Lowered CP4 LLCV from 35% to 34% open because TCs are reading around -30degC.

CP3 filled fast but TCs are reading temps as expected.

After a couple 3.5  hours it seems as though the power adjustment cleared the glitching.  Here is a link the SR3 summary pages; I've also attached the 2 relevant graphs to this alog.  The first attachment is a time series of the oplev SUM, where the glitching is clearly visible, as is my adjustment and the following cessation of the glitching.  The second attachment is a spectrogram of the SUM signal, normalized relative to the median.  Here it the start and end of the glitching can be clearly seen.  I will continue to monitor this over the weekend, but initially it looks like the laser power adjustment cleared the glitching for now.

Andy ran Bruco on this already: https://ldas-jobs.ligo.caltech.edu/~lundgren/aDQ/bruco/H1_1168346418_OMC_DCPD/

Josh, TJ, Jess, Alex

This looks like PI ringing up. In the spectrogram posted above there is a line at 4734.75Hz that grows around the time of the noise. Here are some plots:

Fig 1: the line at 4734.75

Fig2: a BLRMS of this line vs time it grows alot.

Fig3: 8.8 hours of the line growing from onset of noise to really ratty time. 

We will work harder to tie this to the noise. We've started looking back at earlier pages and seeing this line rising around times that noise rises but have work to do to tie them together. 

There are three things that we can try to improve or investigate the drop in range as Patrick relocks.

• We will stop to damp the violin mode harmoinc that Josh noted above (30281 for solution to this problem in the past, note that notching this in the DARM loop before damping it was important).  Jim W and Patrick are currently working on this.
• Based on the BRUCO that Laura linked above it looks like we have higher coherence with things related to SRCL/MICH/PRCL coherence than in the past, so we can try some quick injections to check that our feedforward is still well tuned.
• We also have several reasons to think that we have slowly drifted away from the center of the optics on both ITMs in the horizontal direction (screenshot attached of the a2l gains which indicate the spot position on the optic, PR3 and PR2 yaw witness sensors, POPB yaw which is our out of loop QPD in the PRC, and the range over the last 45 days.)  We can adjust this using the PR3 spot move script to see if things get any better.

Josh Smith, Jess, TJ and I have investigated a bit and found that there is a line around 4700 Hz (Fig. 1) that rings up around the same time as a scratchy forest of lines in mid-range frequencies, right in the bucket. At Josh's suggestion I took a comparative look at the beginnings and ends of three lock stretches on Jan 6 where this pattern rings up (the circled regions in Fig. 1). The last three figures show 10-second averaged spectra of 100 seconds of data gathered at the start and end of these lock stretches (GPS times given in the plots). "Good" periods, where there is no 4.7 kHz line, are shown in blue; "bad" periods where the line shows up are plotted in red. You can clearly see the ASD of H1:GDS-CALIB_STRAIN is systematically worse between 70-500 Hz when the high-frequency line is rung up, compared to when it wasn't. (We also note this behavior on Friday, 13 Jan, and on a couple of other days back in december, all coincident with a wandering downward trend in BNS range.)

Tagging DetChar.

Bubba is out plowing wit the frontloader prior to the GRB alert. His activity clearly shos in the 3-10Hz anthropogenic BLRMS.

The noise reported earlier is still present and the H1 range remains in the same diminished state.

The SR3 oplev is unlikely to be a cause as we do not use oplev damping for this optic, but the glitching still needs to be addressed.  I have a WP filed to address this as soon as H1 loses lock.

Observatory Mode set to ALIGNING for this procedure.

13:08 UTC a2l finished. Images attached only show improvement in alignment. Noise and range are seemingly unchanged at the moment.

Here's a graphical version of what Sheila wrote, showing the time on Oct 11 when the 1083 Hz glitches started. The dither amplitude was reduced at 3:20 UTC, but the servo gain was increased to compensate. There are no 1083 Hz glitches at this time. Severe RF45 noise starts an hour later and lasts until the end of the lock. The 1083 Hz glitches are evident from the beginning of the next lock, and persist in every lock until the recent fix.

The dither amplitude stayed low in the second lock, but the servo gain was reset back to its low value. Apparently, both need to be low to produce the glitches.

Keita tells me that people are concerned about making this change because of the increased noise below 25 Hz in the screenshot attached to the original post.  We did not run the A2L decoupling durring this lock strech, and it was not well tuned.  The shape of the HARD loop cut off at 25Hz is visible in the spectrum, which is one way of identifying bad ASC noise.  The high coherence between CHARD P and DARM at this time is another way of seeing that this is angular noise (attachment). 

So I think that this is unrelated to the OMC gain change and not really a problem. 

1080Hz removal OMC gain/line conditions, does it make more low frequency noise?
Josh, Andy, TJ, Beverly

Conclusion: For two on/off times each for the two OMC gain tests (total 8 times) it looks like the high gain / low line configuration that takes away 1080 Hz (and also takes away some bumps around 6280Hz) coincides with a bit more noise below 25Hz.

Request: We hope this connection with noise below 25Hz is chance (It might have just been drift and we chose times unluckily) and we would like debunk/confirm it. We could do that with a couple cycles of on/off, (e.g. 5 minutes each, with the current configuration vs high gain / low dither configuration). 

See the attached PDF. The pages are: 

• 2,3: January 9th test configuration from Sheila's page: "We ran with the servo gain set to 24 (to give us the nominal 6Hz ugf) and the lowered dither line amplitude from 19:21 UTC to 19:51 UTC Jan 9th." Red/Orange are the test time with low line / high gain, and no 1080Hz
• 4,5: Similar experiment from October. Blue/green are the test time with low line / high gain, and no 1080Hz.

Also: There is no coherence above 10Hz between STRAIN and OMC LSC SERVO/I for any of these test times. So coupling must be non-linear. 
Also: When the 1080Hz bumps disappear we also see a bump around 6280Hz disappear (attached image, sorry no x-axis label but its 6280Hz)

Our post crossed with Sheila's. If possible, we'd still like to see a quick on/off test with the A2L tuned. Could we have five minutes with the gain high and then ramp it down? Maybe with one repeat. Since this is a non-linear effect, we'd like to make sure there's no funny coupling with the CHARD noise. We're not too worried by excess noise below 25 Hz now, but it might be important when we're able to push lower.

While LLO was down I attempted to do a test by increasing the OMC length gain while in lock, which unlocked the IFO.  So on/off tests aren't possible.  Edit:  I broke the lock by changing the gain after the integrator (which had been OK when not on DC readout), we can change the gain upstream instead without unlocking.  

For now I put the new gain into the guardian so the next lock will be with the increased gain, and hopefully see that the low frequency noise is fine. 

Now we have relocked, Patrick ran a2l, and Jeff, Evan, Krishna and I did an on off test by ramping H1:OMC-LSC_I_GAIN:

• high gain for about 15 minutes before 23:55 UTC Jan 12th
• low gain from 25:56:20 Jan 12th to 0:03:21 UTC Jan 13th
• high gain from 0:04 UTC to 0:13:22 UTC
• back to low gain

The attached screen shot using the same color scheme as in the presentation above shows that there is not a difference at low frequency between high gain and low gain.  

We are back in observing in the low gain configuration, but the gain is set in an unusual way (and accepted in SDF so that we can go to obsevering). Keita would like us to hear confirmation from detchar before making this permanent. 

Thank you Sheila, this looks really good. No change at low frequency. 1080Hz gone. The 6280Hz just varies on its own timescale. From our end we're happy with the configuration change since it only does good. Sorry for the red herring about low frequencies.