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Reports until 09:01, Monday 12 October 2015
LHO General
thomas.shaffer@LIGO.ORG - posted 09:01, Monday 12 October 2015 (22434)
Morning Meeting Minutes

10-12-15 Morning Meeting Minutes

·      SEI – Running 45mHz blends are running on the BSC ISIs, may need to change them soon.

·      SUS - Needs charge measurement tomorrow.

·      ISC – Needs noise measurement of LV esd

·      Vac – Leak testing, no N2 deliveries this week.

·      Fac – Work with the water tank, starting again on the crack sealing on beam tube sealing during Observing.

·      PEM – Completed tamper inj at CS, but still needs ends. Needs about a two hours for both end stations.

·      Maintenance items:

o   ESD measurement - Keita

o   Slow controls testing DAQ – JH

o   PCal calibrations X/Y ends (Needs ETMs aligned) – Pcal team

o   Copy HW Inj Infrastructure to PCalX – Jeff K

o   Return HEPI pump pressure sensors to norm – Hugh

o   PEM inj, possible gluing to mirror in PSL – Robert

o   Finish deinstallation of old roof cam - Richard

o   PEM temp sensors on chambers – Richard

o   Fire dept. maintenance on hydrants

o   Plumbing in the LVEA – Jeff B.

o   Phone identification signs in ends/mids – Bubba

o   Jim wants to take some transfer functions on HAM BSC (probably not yet)

o   Load from MY to staging building – Jodi

o   Pulling cables from ends to 250 mark for ion pumps – CDS

o   Sataboy, storage data for DMT, work - Carlos

LHO General
thomas.shaffer@LIGO.ORG - posted 07:15, Monday 12 October 2015 (22431)
Ops Day Transition
H1 AOS
cheryl.vorvick@LIGO.ORG - posted 07:12, Monday 12 October 2015 (22430)
Ops Owl Shift End
	Title:  10/12/2015, Owl Shift 07:00 – 14:30UTC (00:00 – 07:30PT)

State of H1: Locked in NOMINAL_LOW_NOISE at 73.2Mpc, in Observe for 21 hours.

Incoming Operator: TJ

Quick Summary: 

- Nothing to report since Chris Biwer finished his hardware injections 8:10UTC (see his alog)
	- Talked to LLO, and was told their GraceDB is still down, so call if we get an alarm.
- Still to try: Switch ISI Blend Filters after lock loss, and try the Quiet 90 filters on BSC chambers.
 
H1 General
cheryl.vorvick@LIGO.ORG - posted 01:12, Monday 12 October 2015 (22429)
Ops Owl Shift Transition
Title:  10/12/2015, Owl Shift 07:00 – 15:00UTC (00:00 – 08:00PT)

State of H1: Locked in NOMINAL_LOW_NOISE at 76.6Mpc. 

Outgoing Operator: JeffB

Quick Summary: 

Chris Biwer doing hardware injections - ending around 8:10UTC (see his alog)

Instructions to switch ISI Blend Filters after lock loss, and try the Quiet 90 filters on BSC chambers.

H1 INJ (DetChar, INJ)
christopher.biwer@LIGO.ORG - posted 01:09, Monday 12 October 2015 (22428)
Done with hardware injection tests
Finished with hardware injection tests. More details later.
H1 AOS
jeffrey.bartlett@LIGO.ORG - posted 00:24, Monday 12 October 2015 (22427)
Ops Evening Shift Summary
Activity Log: All Times in UTC (PT)

23:00 (16:00) Take over from TJ
02:30 (19:30) Jeff K. left the site
03:57 (20:57) Chris – Running hardware injections for the next 72 minutes. 
			Approved by Mike L., Notified LLO, Will not drop out of Observing
04:30 (21:30) GRB Alert – In one hour stand down
04:58 (21:58) ETMY saturation event during GRB stand down
05:30 (22:30) Finished one hour stand down. 
06:15 (23:15) Chris – Restarted hardware injections
07:20 (00:20) Turn over to Cheryl
	

Title: 10/11/2015, Evening Shift 23:00 – 07:00 (16:00 – 00:00) All times in UTC (PT)

Support: Jeff K, 

Incoming Operator: Cheryl

Shift Summary: 
- 03:57 – Chris running hardware injections. He has Mike’s approval and did not go out of Observing
- 04:30 Received GRB alert. Both sites in Observing mode. In stand down for 1 hour
Chris has stopped the hardware injections he was running.  Per Verbal Alarm - Last injection stopped at 04:28:17, the GRB was time stamped at 04:30:28.  
 	
Good shift. IFO locked in Observing for 14 hours. Low wind and seismic. There were several ETMY saturations. There were 3 mag 5 or greater earthquakes during the shift, but these did not noticeably affect the IFO.      
H1 INJ (DetChar, INJ)
christopher.biwer@LIGO.ORG - posted 22:57, Sunday 11 October 2015 (22426)
Starting hardware injection test again
We're going to continue with the hardware injection tests again after waiting for the external trigger.

The schedule will be updated with the following lines:
1128666017 1 1.0 ringburstinj1b_1126259456_
1128666377 1 1.0 ringburstinj2b_1126259456_
1128666737 1 1.0 ringburstinj3b_1126259456_
1128667097 1 1.0 ringburstinj4b_1126259456_
1128667457 1 1.0 ringburstinj5b_1126259456_
1128667817 1 1.0 ringburstinj6a_1126259456_
1128668177 1 1.0 ringburstinj6b_1126259456_
1128668537 1 1.0 ringburstinj7b_1126259456_
1128668897 1 1.0 ringburstinj8a_1126259456_
1128669257 1 1.0 ringburstinj7a_1126259456_
1128669617 1 1.0 ringburstinj8b_1126259456_
1128669977 1 1.0 ringburstinj9a_1126259456_
1128670337 1 1.0 ringburstinj10a_1126259456_
1128670697 1 1.0 ringburstinj11a_1126259456_
1128671057 1 1.0 ringburstinj12a_1126259456_
1128671417 1 1.0 ringburstinj13a_1126259456_
1128671777 1 1.0 ringburstinj14a_1126259456_
1128672497 1 1.0 ringburstinj15a_1126259456_
H1 INJ (DetChar, INJ)
christopher.biwer@LIGO.ORG - posted 20:54, Sunday 11 October 2015 - last comment - 21:38, Sunday 11 October 2015(22424)
Starting hardware injection test
Beginning hardware injection test. The schedule will be updated with:

1128657857 1 1.0 ringburstinj1a_1126259456_
1128658037 1 1.0 ringburstinj1b_1126259456_
1128658217 1 1.0 ringburstinj2a_1126259456_
1128658397 1 1.0 ringburstinj2b_1126259456_
1128658577 1 1.0 ringburstinj3a_1126259456_
1128658757 1 1.0 ringburstinj3b_1126259456_
1128658937 1 1.0 ringburstinj4a_1126259456_
1128659117 1 1.0 ringburstinj4b_1126259456_
1128659297 1 1.0 ringburstinj5a_1126259456_
1128659477 1 1.0 ringburstinj5b_1126259456_
1128659657 1 1.0 ringburstinj6a_1126259456_
1128659837 1 1.0 ringburstinj6b_1126259456_
1128660017 1 1.0 ringburstinj7a_1126259456_
1128660197 1 1.0 ringburstinj7b_1126259456_
1128660377 1 1.0 ringburstinj8a_1126259456_
1128660557 1 1.0 ringburstinj8b_1126259456_
1128660737 1 1.0 ringburstinj9a_1126259456_
1128660917 1 1.0 ringburstinj10a_1126259456_
1128661097 1 1.0 ringburstinj11a_1126259456_
1128661277 1 1.0 ringburstinj12a_1126259456_
1128661457 1 1.0 ringburstinj13a_1126259456_
1128661637 1 1.0 ringburstinj14a_1126259456_
1128661817 1 1.0 ringburstinj15a_1126259456_
1128661997 1 1.0 ringburstinj16a_1126259456_
Comments related to this report
christopher.biwer@LIGO.ORG - 21:38, Sunday 11 October 2015 (22425)DetChar, INJ
The plan was to do the injections 180s apart. tinj cannot do injections this close together so some injections were skipped.

The following injections were successful:
1128657857 1 1.0 ringburstinj1a_1126259456_
1128658217 1 1.0 ringburstinj2a_1126259456_
1128658577 1 1.0 ringburstinj3a_1126259456_
1128658937 1 1.0 ringburstinj4a_1126259456_
1128659297 1 1.0 ringburstinj5a_1126259456_

The following injections were skipped and thus not successful:
1128658037 1 1.0 ringburstinj1b_1126259456_
1128658397 1 1.0 ringburstinj2b_1126259456_
1128658757 1 1.0 ringburstinj3b_1126259456_
1128659117 1 1.0 ringburstinj4b_1126259456_
1128659477 1 1.0 ringburstinj5b_1126259456_

This injection was scheduled during the GRB alert. It was not injected at H1 but it wasn't stopped in time at L1:
1128659657 1 1.0 ringburstinj6a_1126259456_

A GRB alert happened so I removed the following injections from the schedule:
1128659837 1 1.0 ringburstinj6b_1126259456_
1128660017 1 1.0 ringburstinj7a_1126259456_
1128660197 1 1.0 ringburstinj7b_1126259456_
1128660377 1 1.0 ringburstinj8a_1126259456_
1128660557 1 1.0 ringburstinj8b_1126259456_
1128660737 1 1.0 ringburstinj9a_1126259456_
1128660917 1 1.0 ringburstinj10a_1126259456_
1128661097 1 1.0 ringburstinj11a_1126259456_
1128661277 1 1.0 ringburstinj12a_1126259456_
1128661457 1 1.0 ringburstinj13a_1126259456_
1128661637 1 1.0 ringburstinj14a_1126259456_
1128661817 1 1.0 ringburstinj15a_1126259456_
1128661997 1 1.0 ringburstinj16a_1126259456_
H1 General
jeffrey.bartlett@LIGO.ORG - posted 20:16, Sunday 11 October 2015 (22423)
Ops Evening Mid-Shift
Locked in LOW_NOISE at 22.2W, 68Mpc range. LHO has been in Observation mode for past 10 hours. LLO has also been locked with comparable range during this stretch. Wind is calm (1 to 3 Mph); Seismic activity has been quite during the shift. 

   There have been a few ETMY saturation events, but have not observed any problems with the RF45 EOM.    
H1 AOS
jeffrey.bartlett@LIGO.ORG - posted 16:10, Sunday 11 October 2015 (22422)
Ops Evening Shift Transition
Title:  10/11/2015, Evening Shift 23:00 – 07:00 (16:00 – 00:00) All times in UTC (PT)

State of H1: At 23:00 (16:00) Locked at NOMINAL_LOW_NOISE, 22.2W, 70Mpc. 

Outgoing Operator: TJ

Quick Summary: Environmental conditions are good. IFO in Observing mode. All appears to be normal.       

LHO General
thomas.shaffer@LIGO.ORG - posted 16:00, Sunday 11 October 2015 (22416)
Ops Day Shift Summary

Title: 10/11 Day Shift 15:00-23:00 UTC (8:00-16:00 PST).  All times in UTC.

State of H1: Unlocked, high winds and high microseism

Shift Summary: Locked until 21:24, haven't been able to relock since due to environment issue.

Incoming operator: Jeff B.

Activity log:

H1 DetChar (CAL, CDS, INJ)
jeffrey.kissel@LIGO.ORG - posted 13:25, Sunday 11 October 2015 (22419)
Monitor Test Point for CAL-DELTAL_EXTERNAL Opened Briefly While In Observation Mode
J. Kissel, M. Oliver

Miquel and I are doing some fun Sunday afternoon investigations of the 1 [Hz] comb for the CW group (see LIGO wiki page). Since we see that the power of the comb evolves with time, we've gathered some DELTAL EXTERNAL data as a test point to compare it against the version stored in the frames. Accessing the test point via DTT opened up a test point, naturally. One would have nominally expected opening up test points on any front end to take us out of observation mode BUT CAL-DELTAL_EXTERNAL happens to be computed on the same front end as the continuous wave injections, which -- because they need to access the H1:CAL-INJ_CW_EXC test point continuously -- has been flagged as "OK to have test points activated." (Note, this is why Giles, the H1 IFO automated voice robot, announced that CW injections had started when I hit "start" on the first bit of data gather in the DTT session). Of course, this is a monitor test point, so it had zero effect on the functionality/operation of the IFO, so remaining in observation mode was totally acceptable (even though admittedly, we didn't think of it when getting started). 

The test point was activated at 19:25 UTC (12:25 PDT) to 20:22 UTC (01:22 PDT).

Stay tuned for analysis of the data.
H1 DetChar
thomas.shaffer@LIGO.ORG - posted 13:20, Sunday 11 October 2015 - last comment - 13:53, Sunday 11 October 2015(22418)
~300hz line in the glitch gram

Jeff Kissel noticed a ~300hz line in the glitch gram on the control room wall. Seems like it started with this lock but has been getting worse. Screen shots attached.

Images attached to this report
Comments related to this report
andrew.lundgren@LIGO.ORG - 13:53, Sunday 11 October 2015 (22421)DetChar
Looks like the peaks from the PSL periscope. See this alog for some details. I don't think we need to do anything about them right now, since it seemed like the IFO was hard to lock. I haven't seen them causing many problems for the search.
Images attached to this comment
LHO General
thomas.shaffer@LIGO.ORG - posted 12:07, Sunday 11 October 2015 (22417)
Ops Day Mid Shift Report

Observing at ~70Mpc.

Winds have picked up a bit, around 15-20mph, so I am not sure if this lock will hold with these blends. On the front Striptool AS90 has a fairly large oscillation.

Microseism seems to be on its way down, very slowly. We are now in a range of .3-.5 um/s.

LLO is still observing as well.

H1 General (SEI)
thomas.shaffer@LIGO.ORG - posted 09:58, Sunday 11 October 2015 - last comment - 10:47, Sunday 11 October 2015(22410)
More on the 45mHz blend change

Jim W. suggestd that we try out the 45mHz blends if we can't lock due to the high microseism. Just before I came on shift, Travis and Sheila switched them over for the X and Y directions on BS, ETMY, EMTX, ITMY, and ITMX. They decided to do this because of the low winds and high microseism.  This seemed to have helped us out quite a bit. Before the change, they struggled getting to lock DRMI, with them switched it only took us a few tries before we were all the way to Nominal Low Noise.

I will be accepting these blend changes in the SDF so we can move into Observing.

Furture operators should change them back if the winds pick up (Sheila says anything above 20 might be too high for these blends), but keep in mind this may drop lock.

Comments related to this report
sheila.dwyer@LIGO.ORG - 10:05, Sunday 11 October 2015 (22411)

For future reference the wind had not gusted above 15 mph for about an hour or so before Travis and I changed the blends, and the microseism is at about 0.5 um/sec in the 100-300mHZ BLRMS now.  Before we changed the blends, ITMX optical lever saw about 0.5 urad peak to peak motion, and single arms were not even locking. (ETMX was not moving as much as the ITM) Changing the blends reduced the motion to more like 0.1 urad or even less.  

thomas.shaffer@LIGO.ORG - 10:11, Sunday 11 October 2015 (22413)

Here are SDF shots so operators know what to bring it back to.

Images attached to this comment
jim.warner@LIGO.ORG - 10:47, Sunday 11 October 2015 (22414)

Unfortunately you won't be able to use the SDF system to revert these filters. If you do that you will likely trip the ISI, violently, and it will take longer to recover. You will have open the ST1 BLENDS screen for each ISI and select the appropriate blend from there.

H1 AOS (DetChar, ISC)
joshua.smith@LIGO.ORG - posted 08:48, Sunday 11 October 2015 - last comment - 12:26, Sunday 11 October 2015(22405)
Scattering arches seen by end-Y ASC QPDs, coupling into DARM

Josh, Andy

Yesterday's hveto page found a very constant rate of 30-40Hz glitches in DARM were vetoed by a population of 10-15Hz glitches in the End-Y QPDs (ASC-Y_TR_{A,B}_{NSUM,YAW,PIT}_OUT_DQ). Fig1 shows the time/freq of the DARM triggers and the ASC-Y triggers that vetoed them, Fig 2 shows the time/freq of all DARM triggers and the ones that got vetoed, and Fig 3 shows how the ranking statistic found all ASC-Y channels to be correlated. A .txt file with the times of vetoed DARM triggers is attached in case folks want to look at other channels around those times. 

Fig 4 shows DARM, figs 5,6 show ASC-Y channels, figs 7,8 show zoomed specgrams to see the frequency. What we see are strong scattering arches around 8 and 16 Hz in all of the End-Y QPD channels, at the same time as similarly shaped fringes in DARM around 48Hz. I think DARM (more sensitive to scatter, potentially, at higher frequencies)  is seeing the 6th overtone of the 8Hz arch (multiple reflections?). The higher overtones are sometimes visible in the QPDs too. 

So...what scatterer/optics is moving too much? We know [1] that f_fringe(t) = 􏰁􏰁􏰁􏰁􏰁abs(2 v_sc(t) / lambda)􏰁. Where v_sc(t) is the time derivative of the position of the scatterer x_sc(t). The time between peaks is about 3.5 seconds, but that is half the period because of the abs() so it's a ~7 second period or 0.14Hz motion of the scatterer. 􏰁􏰁􏰁 

Note: We checked and the beam diverter is closed (H1:SYS-MOTION_Y_BDIV_A_POSITION = 1). 

Note 2: Today's hveto page hasn't finished yet, but no reason to believe this isn't happening still.  

[1] Accadia, T., et al. "Noise from scattered light in Virgo's second science run data." Classical and Quantum Gravity 27.19 (2010): 194011.

Images attached to this report
Non-image files attached to this report
Comments related to this report
joshua.smith@LIGO.ORG - 12:26, Sunday 11 October 2015 (22415)DetChar, ISC, SUS

It probably won't surprise anyone to learn that ETMY top-stage motion correlates pretty well with these fringes. Though, likely lots of things are moving in this way following the high microseism. 

  • The first plot shows that the fringe maxima correspond to roughly where the derivative is max on the ETMY longitudinal motion (judging from channel SUS-ETMY_M0_DAMP_L_IN1_DQ).
  • The second plot shows: f_scatter3 = 3*abs(2*velocity/1.064), where velocity is the time derivative of SUS-ETMY_M0_DAMP_L_IN1_DQ in um/s (assuming that channel is in um.).
  • The third plot, which shows a (not normalized) spectrogram of ASC Y TR NSUM and f_scatter3 after lowpassing the L_IN1 channel to remove its 60Hz (which was making the fringe prediction noisy)
  • Don't know what to make of the need for either the 3rd overtone or the motion being 3x what the channel reads in um. 
Images attached to this comment
H1 DetChar (DetChar, ISC, PSL)
gabriele.vajente@LIGO.ORG - posted 12:43, Thursday 08 October 2015 - last comment - 13:45, Sunday 11 October 2015(22343)
Brute force coherence

I ran a BruCo scan for LHO. The full report is available here:

https://ldas-jobs.ligo.caltech.edu/~gabriele.vajente/bruco_lho_1128339017/

Here is my excerpt:

Beside these, there are few more puzzling coherences at higher frequencies:

Images attached to this report
Comments related to this report
eleanor.king@LIGO.ORG - 13:45, Sunday 11 October 2015 (22420)

We see a line in the DARM spectrum at 2450Hz during the first few hours of some lock stretches, which corresponds to the BS butterfly mode frequency (See Keita's alog 17186.)  Could this be the cause of the 2449Hz coherence you see? in PRCL?

H1 ISC
paul.fulda@LIGO.ORG - posted 13:22, Tuesday 22 September 2015 - last comment - 08:16, Monday 12 October 2015(21792)
AS 36 MHz WFS sensing as a function of SR3 RoC (SRC mode matching)

Elli and Stefan showed in aLOG 20827 that the signals measured by AS 36 WFS for SRM and BS alignment appeared to be strongly dependent on the power circulating in the interferometer. This was apparently not seen to be the case in L1. As a result, I've been looking at the AS 36 sensing with a Finesse model (L1300231), to see if this variability is reproducible in simulation, and also to see what other IFO variables can affect this variability. 

In the past when looking for differences between L1 and H1 length sensing (for the SRC in particular), the mode matching of the SRC has come up as a likely candidate. This is mainly because of the relatively large uncertainties in the SR3 mirror RoC combined with the strong dependence of the SRC mode on the SR3 RoC. I thought this would therefore be a good place to start when looking at the alignment sensors at the AS port. I don't expect the SR3 RoC to be very dependent on IFO power, but having a larger SR3 RoC offset (or one in a particular direction) may increase the dependence of the AS WFS signals on the ITM thermal lenses (which are the main IFO variables we typically expect to change with IFO power). This might therefore explain why H1 sees a bigger change in the ASC signals than L1 as the IFOs heat up. 

My first step was to observe the change in AS 36 WFS signals as a function of SR3 RoC. The results for the two DOFs shown in aLOG 20827 (MICH = BS, SRC2 = SRM) are shown in the attached plots. I did not spend much time adjusting Gouy phases or demod phases at the WFS in order to match the experiment, but I did make sure that the Gouy phase difference between WFSA and WFSB was 90deg at the nominal SR3 RoC. In the attached plots we can see that the AS 36 WFS signals are definitely changing with SR3 RoC, in some cases even changing sign (e.g. SRM Yaw to ASA36I/Q and SRM Pitch to ASA36I/Q). It's difficult at this stage to compare very closely with the experimental data shown in aLOG 20827, but at least we can say that from model it's not unexpected that these ASC sensing matrix elements are changing with some IFO mode mismatches. The same plots are available for all alignment DOFs, but that's 22 in total so I'm sparing you all the ones which weren't measured during IFO warm up. 

The next step will be to look at the dependence of the same ASC matrix elements on common ITM thermal lens values, for a few different SR3 RoC offsets. This is where we might be able to see something that explains the difference between L1 and H1 in this respect. (Of course, there may be other effects which contribute here, such as differential ITM lensing, spot position offsets on the WFS, drifting of uncontrolled DOFs when the IFO heats up... but we have to start somewhere). 

Non-image files attached to this report
Comments related to this report
keita.kawabe@LIGO.ORG - 18:33, Tuesday 22 September 2015 (21819)

Can you add a plot of the amplitude and phase of 36MHz signal that is common to all four quadrants when there's no misalignment?

paul.fulda@LIGO.ORG - 10:01, Wednesday 23 September 2015 (21839)

As requested, here are plots of the 36MHz signal that is common to all quadrants at the ASWFSA and ASWFSB locations in the simulation. I also checked whether the "sidebands on sidebands" from the series modulation at the EOM had any influence on the signal that shows up here: apparently it does not make a difference beyond the ~100ppm level. 

Non-image files attached to this comment
paul.fulda@LIGO.ORG - 07:57, Friday 25 September 2015 (21895)

At Daniel's suggestion, I adjusted the overall WFS phases so that the 36MHz bias signal shows up only in the I-phase channels. This was done just by adding the phase shown in the plots in the previous comment to both I and Q detectors in the simulation. I've attached the ASWFS sensing matrix elements for MICH (BS) and SRC2 (SRM) again here with the new demod phase basis. 

**EDIT** When I reran the code to output the sensitivities to WFS spot position (see below) I also output the MICH (BS) and SRC2 (SRM) DOFs again, as well as all the other ASC DOFs. Motivated by some discussion with Keita about why PIT and YAW looked so different, I checked again how different they were. In the outputs from the re-run, PIT and YAW don't look so different now (see attached files with "phased" suffix, now also including SRC1 (SR2) actuation). The PIT plots are the same as previously, but the YAW plots are different to previous and now agree better with PIT plots.

I suspect that the reason for the earlier difference had something to do with the demod phases not having been adjusted from default for YAW signals, but I wasn't yet able to recreate the error. Another possibility is that I just uploaded old plots with the same names by mistake. 

Non-image files attached to this comment
paul.fulda@LIGO.ORG - 14:47, Thursday 24 September 2015 (21899)

To clarify the point of adjusting the WFS demod phases like this, I also added four new alignment DOFs corresponding to spot position on WFSA and WFSB, in ptich and yaw directions. This was done by dithering a steering mirror in the path just before each WFS, and double demodulating at the 36MHz frequency (in I and Q) and then at the dither frequency. The attached plots show what you would expect to see: In each DOF the sensitivity to spot position is all in the I quadrature (first-order sensitivity to spot position due to the 36MHz bias). Naturally, WFSA spot position doesn't show up at WFSB and vice versa, and yaw position doesn't show up in the WFS pitch signal and vice versa. 

For completeness, the yaxis is in units of W/rad tilt of the steering mirror that is being dithered. For WFSA the steering mirror is 0.1m from the WFSA location, and for WFSB the steering mirror is 0.2878m from the WFSB location. We can convert the axes to W/mm spot position or similar from this information, or into W/beam_radius using the fact that the beam spot sizes are at 567µm at WFSA and 146µm at WFSB.

Non-image files attached to this comment
paul.fulda@LIGO.ORG - 10:04, Tuesday 29 September 2015 (22058)

As shown above the 36MHz WFS are sensitive in one quadrature to spot position, due to the constant presence of a 36MHz signal at the WFS. This fact, combined with the possibility of poor spot centering on the WFS due to the effects of "junk" carrier light, is a potential cause of badness in the 36MHz AS WFS loops. Daniel and Keita were interested to know if the spot centering could be improved by using some kind of RF QPD that balances either the 18MHz (or 90MHz) RF signals between quadrants to effectively center the 9MHz (or 45MHz) sideband field, instead of the time averaged sum of all fields (DC centering) that is sensitive to junk carrier light. In Daniel's words, you can think of this as kind of an "RF optical lever".

This brought up the question of which sideband field's spot postion at the WFS changes most when either the BS, SR2 or SRM are actuated.

To answer that question, I:

  • Added 18MHz and 90MHz "WFS" to the Finesse model.
  • Phased these new "WFS" so that all the 18MHz or 90MHz "sum" signals show up in the I-phase (see first two plots).
  • Plotted the new "WFS" responses to BS, SR2 and SRM pitch and yaw (normalized by their "sum" signal), as a function of SR3 RoC (see the rest of the plots).

Some observations from the plots:

  • 90MHz "WFS" show much more response to SRC1 (SR2) and SRC2 (SRM) tilts than 18MHz "WFS". This is somewhat intuitive, because the 45MHz sidebands are resonant in the SRC and the SRC eigenmode may be more sensitive to SR alignments than a beam traced through "single bounce" style.
  • The 90MHz response to SRM/SR2 tilt remain very much in the I phase, with almost no signal in the Q-phase. 
  • 18MHz "WFS" show more response to MICH (BS) than the 90MHz "WFS". This is problably because a BS tilt causes a large coupling of 9MHz HG10/01 mode from the PRC to the SRC relative to the amount of 9MHz HG00 mode already in the SRC (due to high Michelson reflectivity + SRC non-resonance for 9MHz HG00). Since I normalize to the total sideband power, this looks like a bigger response in 18MHz than 90MHz.
  • The 18MHz response to BS tilt is mostly in the Q phase. I'm not sure exactly how to interpret this, but my naive guess is that it's related to the BS tilt being a "differential" mode tilt for the X and Y arms of the small Michelson.
Non-image files attached to this comment
paul.fulda@LIGO.ORG - 08:16, Monday 12 October 2015 (22432)

I looked again at some of the 2f WFS signals, this time with a linear sweep over alignment offsets rather than a dither transfer function. I attached the results here, with detectors being phased to have the constant signal always in I quadrature. As noted before by Daniel, AS18Q looks like a good signal for MICH sensing, as it is pretty insensitive to beam spot position on the WFS. Since I was looking at larger alignment offsets, I included higher-order modes up to order 6 in the calculation, and all length DOFs were locked. This was for zero SR3 RoC offset, so mode matching is optimal.

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