I spent some time this afternoon reviving some of my CESAR/ESCOBAR shenanigans on the Y arm. I found it neccesary to adjust a few things.
Afterwards, ALSY noise levels were good.
I've gone through the SOS suspension document (E970037) and some old elogs to get an idea of all the accesories we need for the process of suspending, aside from the tower itself, which Steve has already put together. Gautam and I have laid our eyes upon most of the critical pieces. Some other objects are unknown, and perhaps not strictly neccesary.
Confirmed to exist:
In addition, I am told that we have a long ribbon cable that can run from the X end to the clean room to enable OSEM damping control while we do the pitch alignment.
Things mentioned in the procedure I have not found:
Some other tasks and their status:
I have updated the vent prep checklist on the wiki. Gautam and I did the following things from it:
Reduce input power to no more than 100mW by adjusting wave plate+PBS setup on the PSL table BEFORE the PMC. (Using the WP + PBS that already exist after the laser.)
The following bullets have not yet been executed:
Check crane functionality & cleanliness
Steve has ordered some teflon parts to take the place of the metal parts in his acetone-soaking jig. They should arrive tomorrow.
So, we will be begin the venting process tomorrow. Doors to come off on Tuesday.
Here are some plans / rough procedures for this week's vent. It is unlikely that I have though of everything, but this should be a reasonable starting point.
The mode cleaner still hasn't been locked in air, we may not want to touch the Y arm optics until we are able to lock to the Y arm and dither align, so we are sure to keep the input pointing from drifting away too much.
For $optic in [ITMX, ITMY, ETMY]:
Rough summary of today's progress:
I didn't really see anything out of the ordinary on the ETMX suspension. Earthquake stops had clearance, OSEMS were secure, no visible glue degredation on face magnets. Inspection with green LED flashlight didn't reveal any obscene dirtieness on either face, just a few particles here and there. The top of the opic barrel unsurprisingly has a good amount of particulate. The wire grooves are way too small to resolve anything at this point, other than that they exist.
The suspension footprint is already marked, tomorrow we can move the suspension closer to the door to get an even closer look at it, before removing it from the chamber.
One glitch was seen to occur without a change in the output voltage monitors in ELOG 11744
It may be advantageous to look at the coil output data from when the OSEM damping is on, to try and reproduce the real output signal amplitude that gets sent to the coils.
Based on Koji's observation of a flat TF, it seems more likely the Vmon channels are looking at the path I've highlighted in green (named "EPICS V Mon"), rather than the path in red (named "DAQ Mon") that Koji initially suspected. This path still lacks any AA for the 16Hz EPICS sampling.
ETMX is currently in the clean room, the barrel is the tiniest bit submerged in acetone that will remove a guide rod, standoff, and side OSEM.
Additional inpsection of the standoffs on the flow bench did not provide any insight, pictures are in picasa. Here is a cropped version of a picture we took:
We should look at them under a microscope.
The magnet, guide rod, and standoff came off without too much force. However, some epoxy residue remains on the barrel. I didn't really want to scrape it off, so I've opted for more soaking. Much of the acetone had evaporated already, so I put some more - just to the point where the residue is submerged.
I was hoping to glue a standoff and guide rod today, but some problems have reared their heads. Story follows:
Upon first placng the optic into the standoff gluing fixture, I was presented with a geometric problem. In the assembly procedure, one glues the rods before the magnets, which prevents a situation like this:
When what you want to do is this:
So, I spun the optic around such that the magnet is on the far side of the scribe line from the side arm, and instead of extending the side arm past the scribe line, will bring it back towards the near side. I also swapped the arms of the fixture such that the guide rod will be glued on the opposite side of the optic than the side magnet, so the side magnet won't get in the way when doing the pitch adjustment of the second standoff.
Then, I found the scribed ruby rods, and took a look at one under a microscope. The groove looks nice and sharp. I placed the standoff in the side arm of the fixture.
However, the fact that the groove does not go all the way around the standoff leads to problem #1: when adjusting the position of the side arm, the standoff seems to roll around unpredictably, making it hard to deterministically position it while keeping the groove facing outwards.
Problem #2 is not too surprising give Steve's finding about the guide rod holding arm in ELOG 12264. Given that the tip is banged up, the guide rod does not sit straight in the arm, making it crooked. This would lead to the second standoff's groove not being well aligned to the suspension wire.
I will meditate on solutions to these problems... I have covered the optic and fixture with the same foil hut Koji made on Friday.
Also, I peeked at the aluminum standoffs under the microscope. Since the groove goes all the way around, we don't really know where the wire was seated before. Still, there are some places where the groove looks kind of worn:
For some reason, all of the non-IOP models on the vertex frontends had crashed.
To get all of the bits green, I ended up restarting all models on all frontends. (This means Gautam's coil tests have been interrupted.)
It took a little time, but I relocked the IMC and realigned to the point where the PRC is flashing, visible on REFL and AS, and tiny flashes are visible in TRY.
I found a note on Steve's desk that R. Abbott left yesterday afternoon about an unidentified slippery substance being present on the floor by cabinet S12, along the X arm. (Steve is away this week)
Just now, I found no trace of the substance in the vicinity of that cabinent (which is one of the cabinets for clean objects). Maybe the janitor cleaned it already?
We have positioned and applied epoxy to one ruby standoff on ETMX, for overnight curing according to the SOS standoff gluing procedure. This included:
Instead of trying to fix up a way of gluing the guiderod with the proper alignment, we chose to be more conservative and glue the standoff today, then switch the gluing fixture's arms tomorrow to glue the guide rod with the good fixture arm.
Additionally, we chose to glue one of the more assymetric standoffs on this first side. What I mean by this is: We have 3 ruby standoffs with grooves. Two of them have the groove about 1/8th of the way along their length, and one has it about 1/4 of the way. Since the second standoff is going to be glued while suspended, after pitch balancing, we figure that we want to use the more centered groove on that side, meaning we used one of the 1/8th standoffs today.
Unfortunately, we neglected to take any pictures :/
The new ETMX ruby guide rods are slightly thicker than the old aluminum ones; specifically 1.27mm vs 1.0mm.
Since we did not change the guide rod location in response to this fact, the vertical position of the suspension point changes, which in turn changes the dynamics of the suspension. Specifically, since the standoff is placed below the guide rod, the suspension point is lowered, which makes the pitch mode softer. I crunched a few numbers and have determined that this effect should not be a problem.
Given the wiki's value of the ETMX pitch resonance frequency of 0.829 Hz, I predict a the new pitch resonance frequency of 0.800 Hz.
(wiki link: https://wiki-40m.ligo.caltech.edu/Suspensions/Mechanical_Resonances)
A useful document about the dynamics of our suspension can be found at T000134
From this document, one will find that the effect of changing the suspension point height over the optic center of mass,`b`, on the pitch resonance frequency (while keeping all other dimensions equal) to be:
The top of the standoff is fixed by the guide rod, so let's say that b' is given by the position of the center of the Ruby standoff. This is then smaller than the previous b by the differences in the radii of the standoffs:
The nominal value of b is 0.985mm. Thus, the pitch resonance frequency is changed by factor of 0.965, i.e. 3.5% smaller. Then, taking the wiki value of 0.829 Hz results in 0.800Hz, a 30mHz decrease.
One step forward, two steps back...
While attempting to suspend ETMX, I broke off a side magnet
It is now gluing
(This is *not* the one that was previously glued. I.e., now both ETMX side magnets have been reglued)
When Koji and I were gluing magnets to ETMY, we decided to position the side magnet based on the empirically observed offsets from the standoff groove seen at other side magnet locations. Specifically, we figured that the magnet should be glued 1.25mm closer to the HR surface than the wire groove.
However, Steve has told me that he believed that this distance should be something like 0.5mm.
I used the 1.25mm figure when gluing the ETMX side magnets, which now do not align well to their OSEM mounts. While it is certainly possible that I made an error when shimming the fixture, I think it is also possible that this figure was incorrect.
Sadly, after poring through the DCC and various elogs, I have not been able to come up with a definitive answer on what this offset should actually be.
One approach is to examine the suspension tower dimensions. I.e. when subject only to gravity, the wire loop should lie in the plane of the back face of the top block of the suspension, as it is constrained by the clamps. Thus, the standoff grooves also lie in this plane. The center of the side OSEM mounting holes are about 1.64mm in front of this plane, which is larger than the 1.25mm figure that Koji and I came up with. Examining the picture Gautam posted of the marginal magnet/OSEM alignement, we see that this figure would in fact move the magnet in the wrong direction...
ELOGs in which the intitial side magnet gluing and fixture shimming are detailed do not reference the absolute position of the side magnet, nor do they include any pictures of their fixture setup. (Some links for the curious: 2652 2654 2668)
The DCC isn't much help either, as it is not clear what version of the gluing fixture we actually have. There is a drawing for a 40m specific version, but it includes swappable side-magnet-pickle-picker-slots to achieve different positions for different (circa 2001) optics; this is not the kind of fixture we currently have in our possesion. (https://dcc.ligo.org/D010131) I have discovered that some versions of this fixture (https://dcc.ligo.org/d990168) include an assumed 0.5deg wedge angle and thus position the two side slots differently. Although the fixture we have has no identifying marks on it whatsoever (naturally), I measured the two side slots to be different in axial position by roughly 0.6mm, which is consistent with a 0.5deg wedge. Furthermore, the sign of this difference indicates that this fixture ring is designed for the opposite wedge orientation than our ETMs, which have a 2.5deg wedge, making this fixture wrong by 3deg (which is ~4mm over the diameter of the optic).
We did not account for this for either ETMX or ETMY, so this is another source of error, but this does not give us much guidance on what the real absolute magnet position should be.
(Full resolution versions of the photos in this ELOG are on picasa)
The OSEM gender changers were not in the box labelled as such, we need these to be able to use the OSEMS to see just how bad the side magnet alignment is, and to do any kind of damping for the fine pitch balancing. The hunt is on.
In the meantime, Gautam and I checked out the standoff seating, and alignment of the face OSEMS (after slightly adjusting the wire length - I guess some sagging is still happening).
With a bit of poking, we convinced ourselves that we sat the standoff in contact with the optic's barrel. Amazingly, we were able to maintain the coarse pitch balance of the optic.
We then partially inserted the face OSEMS, to check their magnet alignment. ("partially" means that the OSEM is not actually enclosing the magnet, we don't want to knock anything off) They seem ok, but not perfect. These magnets were not removed or reglued, so presumably their alignments should be unchanged.
c1susaux (which controls watchdogs and alignments for all non-ETM optics) was down, the last BURT was done yesterday around 2PM.
I restarted via keying the crate. I restored the BURT snapshot from yesterday.
Brief summary, some pictures and such follow in the daytime.
The epoxy needs at least 12 hours of room temperature air curing, so no touchy until 3:30PM on Jul 28!
Tonight's progress on ETMX:
Since the air bake oven we had been using is out of commision, we're not sure where to do our EP30 test runs. If we are fortunate, we can get the fine pitch balance done tomorrow while Bob is still around, so he can help us quickly bake the test dots, so we can do the standoff gluing.
The question arose whether we can get good enough data to diagonize our OSEM sensing matrices in air.
I just took a look at the BS spectra over the last six hours (~10PM-4AM), and the SNR looks good. The BS diagonalization itself doesn't seem so great; the POS is hugely coupled into pitch and yaw, and the angular motions are themselves coupled to each other at around 10%.
NB: use a flat-top window when you really care about peak heights that don't fall exactly on an FFT bin.
I would've liked to check this for the PRM and SRM too, but one of the PRM sensors continues to be dark, and I just noticed that all of the SRM OSEM signals are dark. ughhhh
We have indeed seen numerous tarnished/rusty points along the wires, and just tried to choose lengths free of any of these. I wonder if this can explain the brittleness/ease with which we've been breaking it. My feeling is that we should use the newer wire if feasible.
I turned off the air bake oven at 8:45AM. I'll leave the optics alone for a bit while it cools.
We've seen for some time now that one of the PRM OSEM signals has been gone, and all of the SRM signals seem dark. We had tried squishing various cables to no avail.
Today I played some "musical satellite boxes," in an attempt to see if the problems are in the chambers or in the signal chains. That is, I swapped the OSEM cables from the vacuum feedthroughs between the satellite boxes, and observed what happened.
It seems clear that something is up with SRM inside the chamber. For PRM, it's not so clear...
Somehow, issues with the LR channel follow both the PRM OSEMs and the PRM satellite box.
PRM LR first went dark on Jul 2nd, after the IFO was vented, but before we took any doors off (which happened on the 5th). I'm not sure what may have caused this.
SRM OSEMS first went dark on the evening of Jul 18, the day before ELOG 12310, when ITMY was moved in the same chamber. Maybe this ELOG was written about work the day before, but the sensors show disturbances over the course of hours. I think we need to double check the connections in chamber.
We do indeed have a box of clean spare OSEMs, it should be out with all of the other boxes of clean stuff we had for the suspension building. You could also try swapping in a different satellite box, to see if the circuit powering the OSEM PD is to blame.
ITMX is free, OSEM signals all rougly centered.
This was accomplished by rocking the static alignment (i.e. slow controls) pitch and yaw offsets until the optic broke free. This took a few volts back and forth. At this point, I tried to find a point where the optic seemed to freely swing, and hopefully have signals in all 5 OSEMS. It seemed to be free sometimes but mostly settling into two different stationary states. I realized that it was becoming torqued enough in pitch to be leaning on the top-front or top-back EQ stops. So, I slowly adjusted the pitch from one of these states until it seemed to be swinging a bit on the camera, and three OSEM signals were showing real motion. Then, I slowly adjusted the pitch and yaw alignments to get all OSEMS signals roughly centered at half of their max voltage.
[ericq, Lydia, Teng]
Brief summary of this afternoon's activities:
Addendum: I had a suspicion that the alignment had moved so much, we were missing the TRX PDs. I misaligned the Y arm, and used AS110 as a proxy for X arm power, as we've done in the past for this kind of thing. Indeed, I could maximize the signal and lock a TM00 mode. Both the high gain PD and QPD in the TRX path are totally dark. This needs realignment on the end table.
The misalignment wasn't as bad as I had intially feared; the spot was indeed pretty high on ETMX at first. Both transmon QPDs did need a reasonable amount of steering to center once the dither had centered the beam spots on the optics.
Arms, PRMI and DRMI have all been locked and dither aligned. All oplevs and transmon QPDs have been centered. All AS and REFL photodiodes have been centered.
Green TM00 modes are seen in each arm; I'll do ALS recovery tomorrow.
Just a heads up, it looks like the damping came on at around 8:30pm. Not sure why.
I had hoped to do some ALS work, but I realized too late that we loaned our HP analyzer to Andrew. I decided instead to do some ETMX testing.
I have a script running that'll misalign both ETMs and back by about 0.5mrad with half hour rests in between. It'll be done around 6AM.
Seems like the angular position was fairly stable, though there is some change in the ETMX pitch that could be hysterisis or normal drift. I didn't mention it explicity in the previous log, but the misalignment was purely in pitch. I'll give it another shot with a bigger misalginment, and maybe a mix of pitch and yaw.
PMC was terribly misaligned. The PMCR camera seems to have drifted somewhat off target too, but I didn't touch it.
Realigned ITMX for the nth time today.
Finding ALSY beatnote was easy, ALSX eludes me. I did a rough one-point realignment on the X beat PD which is usually enough, but it's probably been long enough that near/far field alignmnet is neccesary.
ALSY noise is mostly nominal, but there is a large 3Hz peak that is visible in the spot motion, and also modulates the beat amplitude by multiple dBs.
It looked to me that the ETMY oplev spot was moving too much, which led me to measure the oplev OLGs. There is some wierd inter-loop interference going on between OLPIT and OLYAW. With both on (whether OSEM damping is on or off, so input matrix shenanigans can't be to blame) there is a very shallow "notch" at around 4.5Hz, which leads to very little phase at 3Hz, and thus tons of control noise. Turning the OL loop not being measured off makes this dip go away, but the overall phase is still signfinicantly less than we should have. I'm not sure why. I'll just show the PIT plot, but things look pretty much the same for YAW.
I did some more ETMX tests. Locked arm, raised the servo output limit to 15k, then increased the gain to make the loop unstable. I saw the SUS LSC signals go up to tens of thousands of counts when the unlock happened. I did this a dozen times or so, and every time the ETM settled in the same angular position according to the oplev.
Right now, another hysteresis script is running, misaliging in pitch and yaw. Amplitude 1V in each direction. So far, everything is stable after three on/off cycles.
Some things I did last night:
I measured the X PDH OLG, and turned the gain down by ~6dB to bring the UGF back to 10kHz, ~50deg phase margin, 10dB gain margin. However, the error signal on the oscilloscope remained pretty ratty. Zooming in, it was dominated by glitches occuring at 120Hz. I went to hook up the SR785 to the control signal monitor to see what the spectrum of these glitches looked like, but weirdly enough connecting the SR785's input made the glitches go away. In fact, with one end of a BNC connector plugged into a floating SR785 input, touching the other end's shield to any of the BNC shields on the uPDH chassis made the glitches go away.
This suggested some ground loop shenanigans to me; everything in the little green PDH shelves is plugged into a power strip which is itself plugged into a power strip at the X end electronics rack, behind all of the sorensens. I tried plugging the power strip into some different places (including over by the chamber where the laser and green refl PD are powered), but nothing made the glitches go away. In fact, it often resulted in being unable to lock the PDH loop for unknown reasons. This remains unsolved.
As Gautam and Johannes observed, the X green beat was puny. By hooking up a fast scope directly to the beat PD output, I was able to fine tune the alignment to get a 80mVpp beat, which I think is substaintially bigger than what we used to have. (Is this plus the PDH gain changed really attributable to arm loss reduction? Hm)
However, the DFD I and Q outputs have intermittent glitches that are big enough to saturate the ADC when the whitening filters are on, even with 0dB whitening gain, which makes it hard to see any real ALS noise above a few tens of Hz or so. Turning off the whitening and cranking up the whitening gain still shows a reasonably elevated spectrum from the glitches. (I left a DTT instance with a spectrum on in on the desktop, but forgot to export...) The glitches are not uniformly spaced at 120Hz as in the PDH error signal. However, the transmitted green power also showed intermittant quick drops. This also remains unsolved for the time being.
We poked around trying to figure out the X PDH situation. In brief, the glitchiness comes and goes, not sure what causes it. Tried temp servo on/off and flow bench fan on/off. Gautam placed a PD to pick off the pre-doubler AUX X IR light to see if there is some intermittant intensity fluctuation overnight. During non-glitchy times, ALSX noise profile doesn't look too crazy, but some new peak around 80Hz and somewhat elevated noise compared to historical levels above 100Hz. It's all coherent with the PDH control up there though, and still looks like smooth frequency noise...
NB: The IR intensity monitoring PD is temporarily using the high gain Transmon PD ADC channel, and is thus the source of the signal at C1:LSC-TRY_OUT_DQ. If you want to IR lock the X arm, you must change the transmon PD triggering to use the QPD.
[ericq, Gautam, Lydia]
We spent some time tonight trying to revive the PRFPMI. (Why PR instead of DR? Not having to deal with SRM alignment and potentially get a better idea of our best-case PRG). After the usual set up and warm up, we found ourselves unable to hold on to the PRMI while the arms flash. In the past, this was generally solved through clever trigger matrix manipulations, but this didn't really work tonight. We will meditate on the solution.
Tonight, and during last week's locking, we noticed something intermittently kicking the PRM. I've determined that PRM's LR OSEM is problematic again. The signal is coming in and out, which kicks the OSEM damping loops. I've had the watchdog tripped for a little bit, and here's the last ten minutes of the free swinging OSEM signal:
Here's the hour trend of the PRM OSEMS over the last 7 days a plot of just LR since the fix on the 9th of September.
It looks like it started misbehaving again on the evening of the 5th, which was right when we were trying to lock... Did we somehow jostle the suspension hard enough to knock the foil cap back into a bad spot?
Still no luck relocking, but got a little further. I disabled the output of the problematic PRM OSEM, it seems to work ok. Looking at the sensing of the PRMI with the arms held off, REFL165 has better MICH SNR due to its larger seperation in demod angle. So, I tried the slightly odd arrangement of 33I for PRCL and 165Q for MICH. This can indefinitely hold through the buzzing resonance. However, I haven't been able to find the sweet spot for turning on the CARM_B (CM_SLOW) integrator, which is neccesary for turning up the AO and overall CARM gain. This is a familiar problem, usually solved by looking at the value far from resonance on either side, and taking the midpoint as the filter module offset, but this didn't work tonight. Tried different gains and signs to no avail.
I have completed the following non-Steve portions of the pre-vent checklist [wiki-40m.ligo.caltech.edu]
All shutters are closed. Ready for Steve to check nuts and begin venting!
As Gautam mentioned, we had some success locking the PRFPMI last night. (SRM satellite box is still in surgery...)
Unsurprisingly, changing the loss/PRG/CARM finesse means we had to fiddle with the common mode servo parameters a little bit to get things to work. However, before too long, we achieved a first lock on the order of a few minutes. Not long afterwards, we had a nice half hour lock stretch where we could tune up the AO crossover and loop UGFs. The working locking script was committed to SVN. Really, no fundamentally new tactics were used, which is encouraging. (One thing I wondered about was whether a narrower CARM linewidth would still let our direct ALS->REFL11 handoff with no offset reduction work. Turns out it does)
However, the step where we increase the analog CARM gain isn't as bulletproof as it once had been. The light levels "sputter" in and out sometimes if the gain increases are too agressive, and can cause a lockloss. Maybe this is an effect of the narrower linewidth and injecting more ALS noise at high frequencies with the higher CARM bandwidth.
The spatial profiles of the light on the cameras is totally bananas. Here's AS and REFL.
As Koji suggested, here is a 2D histogram of TRY vs REFLDC. It appears that the visibility would max out at 75% or so at arm powers around 400. Indeed, we briefly saw powers that high, but as can be seen on the plot, we were usually a little under 300. Exploring the transmon QPD offset space didn't seem to have much effect here.
One thing that I hadn't looked at in previous locks is coherence with our ground seismometers. It would be cool to have more seismic feedforward, and looking at the frequency domain multiple coherence, it looks like we can win a lot between 1 and 20 Hz. I expected more of a win at 1Hz, though.
I poked around a bit thinking about what we need for a single AS WFS.
New things that we would need:
Things we have:
We'd have 12 new signals to acquire: 4 quadrants x DC, I, Q. In principle the DC part could go into a slow channel, but we have the ADC space to do it fast, and it'll be easier than mucking around with c1iscaux or whatever.
Open question: What to do about AA? A quick search didn't turn up any eurocard AA chassis like the ones we use for the LSC PDs. However, given the digital AA that happens from 64kHz->16kHz in the IOP, we've talked about disabling/bypassing the analog AA for the LSC signals. Maybe we can do the same for the QPD signals? Or, modify the post-demod audioband amplifer in the demod chassis to have some simple, not too agressive lowpass.
It seems that the EX and EY BLRMS banks were missing the BP and LP filters for the 0.03-0.1 and 0.1-0.3 bands. I've copied over the filters from the BS seismometer.
However, if it looks like the integrated C code BLRMS block works out well, we could replace the seismometers' filter module heavy BLRMS blocks and cut down on the PEM model bloat.
This is long overdue, but our burt files for SDF now live in the LIGO userapps SVN, as they should.
The canonical files live in places like /opt/rtcds/userapps/release/cds/c1/burtfiles/c1x01_safe.snap and are symlinked to files like /opt/rtcds/caltech/c1/target/c1x01/c1x01epics/burt/safe.snap
The transients are likely due to doppler interference due to the input laser frequency sloshing due to errant control signals after the IMC unlock. I performed a few "partial" ringdowns by reducing the power by about 80% while keeping the IMC servo locked. (Function generator at 0.5Vpp square wave, 0.25V offet. Turned IMC boosts off to increase the stable range of the servo).
I need to work out how to extract the loss from this, I think having a partial ringdown may change the calculations somewhat; the time constants in the trans and refl signals are not identical.
Thanks to Gautams nice setup, it was very easy to take these measurements. Thanks! Code and data attached.
Yes, writing minute trends causes hourly FB crashes in the current state of things. The "raw" minute trending is turned on, but I think that these are unknown to nds.
In the filter banks there were various version of a 'dewhite' filter. They were all approximately z=150, p=15, g =1 @ DC, but with ~1% differences. I don't trust their provenance and so I've enforced symmetry and fixed their names to reflect what they are (150:15).
The filters were made in response to a measurement of the pentek whitening boards in 2015 (ELOG 11550), but this level of accuracy probably isn't important.
I tried to follow these instructions today to make the Simulink Webview accessible:
controls@nodus|public_html > ln -sfn /users/public_html/FE /export/home/
But...I got a "403 Forbidden" message. What is the secret handshake to get this to work? And why have we added this extra step of security?
This link works for me: https://nodus.ligo.caltech.edu:30889/FE/c1als_slwebview.html. The problem with just /FE/ is that there is no index.html, and we have turned off automatic directory listings.
IIRC, this arrangement was due to the fact that authentication of some of the folders (maybe the wikis) was broken during the nodus upgrade, so there was sensitive information being publicly displayed. This setup gives us discretion over what gets exposed.
EQ: https://nodus.ligo.caltech.edu:30889/FE is live
This was done by adding "Options +Indexes" to /etc/apache/sites-available/nodus
I've added a little more info about the apache configuration on the wiki: ApacheOnNodus