I had set up the 4395 to do this automatically a few years ago, but it looked at the FSS/IMC instead. When the PCDRIVE goes high there is this excess around ~500 kHz in a broad hump.
But the IMC loop gain changes sometimes with alignment, so I don't know if its a loop instability or if its laser noise. However, I think we have observed PCDRIVE to go up without IMC power dropping so my guess is that it was true laser noise.
This works since the IMC is much more sensitive than PMC. Perhaps one way to diagnose would be to lock IMC at a low UGF without any boosts. Then the UGF would be far away from that noise making frequency. However, the PCDRIVE also wouldn't have much activity.
Nice, and we should also permanently install the camera server (c1cam) which is still sitting on the electronics bench. It is running an adapted version of the Python 2/Debian 8 site code. Maybe if COVID continues long enough I'll get around to making the Python 3 version we've long discussed.
There's this elog from Stephen about better 1064 sensitivity from Basler. We should consider getting one if he finds that its actual SNR is as good as we would expect from the QE improvement.
On Monday, I hooked up an AG4395 to the PMC error point (using the active probe). The idea was to take a spectrum of the PMC error point every time the FSS PC drive RMS channel indicated an excursion from the nominal value. An initial look at the results don't suggest that this technique is particularly informative. I'll have to think more about a workaround, but please share your ideas/thoughts if you have some.
Also, the feature in the spectrum at ~110 kHz makes me suspect some kind of loop instability. I'll measure the IMC loop OLG at the next opportunity.
Covid 19 motivated me to revive the summary pages. With Alex Urban's help, the infrastructure was modernized, the wiki is now up to date. I ran a test job for 2020 March 17th, just for the IOO tab, and it works, see here. The LDAS rsync of our frames is still catching up, so once that is up, we can start the old condor jobs and have these updated on a more regular basis.
Ok, now I understand my foolishness. It should definitely be 1/sqrt(f^2+fp^2) .
Might allow for better scatter measurements - not that we need more signal, but it could allow us to use shorter exposure times and reduce blurring due to the wobbly beams.
I've generated specifications for the new BHD optics. This includes the suspended relay mirrors as well as the breadboard optics (but not the OMCs).
To design the mode-matching telescopes, I updated the BHD mode-matching scripts to reflect Koji's draft layout (Dec. 2019) and used A La Mode to optimize ROCs and positions. Of the relay optics, only a few have an AOI small enough for curvature (astigmatism) and most of those do not have much room to move. This reduced the optimization considerably.
These ROCs should be viewed as a first approximation. Many of the distances I had to eyeball from Koji's drawings. I also used the Gaussian PRC/SRC modes from the current IFO, even though the recycling cavities will both slightly change. I set up a running list of items like these that we still need to resolve in the BHD README.
At a glance, all the specifications can be seen in the optics summary spreadsheet.
The LO beam originates from the PR2 transmission (POP), near ITMX. It is relayed to the BHD beamsplitter (and mode-matched to the OMCs) via the following optical sequence:
The resulting beam profile is shown in Attachment 1.
The AS beam is relayed from the SRM to the BHD beamsplitter (and mode-matched to the OMCs) via the following sequence:
A lens is used because there is not enough room on the BHD breadboard for a pair of (low-AOI) telescope mirrors, like there is in the LO path. The resulting beam profile is shown in Attachment 2.
Four nitrogen cylinders replaced the empties in the rack at the west entrance. Additionally, Airgas will now deliver only once a week. Let me know via email or text when the there are four empties in the rack and I'll order the next round.
[Koji / Gautam (Remote)]
sudo /sbin/ifdown eth0
sudo /sbin/ifup eth0
End RTS recovery
rtcds start --all
Vertex RTS recovery
sudo /sbin/ifup eth1
sudo systemctl start modbusIOC.service
sudo /sbin/ifdown eth1
sudo systemctl start modbusIOC.service
RTS recovery ~ part 2
sudo systemctl start open-mx.service
sudo systemctl start mx.service
sudo systemctl start daqd_*
sudo systemctl start MCautolocker.service
sudo systemctl start FSSSlow.service
I just now modified the /etc/rsyncd.conf file as per Dan Kozak's instructions. The old conf file is still there with the file name appended with today's date.
I then enabled the rsync daemon to run on boot using 'enable'. I'll ask Dan to start the file transfers again and see if this works.
controls@nodus|etc> sudo systemctl start rsyncd.service
controls@nodus|etc> sudo systemctl enable rsyncd.service
Created symlink from /etc/systemd/system/multi-user.target.wants/rsyncd.service to /usr/lib/systemd/system/rsyncd.service.
controls@nodus|etc> sudo systemctl status rsyncd.service
● rsyncd.service - fast remote file copy program daemon
Loaded: loaded (/usr/lib/systemd/system/rsyncd.service; enabled; vendor preset: disabled)
Active: active (running) since Mon 2020-04-13 16:49:12 PDT; 1min 28s ago
Main PID: 4950 (rsync)
└─4950 /usr/bin/rsync --daemon --no-detach
Apr 13 16:49:12 nodus.martian.113.168.192.in-addr.arpa systemd: Started fast remote file copy program daemon.
Apr 13 16:49:12 nodus.martian.113.168.192.in-addr.arpa systemd: Starting fast remote file copy program daemon...
[Larry (on site), Koji & Gautam (remote)]
Network recovery (Larry/KA)
Asked Larry to get into the lab.
14:30 Larry went to the lab office area. He restarted (power cycled) the edge-switch (on the rack next to the printer). This recovered the ssh-access to nodus.
Also Larry turned on the CAD WS. Koji confirmed the remote access to the CAD WS.
Nodus recovery (KA)
Apr 12, 22:43 nodus was restarted.
Apache (dokuwiki, svn, etc) recovered along with the systemctl command on wiki
ELOG recovered by running the script
Control Machines / RT FE / Acromag server Status
Judging by uptime, basically only the machines that are on UPS (all control room workstations + chiara) survived the power outage. All RT FEs are down. Apart from c1susaux, the acromag servers are back up (but the modbus processes have NOT been restarted yet). Vacuum machine is not visible on the network (could just be a networking issue and the local subnet to valves/pumps is connected, but no way to tell remotely).
KA imagines that FB took some finite time to come up. However, the RT machines required FB to download the OS. That made the RTs down. If so, what we need is to power cycle them.
Acromag: unknown state
The power was lost at Apr 12 22:39:42, according to the vacuum pressure log. The power loss was for a few min.
In the past year, pygwinc has expanded to support not just fundamental noise calculations (e.g., quantum, thermal) but also any number of user-defined noises. These custom noise definitions can do anything, from evaluating an empirical model (e.g., electronics, suspension) to loading real noise measurements (e.g., laser AM/PM noise). Here is an example of the framework applied to H1.
Starting with the BHD review-era noises, I have set up the 40m pygwinc fork with a working noise budget which we can easily expand. Specific actions:
I set up our fork in this way to keep the 40m separate from the main pygwinc code (i.e., not added to as a built-in IFO type). With the 40m code all contained within one root-level directory (with a 40m-specific name), we should now always be able to upgrade to the latest pygwinc without creating intractable merge conflicts.
I have a query out to Dolphin asking:
Answers from Dolphin:
Since upgrading every front end is out of the question, our only option is to install an old OS (Linux kernel < 3.x) on the two new machines. Based on Keith's advice, I think we should go with Debian 8. (Link to Keith's Debian 8 instructions.)
I don't have a recent measurement of the optical gain of this config so I can't undo the loop, but in-loop performance doesn't suggest any excess in the 10-100 Hz band. Interestingly, there is considerable improvement below 10 Hz. Maybe some of this is reduced A2L noise because of the better angular stability, but there is also improvement at frequencies where the FF isn't doing anything, so could be some bilinear coupling. The two datasets were collected at approximately the same time in the evening, ~5pm, but on two different days.
I wonder how much noise is getting injected into PRC length at 10-100 Hz due to this. Any change the PRC ERR?
that's pretty great performance. maybe you can also upload some code so that we can do it later too - or maybe in the 40m GIT
Using the data I collected yesterday, the POP angular FF filters have been trained. The offline time-domain performance looks (unbelievably) good, online performance will be verified at the next available opportunity(see update).
The sequence of steps followed is the same as that done for the MCL FF filters. The trace that is missing from Attachment #1 is the measured online subtraction. Some rough notes:
Update Apr 5 1145pm:
This afternoon, I kept the PRM locked for ~1hour and then measured transfer functions from the PRM angular actuators to the POP QPD spot motion for pitch and yaw between ~1pm and 4pm. After this work, the PRM was misaligned again. I will now work on the feedforward filter design.
I wanted to pass along a complication pointed out by K. Thorne re: our plan to use Gen1 (old) Dolphin IPC cards in the new real-time machines: c1bhd, c1sus2. The implication is that we may be forced to install a very old OS (e.g., Debian 8) for compatibility with the IPC card driver, which could lead to other complications like an incompatibility with the modern network interface.
I'll add more info if I hear back from them.
We want to migrate the end shutter controls from c1aux to the end acromags. Could you include them to the list if not yet?
This will let us remove c1aux from the rack, I believe.
Yehonathan's list does include C1:AUX-GREEN_Y_Shutter and I copied its definition from /cvs/cds/caltech/target/c1aux/ShutterInterlock.db into the new ETMYaux.db file.
I noticed ShutterInterlock.db still contains about a dozen channels. Some of them appear to be ghosts (like the C1:AUX-PSL_Shutter[...] set, which has since become C1:PSL-PSL_Shutter[...] hosted on c1psl) but others like C1:AUX-GREEN_X_Shutter appear to still be in active use.
I have made a wiring + channel list that need to be included in the new C!AUXEY Acromag.
I used Yehonathan's wiring assignments to lay the rest of groundwork for the final slow controls machine upgrade, c1auxey. Actions completed:
The "1" will be dropped after the new system is permanently installed.
Hardware-wise, this system will require:
I know that we do have these quantities left on hand. The next steps are to set up the Supermicro host and begin assembling the Acromag chassis. Both of these activities require an in-person presence, so I think this is as far as we can advance this project for now.
Retraining the MCL filters resulted in a slight improvement in the performance. Compared to no FF, the RMS in the 0.5-5 Hz range is reduced by approximately a factor of 3.
Attachment #1 shows my re-measurement of the MC2 position drive to MCL transfer function.
Attachment #2 shows the IIR fits to the FIR filters calculated here.
Attachment #3 shows several MCL spectra.
Conclusions + next steps
The problem is that foton does not inherit the model sample rate when launched from DTT/awggui. This is likely some shared/linked/dynamic library issue, the binaries we are running are precompiled presumably for some other OS. I've never gotten this to work since we changed to SL7 (but I did use it successfully in 2017 with the Ubuntu12 install).
do you really mean awggui cannot make shaped noise injections via its foton text box ? That has always worked for me in the past.
If this is broken I'm suspicious there's been some package installs to the shared dirs by someone.
I'd like to re-measure the transfer function from driving MC2 position to the MC_L_DQ channel (for feedforward purposes). Swept sine would be one option, but I can't get the "Envelope" feature of DTT to work, the excitation amplitude isn't getting scaled as specified in the envelope, and so I'm unable to make the measurement near 1 Hz (which is where the FF is effective). I see some scattered mentions of such an issue in past elogs but no mention of a fix (I also feel like I have gotten the envelope function to work for some other loop measurement templates). So then I thought I'd try broadband noise injection, since that seems to have been the approach followed in the past. Again, the noise injection needs to be shaped around ~1 Hz to avoid knocking the IMC out of lock, but I can't get Foton to do shaped noise injections because it doesn't inherit the sample rate when launched from inside DTT/awggui - this is not a new issue, does anyone know the fix?
Note that we are using the gds2.15 install of foton, but the pre-packaged foton that comes with the SL7 installation doesn't work either.
The envelope feature for swept-sine wasn't working because i specified the frequency grid in the wrong order apparently. Eric von Reis has been notified to include a sorting algorithm in future DTT so that this can be in arbitrary order. fixing that allows me to run a swept sine with enveloped excitation amplitude and hence get the TF I want, but still no shaped noise injections via foton 😢
Yesterday evening I took nearly all of the masks, gloves, gowns, alcohol wipes, hats, and shoe covers. These were the ones in the cleanroom cabinets at the east end of the Y-arm, as well as the many boxes under the yarm near those cabinets.
This photo album shows the stuff, plus some other random photos I took around the same time (6-7 PM) of the state of parts of the lab.
It was mostly copied from C1AUXEX.
I ignored the IPANG channels since it is going to be removed from the table.
Since there has been a proliferation of BHD Google docs recently, I've linked them all from the BHD wiki page. Let's continue adding any new docs to this central list.
The email address in the N2 checking script wasn't right - I now updated it to email the 40m list if the sum of reserve tank pressures fall below 800 PSI. The checker itself is only run every 3 hours (via cron on c1vac).
I reset the remote of this git repo to the 40m version instead of Jon's personal one, to ensure consistency between what's on the vacuum machine and in the git repo. There is now a N2 checker python mailer that will email the 40m list if all the tank pressures are below 600 PSI (>12 hours left for someone to react before the main N2 line pressure drops and the interlocks kick in). For now, the script just runs as a cron job every 3 hours, but perhaps we should integrate it with the interlock process
I think the feedforward filters used for stabilizing MCL with vertex seismometers would benefit from a retraining (last trained in Sep 2015).
I wanted to re-familiarize myself with the seismic feedforward methodology. Getting good stabilization of the PRC angular motion as we have been able to in the past will be a big help for lock acquisition. But remotely, it is easier to work with the IMC length feedforward (IMC is locked more often than the PRC). So I collected 2 hours of data from early Sunday morning and went through the set of steps (partially).
Attachment #1 shows the performance of a first attempt.
Attachment #2 shows a comparison between the filter used in Attachment #1 and the filters currently loaded into the OAF system.
Attachment #3 is the asd after implementing a time domain Wiener filter, while Attachment #4 is an actual measurement from earlier today - it's not quite as good as Attachment #3 would have me expect but that might also be due to the time of the day.
Conclusions and next steps:
On the basis of Attachments #3 and #4, I'd say it's worth it to complete the remaining steps for online implementation: FIR to IIR fitting and conversion to sos coefficients that Foton likes (prefereably all in python). Once I've verified that this works, I'll see if I can get some data for the motion on the POP QPD with the PRMI locked on carrier. That'll be the target signal for the PRC angular FF training. Probably can't hurt to have this implemented for the arms as well.
While this set of steps follows the traditional approach, it'd be interesting if someone wants to try Gabriele's code which I think directly gives a z-domain representation and has been very successful at the sites.
* The y-axes on the spectra are labelled in um/rtHz but I don't actually know if the calibration has been updated anytime recently. As I type this, I'm also reminded that I have to check what the whitening situation is on the Pentek board that digitizes MCL.
Some short notes, more details tomorrow.
Attachment #1 shows time series of some signals, from the time I ramp of ALS CARM control to a lockloss. With this limited set of signals, I don't see any clear indication of the cause of lockloss, but I was never able to keep the lock going for > a couple of mins.
Attachment #2 shows the CARM OLTF. Compared to last week, I was able to get the UGF a little higher. This particular measurement doesn't show it, but I was also able to engage the regular boost. I did a zeroth order test looking at the CM_SLOW input to make sure that I wasn't increasing the gain so much that the ADC was getting saturated. However, I did notice that the pk-to-pk error signal in this locked, 5kHz UGF state was still ~1000 cts, which seems large?
Attachment #3 shows the DTT measurement of the relative gains of DARM A and B paths. This measurement was taken when the DARM_A gain was 1, and DARM_B gain was 0.015. On the basis of this measurement, DARM_B (=AS55) sees the excitation injected 16dB above the ALS signal, and so the gain of the DARM_B path should be ~0.16 for the same UGF. But I was never able to get the DARM_B gain above 0.02 without breaking the lock (admittedly the lockloss may have been due to something else).
Attachment #4 shows a zoomed in version of Attachment #1 around the time when the lock was lost. Maybe POP_YAW experienced too large an excursion?
Some other misc points:
I was in the lab at the time. But did not notice anything (like turbo sound etc). I was around ETMX/Y (1X9, 1Y4) rack and SUS rack (1X4/5), but did not go into the Vac region.
There was a jump in the main volume pressure at ~6pm PDT yesterday. The cause is unknown, but the pressure doesn't seem to be coming back down (but also isn't increasing alarmingly).
I wanted to look at the RGA scans to see if there were any clues as to what changed, but looks like the daily RGA scans stopped updating on Dec 24 2019. The c0rga machine responsible for running these scans doesn't respond to ssh. Not much to be done until the lockdown is over i guess...
No real progress tonight - I made it a bunch of times to the point where CARM was RF only, but I never got to run a measurement to determine what the DARM_B loop gain should be to make the control fully RF.
I measured the cross-calibration of the two PDs on the PSL table.
I used the existing flip mounted BS that routes the beam into a PDA255, the same as in the IMC transmission.
I placed a PDA520, the same as the one measuring TRY_OUT on the ETMY table, on the transmission of the BS (Attachment 1).
I used the SR785 to measure the frequency response of PDA520 with reference to PDA255 (Attachment 2). Indeed, calibration is quite significant.
I calibrated the Y arm frequency response measurement.
However, the data seem to fit well to 1/sqrt(f^2+fp^2) - electric field response - but not to 1/(f^2+fp^2) - intensity response. (Attachment 3).
Also, the extracted fp is 3.8KHz (Finesse ~ 500) in the good fit -> too small.
When I did this measurement for the IMC in the past I fitted the response to 1/sqrt(f^2+fp^2) by mistake but I didn't notice it because I got a pole frequency that was consistent with ringdown measurements.
I also cross calibrated the PDs participating in the IMC measurement but found that the calibration amounted for distortions no bigger than 1db.
Today I finished implementing loopback monitors of the up/down state of the slow controls machines. They are visible on a new MEDM screen accessible from Sitemap > CDS > Slow Machine Status (pictured in attachment 1). Each monitor is a single EPICS binary channel hosted by the slow machine, which toggles its state at 1 Hz (an alive "blinker"). For each machine, the monitor is defined by a separate database file named c1[machine]_state.db located in the target directory.
Sitemap > CDS > Slow Machine Status
This is implemented for all upgraded machines, which includes every slow machine except for c1auxey. This is the next and final one slated for replacement.
The blinkers are currently implemented as soft channels, but I'd like to ultimately convert them to hard channels using two sinking/sourcing BIO units. This will require new wiring inside each Acromag chassis, however. For now, as soft channels, the monitors are sensitive to a failure of the host machine or a failure of the EPICS IOC. As hard channels, they will additionally be sensitive to a failure of the secondary network interface, as has been known to happen.
Each slow machine's IOC had to be restarted this afternoon to pick up the new channels. The IOCs were restarted according to the following procedure.
The intial recovery of c1susaux did not succeed. Most visibly, the alignment state of the IFO was not restored. After some debugging, we found that the restart of the modbus service was partially failing at the final burt-restore stage. The latest snapshot file /opt/rtcds/caltech/c1/burt/autoburt/latest/c1susaux.snap was not found. I manually restored a known good snapshot from earlier in the day (15:19) and we were able to relock the IMC and XARM. GV and I were just talking earlier today about eliminating these burt-restores from the systemd files. I think we should.
I want to monitor the PMC TRANS and REFL levels on the PSL table - previously there were some cables going to the oscilloscope on the shelf but someone had removed these. I re-installed them just now. While there, I disconnected the drive to the AOM - there must've been some DC signal going to it because when I removed the cable, the PMC and IMC transmission were recovered to their nominal levels.
Updated the cert in /etc/httpd/ssl. The new cert is good until March 12, 2022.
Finally, some RF only CARM, see Attachment #1. During this time, DARM was also on a blend of IR and ALS control, but I couldn't turn the ALS path off in ~4-5 attempts tonight (mostly me pressing a wrong button). Attachment #2 shows the CARM OLTF, with ~2kHz UGF - for now, I didn't bother turning any boosts on. PRCL and MICH are still on 3f signals.
The recycling gain is ~7-8 (so losses >200ppm), but there may be some offset in some loop. I'll look at REFL DC tomorrow.
Can we please make an effort to keep the IFO in this state for the next week or two
- it really helped tonight I didn't have to spend 2 hours fixing some random stuff and could focus on the task at hand.
the long DB25 cable to connect the Acromag chassis to the temperature sensor interface box arrived. We laid it out today. This cable does the following:
Both signals now show up in the EPICS channels, but are noisy - I suspect this is because the return pin of the Acromag is not shorted to ground (this is a problem I've seen on the bench before). We will rectify this tomorrow as well.
We took this opportunity to remvoe the bench supply and temporary Acromag crate (formerly known as c1psl2) from under the PSL table. While trying to find some space to store the bench supply, we came across a damaged Oscilloscope in the second "Electronics" cabinet along the Y-arm, see Attachment #1.
After this work, I found that the IMC autolocker was reliably failing to run the mcup script at the stage where the FSS gains are ramped up to their final values. I was, however, able to smoothly transition to the low-noise locked state if I was manipulating the EPICS sliders by hand. So I added an extra 2 seconds of sleep time between the increasing of the VCO gain to the final value and the ramping of the FSS gains in the mcup script (where previously there was none). Now the autolocker performs reliably.
Of course the reboot wiped any logs we could have used for clues as to what happened. Next time it'll be good to preserve this info. I suspect the local subnet went down.
P.S. for some reason the system logs are priveleged now - I ran sudo sysctl kernel.dmesg_restrict=0 on c1psl to make it readable by any user. This change won't persist on reboot.
I restarted the IOC but it didn't help.
I am now rebooting c1psl... That seemed to help. PMC screen seem to be working again. I am able to lock the PMC now.
Came this morning to find the PMC was unlocked since 6AM. Laser is still on, but PMC REFL PD DC shows dead white constant 0V on PMC screen. All the controls on the PMC screen show constant 0V actually except for the PMC_ERR_OUTPUT which is a fast channel.
Is PSL Acromag already failing?
IMC was locking easily once some switches on the MC servo screen were put to normal states.
TTs were grossly misaligned. Onces they where aligned, arm cavities were locking easily. Dither align for the X arm is very slow though...
when doing the AM sweeps of cavities
make sure to cross-calibrate the detectors
else you'll make of science much frivolities
much like the U.S. elections electors
It's been a while since I've attempted any locking, so tonight was mostly getting the various subsystems back together.
I opened the packages send from Syracuse.
- The components are not vacuum clean. We need C&B.
- Some large parts are there, but many parts are missing to build complete SOSs.
- No OSEMs.
- Left and right panels for 6 towers
- 3 base blocks
- 1 suspension block
- 8 OSEM plates. (1 SOS needs 2 plates)
- The parts looks like old versions. The side panels needs insert pins to hold the OSEMs in place. We need to check what needs to be inserted there.
- An unrelated tower was also included.
Attachment #1 shows the relevant parts of the schematic of the WFS demod board (not whitening board).
Before removing the boards from the eurocrate:
After Koji effected the fix, the boards were re-installed, HV supplies were dialled back up to nominal voltage/currents, and the PMC/IMC were re-locked. The WFS DC channels now no longer saturate even when the IMC is unlocked 👏 👏 . I leave it to Yehonathan / Jon to calibrate these EPICS channels into physical units of mW of power. We should also fix the MEDM screen and remove the un-necessary EPICS channels.
Later in the evening, I took advantage of the non-saturated readbacks to center the beams better on the WFS heads. Then, with the WFS servos disabled, I manually aligned the IMC mirrors till REFLDC was minimized. Then I centered the beam on the MC2 transmission QPD (looking at individual quadrants), and set the WFS1/2 RF offsets and MC2 Trans QPD offsets in this condition.
WFS DC channels are saturating when the IMC is unlocked.
My old scheme was flawed as I used pitch as the readback. The pitch signal could not distinguish the cross-coupling due to coil imbalance and that due to the natural suspension L2P. A new scheme based on yaw alone has been developed and will be integrated into ifo_test. For now we revert the C1:SUS-MC2_UL/UR/LR/LLCOIL gains back to 1, -1, 1, -1.
We did some quick DC balancing of the MC2 coil drivers to reduce the l2a coupling. We updated the gains in the C1:SUS-MC2_UL/UR/LR/LLCOIL to be 1, -0.99, 0.937,-0.933, respectively. The previous values were 1, -1, 1, -1.
The procedures are the following:
Drive UL+LR and change the gain of LR to zero pitch.
Drive UR+LL and change the gain of LL to zero pitch.
Lastly, drive all 4 coils and change UR & LR together to zero yaw.
We used C1:SUS-MC2_LOCKIN1_OSC to create the excitations at 33 Hz w/ 30,000 cts. The angular error signals were derived from IMC WFSs.
While this time we did things by hand, in the future it should be automated as the procedure is sufficiently straightforward.
I want to measure the transfer function of the arm cavities to extract the pole frequencies and get more insight into what is going on with the DC loss measurements.
The idea is to modulate the light using the PSL AOM. Measure the light transmitted from the arm cavities and use the light transferred from the IMC as a reference.
I tried to start measuring the X arm but the transmission PD is connected to the QPD whitening filter board with a 4 pin Lemo for which I couldn't find an adapter.
Could this be because of the PDA520 limited BWs? I tried playing with the PD gain/bandwidth switch but it seems like it was already set to high bandwidth/low gain.
In any case, the extracted pole frequency ~ 2.9kHz implies a finesse > 600 (assuming FSR = 3.9MHz) which is way above the maximal finesse (~ 450) for the arm cavities.
I disconnected the source from the AOM. But left the other two BNCs connected to the SR785. Also, TRY PD is still teed off. Long BNC cable is still on the ground.