This is a comparison of a few hours of consecutive locks with and without the offset on the pointing, given that the automation resets the offset after an unlock.

Fig 1: the known difference in the resonant power. It has been classified as an effect of a different absorption, becuase it is sistematically dependent on the defined position and not on the beam orientation or shift. I suppose that the higher optical gain is more likely correlated to the higher power and it is not an effect of a possible alignment improvement.

Fig 2: there is a general improvement of the BNS range, as expected because of a reduction of the mystery noise, but some other fluctuating noise seems to have an impact, because the difference between the two cases is less evident than the difference of the optical gain.

Fig 3: B1p and B1s level and fluctuation are not really different in the two segments. B2_DC is lower in the case of standard pointing: if I'm not wrong, it could correspond to a better matching of the beam on the arm optical axes. I don't see any evidence of an alignment improvement. In particular, the asymmetric fluctuations of B1p and B1s wrt a DIFFp_TY alignment monitor seems unchanged  (fig 4, fig 5). I would conclude that this horizontal asymmetry of the working point cannot be cured by a change of pointing on the arms or a shift of the beam.

It remains true that moving the spot on WE out of some excess of absorption produces a general improvement of the range. I would make a test also with a vertical offset, becuase I remember that there was an influence on the power also along this dof. An increase should be obtained by a negative offset.

This entry went back into drafts.

Yesterday (Jan 8) around 9:30 UTC I have put an offset of +1.5mm on WE and -1.5mm on NE with a 30 minute ramp. As the lock last night was not very long due to bad weather to confirm the effect of the offset on the optical gain and sensitivity stability.

Figure 1. Last night the offset of +/-3mm on WE/NE for sure increased the coupling of angular control noise. The noise between 20Hz and 40Hz increased between ~19:30 UTC and the unlock at ~22:00 UTC

At 18:14 UTC I have changed the offset ot +2mm on WE and -1mm on NE with a 20min ramp, to check if there is a bit of differential offset that we are putting in, as adding the offsets this morning has put the out-of-loop DIFFp signal based on the 491Hz line in B1 away from zero for TY.

Today (Jan 9) at 07:27 I put +2mm offset on WE with a 30 minute ramp, and no offset on NE. To see if in that case there is less impact on the B1 based DIFFp TY out-of-loop error signal. Last night test did not show much difference, but it was only a 0.5mm change in beam spot on NE.

13:55 UTC Put an offset of +2mm on NE with a 30 minute ramp

17:28 UTC put an offset of -2mm on NE with a 1h ramp

Figure 1 shows the data from yesterday when three steps of offset on WE/NE where done in a single lock. Changing the beam spot position on WE only at ~8 UTC clearly changes the beam spot seen on the B8 quadrant, increase the carrier gain (B4), changes  DIFF TY INPUT and moves away from zero DIFFp TY DCP HF mag. In contrary, the subsequent two changes in NE beam spot position at ~14 UTC and ~18 UTC did not have impact on neither of these DIFFp signals.

Moving the beam spot on NE also changes the carrier gain, with the common mode changes on NE/WE (which should corresopnd to opposite direction on the end mirrors) bringing the highest carrier gain (B4). As the beam spot doesn't change on WE, this seems to exclude the possibility that the change in gain is due to a point absorber on WE. Although it could just mean that we have also a point absorber on NE, and it just a coincidence that moving in the common mode moves the beam away from point absorbers both at NE and WE at the same time. Note that the dither signals are not being biased and missing some beam motion on WE, as the B8 quadrant confirms that the beam moves significantly only when the offset on the WE dither is changed.

So the conclusion so far is that the NE and WE beam spot should be moved in a common way, and that only WE beam spot affects the DIFFp TY error signals.

Figure 2 includes Jan 8 and Jan 9, with an unlock in between. On Jan 8 two other steps of beam spots position on NE/WE were made. It confirms that the effect on B4 DC is systematic, the impact on the BNS range is hard to judge as the weather condition change during those two days, with the interferometer unlocking at the end of Jan 9 due to bad weather.

Continuation of explaration of alignment offsets.

Around 8:20 UTC put an offset of +3mm on WE and -3mm on NE with a 1h ramp. Then around 13:50 UTC shifted PR X by +100um with a 20min ramp.

Figure 1 shows the trend of the data during the day up to the unlock. What is new is that the shift in PR X seems to have increased further the power in the arms, and at the same time decrease the power in reflection of B2. It might have also brought back the DIFFp TY signal based on height of DCP line in B1 back towards zero (which is moved away from zero  when the offset is put on WE), and reduced B1s. These effects need to be confirmed whether they are steady, or if they are just transient as it was during the shift on Tuesday.

AT ~14:30 UTC there was an unlock, and in the 30 minutes before there were several times saturations of B1 photodiodes that then create kicks in the SR and SDB1 alignment signals. It is unclear if this is due to the additional shift of beam spot on PR, or due to the sharp increase in wind speed that happened at the same time (starting around 14:00 UTC).

For the B2 DC power, it is always difficult to interpret if the power increasing/decreasing is a good or a bad thing. The interferometer (PR + arm cavities), is an over-coupled configuration. Which means that the dominant source of losses in the cavity is the PR transmission itself. In that condition when the losses in the arms increase, the inteferometer moves closer to the critically coupled configuration where the PR transmission and the effect of the losses is the same, in which there is no carrier TEM00 reflected by the interferometer. So in principle the power increasing on B2 is good, as it means less losses. However, if the beam is just misaligned at the interferometer input, then an order 1 mode is reflected, and the B2 power increases, and that is a bad thing. So depending on whether the power increases in the TEM00 mode or in the order 1 mode the conclusion whether we improve the working point is opposite. So unfortunately the B2 power itself is not a conclusive indicator.

In this case the change in input beam alignment makes the B2 power lower, after the power increased with the arm offsets, and the power in the arms keep increasing. So I would say the increase in B2 when putting offsets on the NE/WE beam spots corresponds to more order 1 mode reflected by the interferometer. And that this is compensated when shifting PR.

Today we have continued the investigation of NE/WE and PR offsets. First a 2mm common offset on NE/WE, then PR moved by 100um, then the NE/WE common offset increased to 3mm, then the PR offset removed, and finally the NE/WE common offset removed. The interferometer unlocked during that last step.

Figure 1. In this case the PR offset reduced the B2 power only by a small amount, and increased significanly the B2 power fluctuations at low frequency (much below 1Hz). The PR offset may have reduced the B1s power by a few percent. The increase in carrier gain and DARM gain from the NE/WE common offset was clearly present again.

Today we have move the end mirrors by 3mm in the horizontal direction in a common way. The displacement of the mirrors was done around 10:30 UTC when the interferometer was unlocked, and the mirrors have been returned to their normal position with a 1 hour long ramp starting from 17:35 UTC. In the middle of that tests the wind speeds were high preventing stable locking for several hours.

Figure 1 shows the trend of data during that test. On the first longer relock around 14:30 UTC, the power in the arms was higher by a few percent and so was the DARM optical gain. But this was not reproduced at the following lock. Looking at the time when the mirror position was returned in a smooth way from 17:35-18:35 there has been no change in arm power nor in DARM optical gain. This would mean that the change in power seen during offset changes on the mirrors is due to a point absorber on west end.

Figure 2-4 compare the beam camera images for three different times, figure 2 on Wednesday when the offset on the beam centering was put in place, figure 3 this morning at the normal mirror position without offset, and figure 4 in the middle of the day with the mirror moved by 3mm. The camera image barycenter was indicating that on Wednesday and today the beam was moving in opposite directions, but looking at the camera image the beam is moving in the same direction (once with the mirror and once by itself). What maybe confusing the image barycenter is the bright spot in the bottom left corner of the image which must be some reflection in the viewport, which changes brightness. So the mirrors were moved in the correct direction.

Looking at the camera images the aluminium markers on the baffle are much brighter on WE than on NE. This could be the explanation of why there are more issues with scattered light on WE, if there is more scattered coming there, for example from WI (or scattered light from WE reflected by WI back towards the baffle on WE).