Right, I doubt they were ever recorded or used for interlocks. But the readbacks did work at one point in the past. There's a photo of the old vac monitor screen on p. 19 of E1500239 (last updated 2017) which shows the fields once alive.
I don't disagree that the pressure gauges would register the change. What I'm not sure about is whether the change would violate any of the existing interlock conditions, triggering a shutdown. Looking at what we have now, the only non-pump-related conditions I see that might catch it are the diffpres conditions:
abs(P2 - PTP2) > 1 torr (for a TP2 failure)
abs(P3 - PTP3) > 1 torr (for a TP3 failure)
abs(P1a - P2) > 1 torr (for either a TP2 or TP3 failure)
For the P1a-P2 differential, the threshold of 1 torr is the smallest value that in practice still allows us to pump down the IFO without having to disable the interlocks (P1a-P2 is the TP1 intake/exhaust differential). The purpose of the P2-PTP2/P3-PTP3 differentials is to prevent V4/5 from opening and suddenly exposing the spinning turbo to high pressure. I'm not aware of a real damage threshold calculation that any one has done; I think < 1 torr is lore passed down by Steve.
If a turbo pump fails, the rate it would backstream is unknown (to me, at least) and likely depends on the failure mode. The scenario I'm concerned about is if the backstream rate is slower than the conduction time through the pumspool and into the main volume. In that case, the pressure gauges will rise more or less together all the way up to atmosphere, likely never crossing the 1 torr differential pressure thresholds.
There's already a channel C1:Vac-error_status, where if the value is anything other than an empty string, there is an interlock tripped. Does that work?
I agree there were MEDM fields, but I can't find any record of these channels being recorded till 2018 December, so I don't agree that they were being digitally monitored. You can also look back in the elog (e.g. here and here) and see that the display fields are just blank. I would then assume that no interlocks were dependent on these channels, because otherwise the vacuum interlocks would be perpetually tripped.
Sorry but I'm having trouble imagining a scenario how the pressure gauges wouldn't register this before the IFO volume is compromised. Is there some back of the envelope calculations I can do to understand this? Since both the pressure gauges and the TP diagnostic channels are being monitored via EPICS, the refresh rate is similar, so I don't see how we can have a pump temperature / speed / current threshold tripped but NOT have this be registered on all the pressure gauges, seems like a bit of a contrived scenario to me. Our thresholds currently seem to be arbitrary numbers anyway, or are they based on some expected backstreaming rate? Isn't this scenario degenerate with a leak elsewhere in the vacuum envelope that would be caught by the differential pressure interlocks?
For the email alert, can you expose a soft channel that is a flag - if this flag is not 1, then the service will send out an email.
Looking at images of the old vac screens, the TP2/3 rotation speed and status string were digitally monitored. However I don't know if there were software interlocks predicated on those.
The temperature and current interlocks are implemented precisely because the pumps can shut themselves off. The concern is not about damaging the pumps (their internal logic protects against that); it's that a pump could automatically shut down and back-vent the IFO to atmosphere. Another interlock (e.g., the pressure differentials) might catch it, but it would depend on the back-vent rate and the scenario has never been tested. The temperature and current interlocks are set to trip just before the pump reaches its internal shut-down threshold.
One way we might be able to reduce our reliance on the flaky serial readbacks is to implement rotation-speed hardware interlocks. The old vac documentation alludes to these, but as far as Chub and I could determine in 2018, they never actually existed. The older turbo controllers, at least, had an analog output proportional to speed which could be used to control a relay to interrupt the V4/5 control signals. I'll look into this for the new controllers. If it could be done, we could likely eliminate the layer of serial-readback interlocks altogether.
That would be awesome if you're willing to volunteer. I agree this would be great to have.
I will be at the 40m today from 9:30am to 4pm.
I missed the vacuum discussion on the call today, but I have some questions/comments:
At the very least, I think we should consider making the interlock code have levels (like interrupts on a micro controller). So if the pressure gauges are communicating and are reporting acceptable pressure readings, we should be able to reject unphysical readbacks from the TP controllers.
I still don’t understand why TP2 can’t back TP1, but we just disable all the software interlock conditions contingent on TP2 readbacks. This pump is far newer than TP3, and unless I’ve misunderstood something major about the vacuum infrastructure, I don’t really see why we should trust this flaky serial readbacks for any actionable interlocks, at least without some AND logic (since temperature, current and speed aren’t really independent variables).
I also think we should finally implement the email alert in the event the vacuum interlock is tripped. I can implement this if no one else volunteers.
This might also be a good reminder to get the documentation in order about the new vacuum system.
I replaced an empty N2 cylinder, there are now two empty tanks in the outside rack.
[Jon, Jordan, Koji]
Today Jordan reconfigured the vac system to allow pumping of the main volume resume, with Jon and Koji remotely advising. All clear to resume normal IFO activities. However, the vac system is operating in a temporary configuration that will have to be reverted as we locate replacement components. Details below.
Since serial readback of the TP2 controller seems to be failing, we reconfigured the system with TP3 now backing for TP1. TP2 was valved off (at V4) and shut down until we can replace its controller.
TP3 has its own problems, however. It was valved off in January after its temperature readback began glitching and spuriously triggering the interlocks [ELOG 15140]. However the problem appears to be limited only one readback (rotation speed, current, voltage are fine) and there is enough redundancy in the pump-dependent interlock conditions to safely connect it to the main volume.
We also discovered that sometime since January, the TP3 dry pump has failed. The foreline pressure had risen to 165 torr. Since the TP2 and TP3 dry pumps are not interchangeable (Agilent vs. Varian), we instead valved in the auxiliary dry pump and disconnected the failed dry pump using a KF blank. This is a temporary arrangement until the permanent dry pump can be repaired. Jordan removed it to replace the tip seals and will test it in the bake lab before reinstalling.
With this configuration in place, we proceeded to pump down the main volume without issue (attachment 1). We monitored the pumpdown for about 45 min., until the pressure had reached ~1E-5 torr and TP3 had been transitioned to standby (low-speed) mode.
ITMU01 / ITMU02 as well as the five E1800089 mirrors came back to the 40m. Instead, the two ETM spares (ETMU06 / ETMU08) were delivered to GariLynn.
Jordan worked on transportation.
Note that the E1800089 mirrors are together with the ITM container in the precious optics cabinet.
I will be at the 40m today at 10am to deliver optics to Downs and to replace the TP2 controller.
Didn't mean to sound whiny. I will wait until the vacuum team tells me it is okay.
The vacuum safety policy and design are not clear to me, and I don't know what the first and second defense is. Since we had limited time and bandwidth during the remotely-supported recovery work today, we wanted to work step by step.
The pressure rising rate is 20mtorr/day, and turning on TP3 early next week will resume the main-volume pumping without too much hustle. If you need the IFO time now, contact with Jon and use backing with TP3.
I still don't understand why restoring the vacuum is contingent on this functionality working. All the TPs have their own internal logic to shutdown the pump if some damage threshold is exceeded. Plus, we have the pressure-sensor based interlocks to protect the main volume as well as pumps. While the extra redundancy from the readbacks from the controller is useful, clearly it isn't the first line of defense?
The main volume pressure is currently ~10mTorr. If we pump down before this reaches 500mTorr, the procedure is pretty straightforward. Otherwise, we have to do the dance with the manual throttling valve (judging by current rate of increase, unlikely to exceed this over the weekend, but I lose IFO time).
Obviously I don't want to rush this and have some permanent damage, so I'll stay out of this unless otherwise instructed.
Jon and Koji remotely supported Jordan's resetting the TP2 controller.
From the operator's console in front of the vac rack:
Open a terminal window (click the LXTerminal icon on the desktop)
Type "control" + enter to open the vac controls screen
Toggle all the open valves closed (edit by KA: and manually close RV2 by rotating the gate valve handle )
Turn OFF TP2 by clicking the "Off' button. Make sure the status changes and the rotation speed falls to zero (you'll also hear the pump spinning down)
The other pumps (TP1, TP3) can be left running
Once TP2 has stopped spinning, go to the back of the rack and locate the ethernet cable running from the back of the TP2 controller to the IOLAN server (near the top of the rack). Disconnect and reconnect the cable at each end, verifying it is firmly locked in place.
From the front of the rack, power down the TP2 controller (I don't quite remember for the Agilent, but you might have to move the slider on the front from "Remote" to "Local" first)
Wait about 30 seconds, then power it back on. If you had to move the slider to shut it down, revert it back to the "Remote" position.
Go back to the controls screen on the console. If the pump came back up and is communicating serially again, its status will say something other than "NO COMM"
Turn TP2 back on. Verify that it spins up to its nominal speed (66 kRPM)
At this point you can reopen any valves you initially closed (any that were already closed before, leave closed)
TP2 was stopped and at this moment the glitches were gone. Jordan powercycled the TP2 controller and we brought up the TP2 back at the full speed.
However, the glitches came back as before. Obviously we can't go on from here, and we've decided to stop the recovery process here today.
- We left TP1/2/3 running while the valves including RV2 were closed.
- When Jordan is back in the lab next week, we'll try to use TP3 as the backing of TP1 so that we can resume the main volume pumping.
- Currently, TP3 does not have interlocking and that is a risk. Jon is going to implement it.
- Meanwhile, we will try to replace the controller of TP2. We are supposed to have this in the lab. Ask Chub about the location.
- Once we confirm the stability of the diagnostic signals for TP2, we will come back to the nominal pumping scheme.
I will be in the Clean and Bake lab today from 12pm to 4pm.
1. I agree that it's likely that it was the temp signal glitch.
Recom #2: I approve to reopen the valves to pump down the main volume. As long as there is no frequent glitch, we can just bring the vacuum back to normal with the current software setup.
2. Recom #1 is also reasonable. You can use simple logic like if we register 10 consecutive samples that exceed the threshold, we can activate the interlock. I feel we should still keep the temp interlock. Switching between pumping mode and the normal operation may cause unexpected omission of the interlocks when it is necessary.
3. We should purchase the UPS battery / replacement rotary TIP seal. Once they are in hand, we can stop the vacuum and execute the replacement. Can one person (who?) accomplish everything with some remote help?
4. The lab temp: you mean, 12degC swing with the AC on!?
The pumpspool UPS has its "Replace Battery" indicator light on. Might be a good chance to change the UPS, but at the very least, we should put in fresh batteries (last replacement was in Aug 2017).
I'll say this again - the pumpspool area is noisier than I remember it being, I think one / both of the roughing pumps backing TP2 / TP3 need tip-seal replacements.
BTW, EX is 5C hotter than EY, by virtue of the tarnac outside? In fact, judging by Steve's thermometers, EX reports a 12C swing in 24 hours between 30 C and 18 C (so almost no temperature control) while EY reports a 5C swing between 20 and 25 C. This is borne out by the ETM Oplev data I think...
It looks like the main vacuum interlock was tripped due to a serial communication error from the TP2 controller. With Rana/Koji's permission, I will open V1 and expose the main volume to TP1 again (#2 in last section).
Recommended course of action:
There appears to have been some sort of vacuum failure.
ldas-pcdev1 was down, so the summary pages weren't being generated. I have now switched over to ldas-pcdev6. I suspect some forepump failure, will check up later today unless someone else wants to take care of this.
There was no interlock action, and I don't check the vacuum status every half hour, so there was a period of time last night there was high circulating power in the arm cavities when the main volume pressure was higher than nominal. I have now closed the PSL shutter until the issue is resolved.
I will be in the Clean and Bake lab today from 11am to 4pm.
After further astigmatism/tolerance analysis [ELOG 15380, 15387] our conclusion is that the stock-optic telescope designs [ELOG 15379] are sufficient for the first round of BHD testing. However, for the final BHD hardware we should still plan to procure the custom-curvature optics [DCC E2000296]. The optimized custom-curvature designs are much more error-tolerant and have high probability of achieving < 2% mode-matching loss. The stock-curvature designs can only guarantee about 95% mode-matching.
Below are the final distances between optics in the relay paths. The base set of distances is taken from the 2020-05-21 layout. To minimize the changes required to the CAD model, I was able to achieve near-maximum mode-matching by moving only one optic in each relay path. In the AS path, AS3 moves inwards (towards the BHDBS) by 1.06 cm. In the LO path, LO4 moves backwards (away from the BHDBS) by 3.90 cm.
I will be in the Clean and Bake lab from 10am to 4pm today. I will also replace an empty N2 cylinder.
Using the updated AOI's for the LO path: (4.8, 47.9, 2.9, 4.5) deg for (LO1, LO2, LO3, LO4), we obtain the following results.
First two plots are scattering plots for the t and s planes, respectively. Note that here we have changed to 0.5% fractional RoC error and 3 mm positional error. We have also changed the meaning of the colors: pink:MM>0.98; olive 0.95<MM<=0.98, and grey MM<=0.95. It seems that both planes would benefit statistically if we make the LO3-LO4 distance longer by a few mm.
We also consider how much we could compensate for the MM error in the last plot. We have a few mm window to make both planes better than 0.95.
I don't think we ever discussed why the angular RMS of the ETMs is so much higher than the ITMs. Maybe that's a separate matter because, even assuming the worst case, the actuation range requirement is
(0.82 μrad RMS) x (15 μrad/μrad) x (10 safety factor) = 0.12 mrad
which is still only order 1% of the pitch/yaw pointing range of the Small Optic Suspensions, according to P1600178 (sec. IV. A). Can we check this requirement off the list?
We computed the required actuation range for the telescope design in elog:15357. The result is summarized in the table below. Here we assume we misalign an IFO mirror by 1 urad, and then compute how many urad do we need to move the (AS1, AS4) or (LO1, LO2) mirrors to simultaneously correct for the two gouy phases.
The most demanding ifo mirrors are the ETMs and the BS, for every 1 urad misalignment the telescope needs to move 10-15 urad to correct for that. However, it is unlikely for those mirrors to move more 100 nrad for a locked ifo with ASC engaged. Thus a few urad actuation should be sufficient. For the recycling mirrors, every 1 urad misalignment also requires ~ 1 urad actuation.
As a result, if we could afford 10 urad actuation range for each telescope suspension, then the gouy phase separations we have should be fine.
We looked at the oplev spectra from gps 1274418500 for 512 sec. This should be a period when the ifo was locked in the PRFPMI state according to elog:15348. We just focused on the yaw data for now. Please see the attached plots. The solid traces are for the ASD, and the dotted ones are the cumulative rms. The total rms for each mirror is also shown in the legend.
I am now confused... The ITMs looked somewhat reasonable in that at least the < 1 Hz motion was suppressed. The total rms is ~ 0.1 urad, which was what I would expect naively (~ x100 times worse than aLIGO).
There seems to be no low-freq suppression on the ETMs though... Is there no arm ASC at the moment???
I will be in the Clean and Bake lab today from 9:30am to 4pm.
Hmm? T1300364 suggests MM_total = Sqrt(MM_Vert * MM_Horiz)
Around 5pm local time, the three vertex FEs crashed. AFAIK, no one was in the lab or working on anything CDS related, so this is worrying.
MM_total = (MM_vert + MM_horiz) / 2.
The large astigmatic MM loss in the LO case is mainly due to the strong LO4 curvature (R=0.15m) with a 10 deg AOI. I looked again at whether LO1 could be increased from R=5m to the next higher stock value of 7.5m, as this would allow weaker curvatures on LO3 and LO4. However, no, that is not possible---it reduces the LO1-LO2 Gouy phase separation to only 18 deg.
There is, however, a good stock-curvature option if we want to reconsider actuating LO4 instead of LO2 (attachment 1). It achieves 99.2% MM with the OMCs, allowing positions to vary +/-1" from the current design. The LO1-LO4 Gouy phase separation is 72 deg.
Alternatively, we could look at reducing the AOI on LO3 and LO4 (keeping LO1-LO2 actuation).
Can you describe the mode matching in terms of the total MM? Is MM_total = sqrt(MM_vert * MM_horiz)?
We consider the astigmatism effects of the stock options. The conclusions are:
1. For the AS path, the stock should work fine for the phase-one of BHD, if we could tolerate a few percent MM loss. The window for length adjustment to achieve >99% MM for both s and t is only 1 mm for 1% RoC error (compared to ~ 1 cm in the customized case).
2. The LO path seemed tricky. As LO3 & LO4 are both significantly curved (RoC<=0.5 m), the non-zero angle of incidence makes the astigmatism quite sever. For the t-plane the nominal MM can be 0.98, yet for the s-plane, the nominal MM is only 0.72. We could move things around to achieve a MM ~ 0.85, which is probably fine for the phase-one implementation but not long term.
Attachments 1-3 are for the AS path; 4-6 are for the LO path.
1 & 4. Marginalized MM distribution for the AS/LO paths. Here we assumed 5 mm positional error and 1% fractional RoC error. Due to the astigmatism, the nominal s-plane MM is only 0.72 for the LO path.
2 & 5. Scattering plots for the AS/LO paths. We color coded the points as the following: pink: MM>0.99; olive: 0.98<MM<=0.99; grey: MM<=0.98. For the AS path, MM is mostly sensitive to the AS1 RoC and can be adjusted by changing AS1-AS3 distance. For the LO path, the LO3 RoC and LO3-LO4 distance are most critical for the MM.
3 & 6. Assuming +- 1% AS1 (LO3) fractional RoC error, how much can we compensate for it using AS1-AS3 (LO3-LO4) distance. For the AS path, there exists a ~ 1 mm window where the MM for s and t can simultaneously > 99%. For the LO path, the best we can do is to make s and t both ~ 85%.
For the initial phase of BHD testing, we recently discussed whether the mode-matching telescopes could be built with 100% stock optics. This would allow the optical system to be assembled more quickly and cheaply at a stage when having ultra-low loss and scattering is less important. I've looked into this possibility and conclude that, yes, we do have a good stock optics option. It in fact achieves comprable performance to our optimized custom-curvature design [ELOG 15357]. I think it is certainly sufficient for the initial phase of BHD testing.
It turns out our usual suppliers (e.g., CVI, Edmunds) do not have enough stock options to meet our requirements. This is for two reasons:
However I found that Lambda Research Optics carries 1" and 2" super-polished mirror blanks in an impressive variety of stock curvatures. Even more, they're polished to comprable tolerances as I had specificied for the custom low-scatter optics [DCC E2000296]: irregularity < λ/10 PV, 10-5 scratch-dig, ROC tolerance ±0.5%. They can be coated in-house for 1064 nm to our specifications.
From modeling Lambda's stock curvature options, I find it still possible to achieve mode-matching of 99.9% for the AS beam and 98.6% for the LO beam, if the optics are allowed to move ±1" from their current positions. The sensitivity to the optic positions is slightly increased compared to the custom-curvature design (but by < 1.5x). I have not run the stock designs through Hang's full MC corner-plot analysis which also perturbs the ROCs [ELOG 15339]. However for the early BHD testing, the sensitivity is secondary to the goal of having a quick, cheap implementation.
The following tables show the best telescope designs using stock curvature options. It assumes the optics are free to move ±1" from their current positions. For comparison, the values from the custom-curvature design are also given in parentheses.
The AS relay path is shown in Attachment 1:
The LO relay path is shown in Attachment 2:
I've created a new tab in the BHD procurement spreadsheet ("Stock MM Optics Option") listing the part numbers for the above telescope designs, as well as their fabrication tolerances. The total cost is $2.8k + the cost of the coatings (I'm awaiting a quote from Lambda for the coatings). The good news is that all the curved substrates will receive the same HR/AR coatings, so I believe they can all be done in a single coating run.
I will be at the 40m, in the Clean and bake lab today from ~9am to ~3pm.
We can limit the EPICS values giving some parameters to the channels. cf https://epics.anl.gov/tech-talk/2012/msg00147.php
But this does not solve the MC1 issue. Only we can do right now is to make the output resister half, for example.
I found that there is an issue with the MC1 slow bias voltages.
I usually offload the DC part of the output voltage from the WFS servos to the slow bias voltage sliders, so as to preserve maximum actuation range from the fast system. However, today, I found that this servo wasn't working well at all. So I dug a little deeper. Looking at the EPICS database records:
GariLynn worked on the measurement of E1800089 mirrros.
The result of the data analysis, as well as the data and the codes, have been summarized here:
This EQ seems to have knocked all suspensions out. ITMX was stuck. It is now released, and the IMC is locked again. It looks like there are some serious aftershocks going on so let's keep an eye on things.
I am inclined to believe that the arm cavity losses are such that the IFO is overcoupled. Some calculations, validated with Finesse modeling also suggest that there isn't a sign change for the CARM error signal when the IFO goes from being undercoupled to overcoupled, but I may have made a mistake here?
Thoughts from others?
For these initial attempts, I was just trying to transition MICH to REFL55Q. I agree, the PRCL situation may be more complicated.
Which 1f signals are you going to use? PRCL has sign flipping at the carrier critical coupling. So if the IFO is close to that condition, 1f PRCL suffers from the sign flipping or large gain variation.
The 40m summary pages have been revived. I've not had to make any manual interventions in the last 5 days, so things seem somewhat stable, but I'm sure there will need to be multiple tweaks made. The primary use of the pages right now are for vacuum, seismic and PSL diagnostics.
I implemented an ASC servo for the PRC, with the POP QPD as a sensor, and the PRM as the actuator. This has improved the stability of the lock (longer locks are possible), and also reduced the RIN of the arm transmission.
Attachment #1 shows the in-loop error signal suppression, and some out-of-loop monitors (POP22 and POPDC).
Attachment #2 compares the arm transmission RIN with the PRFPMI locked, with and without PRC ASC. The 3 Hz bump is definitely squished, but I think we can do better yet.
Attachments #3-5 are in the style of elog15361. No Oplev signals yet, I'll add them soon.
I guess what this means is that the stability of the lock could be improved by turning on some POP QPD based feedback control, I'll give it a shot
Attachments #1 and Attachments #2 are in the style of elog15356, but with data from a more recent lock. It'd be nice to calibrate the ASDC channel (and in general all channels) into power units, so we have an estimate of how much sideband power we expect, and the rest can be attributed to carrier leakage to ASDC.
On the basis of Attachments #1, the PRG is ~19, and at times, the arm transmission goes even higher. I'd say we are now in the regime where the uncertainty of the losses in the recycling cavity - maybe beamsplitter clipping? is important in using this info to try and constrain the arm cavity losses. I'm also not sure what to make of the asymmetry between TRX and TRY. Allegedly, the Y arm is supposed to be lossier.
This is very interesting. Do you have the ASDC vs PRG (~ TRXor TRY) plot? That gives you insight on what is the cause of the low recycling gain.
The CARM loop now has a UGF of ~12 kHz with a phase margin of ~60 degrees. These values of conventional stability indicators are good. The CARM optical gain that best fits the measurements is 9 MW/m.
I've been working on understanding the loop better, here are the notes.
Attachment #1 shows a block diagram of the loop topology.
Attachment #2 shows the OLGs of the two actuation paths.
Attachment #3 and #4 show the model, overlaid with measurements of the loop OLG and crossover TF respectively.
Attachment #5 shows the evolution of the CARM OLG at a few points in the lock acquisition sequence.
Now the I have a model I believe, I need to think about whether there is any benefit to changing some of these loop shapes. I've already raised the possibility of changing the shape of the boosts on the CM board, with which we could get a bit more suppression in the 100 Hz - 1kHz region (noise budget of laser frequency noise --> DARM required to see if this is necessary).
There were many locklosses from the point where the arm powers were somewhat stabilized. Attachments #1 and #2 show two individual locklosses. I think what is happening here is that the BS seismometer X channel is glitching, and creating a transient in the angular feedforward filter that blows the lock. The POP QPD based feedback loop cannot suppress this transient, apparently. For now, I get around this problem by boosting the POP QPD feedback loop a bit, and then turning the feedforward filters off. The fact that the other seismometer channels don't report any transient makes me think the problem is either with the seismometer itself, or the readout electronics. The seismometer masses were recently recentered, so I'm leaning towards the latter.
I didn't explicitly check the data, but I am reasonably certain the same effect is responsible for many PRMI locklosses even with the arms held off resonance (though the tolerance to excursions there is higher). Pity really, the feedforward filters were a big help in the lock acquisition...
how bout corner plot with power signals and oplevs? I think that would show not just linear couplings (like your coherence), but also quadratic couplings (chesire cat grin)
I agree, I think the PRC excess angular motion, PIT in particular, is a dominant contributor to the RIN. Attachments #1-#3 support this hypothesis. In these plots, "XARM" should really read "COMM" and "YARM" should really read "DIFF", because the error signals from the two end QPDs are mixed to generate the PIT and YAW error signals for these ASC servos - this is some channel renaming that will have to be done on the ASC model. The fact that the scatter plot between these DoFs has some ellipticity probably means the basis transformation isn't exactly right, because if they were truly orthogonal, we would expect them to be uncorrelated?
I guess what this means is that the stability of the lock could be improved by turning on some POP QPD based feedback control, I'll give it a shot.
- PRC TT misalignment (~3Hz)
Don't can you check the correlation between the POP QPD and the arm RIN
I see. At the 40m, we have the direct transition from ALS to RF. But it's hard to compare them as the storage time is very different.
My speculation for the worse RIN is:
- Unoptimized alignment -> Larger linear coupling of the RIN with the misalignment
- PRC TT misalignment (~3Hz)
Don't can you check the correlation between the POP QPD and the arm RIN?