon fringes are not in phase with the BNS range fringes, but looking at the individual modes the BNS range is higher for low power on the order 8 mode. I don't see a physical reasons for it, so I expect it is a coincidence. Also the BNS range is higher whenever the order 9 mode is higher. The fact that the order 8 and 9 are out phase could make sense, as they are both close to co-resonating in the arm cavities, so we could be oscillating between being close to the order 8 mode or order 9 mode. But how the etalon effect does that is puzzling.

/users/mwas/OMC/EDB_OMC_fast_scan_20260114/EDB_OMC_slow_scan.m

I found the ITF locked at LOW_NOISE_2 and the planned activity on "SQZ recovery" concluded without particular issues at 20:00 UTC.
ITF unlocked at 21:13 UTC (TBC). Relocked at first attempt at LN2 at 21:45 UTC.

After an inspection of the Det EE room, we found that the DDS5 generator was off. This was due to its internal power supply being damaged.  Consequently, since the DDSs are connected in series to the external Wenzell clock, all the downstream DDSs were without a frequency reference.

After replacing the power supply and resetting DDS5, all the PLLs started working again. This started around 2:00 PM local time.

After the recovery of PLLs. Henning managed to relock the squeezer with a MZ offset of 0V. I.e. a parametric gain of 2.8 and 8.8dB of generated SQZ. In this moment we are not able to generate more than 9dB of squeezing but it is already a good value for the optimization of the system.

After that we switched on the hardware below EQB1 in order to be able to move everything also from remote.

We controlled again also the SQB1 suspended bench

Finally we aligned the HD detector with the squeezed light reflected by the retroreflector. We measured 7.9dB of AntiSqueezing and 5.6 dB of squeezing. These value are slightly lower respect what we had in september 2022 https://logbook.virgo-gw.eu/virgo/?r=57124

In the next shift we will do the fine tuning of everything starting from the delay line. 

At the end of the activity we unlocked the 1064nm mode cleaner and closed all the shutter. We left the EQB1 stepper motor driver on.

The etalon scan going on since yesterday produces a large fluctuation of B1p_DC, apparently correlated to the finesse asymmetry (fig 1) - relying on the calibration of B7 and B8 photodiodes. The relationship is a very straight line: the edge seems quite far from being a minimum, so a good symmetry of the two cavities seems not accessible with our test masses. This fact was already observed during O3, but now the signal recycling amplifies the effect visible on the dark fringe.

The ITF noise follows the same line. In fig 2 the correlation between the finesse asymmetry and the BNS range is shown. Considering that BNS is also influenced by the DCP, which was non perfectly constant, a sort of normalized range has been computed, multiplying BNS and DCP/420. In fig 3 the corresponding correlation is shown: it seems a bit sharper. The same behaviour is visible correlating B1p_DC to the normalizad BNS, but it looks a bit more like an asymmetric ball.

Looking at the times of the DAS translation I have the impression that the spot that appears on the left becomes also visible on B1p.
Figure 1 shows with red circle where the spot on the left appears in B1p when there is also a spot on the left on the B1s camera.

Figure 2 is the same without the circle for ease of comparison with the following figures

Figure 3 is before the tuning where the spot on B1s is on the right, and there is no spot on the left on B1p.

Figure 4 is halfway through the tuning, when the spot on B1s on the left and right become more comparable and there is a hint of an increase on B1p on the left.

Figure 5 is this morning when again the spot was a bit brighter on the left of both B1s and B1p.

An interpreation of this is that the spot seen on B1s is real, but when it is on the right it is clipped before reaching the B1p camera which explains why it is only see on B1s.

A way of confirming it would be to test the DAS translation again, and move it so the spot on the left becomes very bright, and if it becomes bright on both B1s and B1p it would confirm it is real. One could also move the DAS up and down and check if spots appear below and above, but that may be less reliable as that may be clipped by the SR diaphragm

Figure 6 show the situation in LN2 from December, we actually already had a bright spot on the right, it was just not as bright. Comparing B1s to the other images, it looks like the top and bottom of the pattern is missing, which would makes sens as it would be clipped by the rectangular SR diaphragm. 

Figure 7 and 8 correspond to the time in LN2 in October when the OMC mode matching was mistuned on purpose. What is interesting there is that on B1s the pattern looks the same in both cases, but it is shrunk down a little when the OMC mode matching is mistuned, which makes sense as that changes the magnification of the telescope and where the beam waist is compared to the B1s camera.

Following the changes of SDB1 B5 QD2 offsets performed yesterday ( https://logbook.virgo-gw.eu/virgo/?r=68482 ), I updated the thresholds used in the DMS flag "SDB1_alignment".

DetMoni was restarted at 15h46 utc.

Last night the etalon loop has been switched on and the BNS range has been going up and down following the etalon fringes. One can try to estimate the mystery noise for that configuration, and compare it what was achieved in LN2 last year. Using for that the code in /users/mwas/detchar/toySensitivity_20260115/toySensitivity.m and adjusting the level of mystery noise by hand until the noise budget is roughly equal to the measured noise, this automatically includes the measured level of the DARM optical gain and the DCP pole, so the mystery noise level is set in units of mW/rtHz, and the adjustment is done by a factor compared to the mystery noise level measured in LN3 in July 2025.

Figure 1 shows LN2 last year in October, when the defect state of the interferometer was still good. Mystery noise is 1.7 times the LN3 reference level.

Figure 2 shows LN2 on the same day, when the OMC mode matching was mistuned which suprisingly increased the sensitivity. Mystery noise is 1.45 times the LN3 reference level.

Figure 3 shows LN2 in December, after the defect state of the interferometer has changed. Mystery noise is 1.9 times the LN3 reference level, at that time the level was no longer improving when the OMC mode matching was mistuned.

Figure 4 shows LN2 last night, mystery noise is 1.3 times the LN3 reference level.

So depending which reference time one uses the current mystery noise level is between 10% and 30% lower than last year. In all cases SR is well aligned with the mean DCP frequency between 415Hz and 425Hz. 

Figure 5 shows the strain data ASD for the preceeding four times for comparison. The difference between last night and the time in October with the OMC mode matching mistuned is small.

This morning I found the ITF locked at LOW_NOISE_2, it kept the lock for all the shift; the main work was the recovery of the squeezing system, the activity went on without problem and it will go on in the afternoon...

ISC
- 9:46UTC: NI etalon set from 19.29 to 19.00, 14400 sec ramp time.
- 9:46UTC: WI etalon set from 19.74 to 19.23, 14400 sec ramp time.

For consistency with the magnetic CF computed from sweep sine data, here attached are CF with low frequency lines computed using B field modulus measured by the 3D magnetometer in proximity of the NE and WE towers. For CEB the Bd. magnetometer (Metronix) is kept.  Plots compare these new values (red) with old values (blue) computed using magnetic field modulus from Bd. magnetometers. Uncertainties are computed as sigma = value / SNR.

Yesterday (Jan 14) around 21:00 UTC I have started continous scan of the EDB OMC, these are still ongoing. By mistake I have made them 4 times slower than OMC scans done in the past, so the location of the various HOM is not the same as a function of temperature. However, a good side of that is that there should be much less aberration created inside the OMC itself due to a thermal gradient due to rapid changes in temperature.

Figure 1-12 show the carrier HOM from 1 to 12, the color just split the scans into 5 groups as a function of time. Figure 13 and 14 shows the 56MHz TEM00 upper and lower sidebands. And finally figure 15 shows the carrier TEM00.

On figure 1 and 15, carrier TEM00 and order 1 mode, there is an oscillation superposed to each peak. These have always been present, and I expect that these correspond to alignment fluctuations. It also means that the TEM00 present on the EDB OMC is dominated by HOM converted back into TEM00 by alignment oscillations.

These measurements are done in parallel of the etalon loop being turned on a going through fringes, which are clearly visible in the power in the arms, B1p, BNS range, etc. On figure 3 one can see that on the black lines between 1:30 UTC and 3;00 UTC the power of the order 3 mode decreases by up to a factor 3 compared to other times. On figure 9 and to a lesser extent figure 7, one can see that the order 9 mode and to a lesser extent the order 7 mode increases at the same time as the order 3 mode decreases.

/users/mwas/OMC/EDB_OMC_fast_scan_20260114/EDB_OMC_fast_scan.m

today the SR_PAY on the DMS flag was red. By looking at the data the there was a slight misbalancing of the MAR actuation (LF coherence between Zcorr and TYcorr = 1). At 09.06 UTC the coefficient MAR_Z_CORR -> MAR TY CORR has been adjusted to improve the actuation balancing.

ITF found in Commissioning mode and in LN2 ISC and TCS activities in progress.

The activities of the shift were the following: WI DAS movement and Work on the lock acquisition robustness.

All the activities concluded at 19:40 UTC; ITF left in LOW_NOISE_2 and Autorelock enabled.

Software

ComputingDMS does not restart; to be investigated.

On the attached figure we can see the comparison between the various B1p and B1s photodiodes, in Low Noise 2, on Nov 4 and Jan 8.

It is interesting to note that while the power on the main B1s photodiode has increased after the SR diaphragm installation, the power on the B1sP photodiode (monitoring the P polarization) has decreased.