The summary pages were in a sad state of disrepair - the daily jobs haven't been running for > 1 month. I only noticed today because Jordan wanted to look at some vacuum trends and I thought summary pages is nice for long term lookback. I rebooted it just now, seems to be running. @Tega, maybe you want to set up some kind of scripted health check that also sends an alert.
As discussed at the meeting, I commenced the recovery of the CDS status at 1750 local time.
Single arm POX/POY locking was checked, but not much more. Our IMC WFS are still out of service so I hand aligned the IMC a bit, IMC REFL DC went from ~0.3 to ~0.12, which is the usual nominal level.
FYI, there is this. Seems pretty well maintained, and so might be more useful in the long run. The available catalog of instruments is quite impressive - TC200 temp controller and SRS345 func gen are included and are things we use in the lab. maybe you can make a pull request to add MDT694B (there is some nice API already built I think). We should also put our netgpibdata stuff and the vacuum gauge control (basically everything that isn't rtcds) on there (unless there is some intellectual property rights issues that the Caltech lawyers have to sort out).
Given the similarities between the MDT694B (single channel piezo controller) and TC200 (temperature controller) serial interfaces, I added the pyserial driver here.
*Warning* this first version of the driver remains untested
Now that the new Agilent full-range gauges (FRGs) have been received, I'm putting together an installation plan. Since my last planning note in Sept. (ELOG 15577), two more gauges appear to be malfunctioning: CC2 and PAN. Those are taken into account, as well. Below are the proposed changes for all the sensors in the system.
For replacements, I recommend we consider the Agilent FRG-700 Pirani Inverted Magnetron Gauge. It uses dual sensing techniques to cover a broad pressure range from 3e-9 torr to atmosphere in a single unit. Although these are more expensive, I think we would net save money by not having to purchase two separate gauges (Pirani + hot/cold cathode) for each location. It would also simplify the digital controls and interlocking to have a streamlined set of pressure readbacks.
For controllers, there are two options with either serial RS232/485 or Ethernet outputs. We probably want the Agilent XGS-600, as it can handle all the gauges in our system (up to 12) in a single controller and no new software development is needed to interface it with the slow controls.
I don't think your simulation looked inaccurate (at least not to me). In my opinion, we just want to minimize any excess noise from intermodulation. Of course, its possible that stuffing too many notches will make it difficult to have the same low noise as a simple circuit, so that's worth considering.
Also, the intermodulation is mainly a problem when the other peaks are not suppressed by some feedback: e.g. POP55_I can have excess noise if POP55_Q or POP11_I are not controlled by some MICH/PRCL/SRCL loops.
For the WFS, perhaps this is not a significant issue, but I'm not sure. My suggestion is to stuff 11 & 55 for sure, and then the others depending on the amplitude of the peaks and the consequent intermodulation. IF it works with all stuffed, that seems good. If its tricky to get it to work with all stuffed, I'd back off on a couple of them...but it probably takes more careful thought to figure out which ones are least important.
An 8 channel whitening chassis was prepared and tested. I measured:
Whitening chassis. Waiting for front panels to arrive, PCBs and interface board are in hand, stuffed and ready to go. A question here is how we want to control the whitening - it's going to be rather difficult to have fast switchable whitening. I think we can just fix the whitening state. Another option would be to control the whitening using Acromag BIO channels.
I am confused by the discussion during the call today. I revisited Hartmut's paper - the circuit in Fig 6 is essentially what I am calling "only 2f_2 notch stuffed" in my previous elog. Qualitatively, the plot I presented in Attachment #2 of the preceeding elog in this thread shows the expected behavior as in Fig 8 of the paper - the impedance seen by the photodiode is indeed lower. In Attachment #1, I show the comparison - the "V(anode)/I(I1)" curve is analogous to the "PD anode" curve in Hartmut's paper, and the "V(vout)/I(I1)" curve is analogous to the "1f-out" curve. I also plot the sensitivity analysis (Attachment #2), by varying the photodiode junction capacitance between 100pF and 200 pF (both values inclusive) in 20 pF steps. There is some variation at 55 MHz, but it is unlikely that the capacitance will change so much during normal operation?
I understand the motivation behind stuffing the other notches, to reduce intermodulation effects. But the impression I got from the call was that somehow, the model I presented was wrong. Can someone help me identify the mistake?
I didn't bother to export the LTspice data and make a matplotlib plot for this quick analysis, so pardon the poor presentation. The colors run from green=100pF to grey=200pF.
On the call last week, I claimed that there isn't much hope of directly measuring Ponderomotive Squeezing in aLIGO without some significant configurational changes. Here, I attempt to quantify this statement a bit, and explicitly state what I mean by "significant configurational changes".
The I/O relations will generally look something like:
The. magnitudes of the matrix elements C_12 and C_21 (i.e. phase to amplitude and amplitude to phase coupling coefficients) will encode the strength of the Ponderomotive squeezing.
For the inital study, let's assume DC readout (since there isn't a homodyne readout yet even in Advanced LIGO). This amounts to setting in the I/O relations, where the former angle is the "homodyne phase" and the latter is the "SRC detuning". For DC readout, the LO quadrature is fixed relative to the signal - for example, in the usual RSE operation, . So the quadrature we will read out will be purely (or nearly so, for small detunings around RSE operation). The displacement noises will couple in via the matrix element. Attachment #1 and Attachment #2 show the off-diagonal elements of the "C" matrix for detunings of the SRC near RSE and SR operation respectively. You can see that the optomechanical coupling decays pretty rapidly above ~40 Hz.
In this particular case, there is no benefit to detuning the SRC, because we are assuming the homodyne angle is fixed, which is not an unreasonable assumption as the quadrature of the LO light is fixed relative to the signal in DC readout (not sure what the residual fluctuation in this quantity is). But presumably it is at the mrad level, so the pollution due to the orthogonal anti-squeezed quadrture can be ignored for a first pass I think. I also assume ~10 degrees of detuning is possible with the Finesse ~15 SRC, as the linewidth is ~12 degrees.
To see how this would look in an actual measurement, I took the data from Lee's ponderomotive squeezing paper, as an estimate for the classical noises, and plotted the quantum noise models for a few representative SRC detunings near RSE operation - see Attachment #3. The curves labelled for various phis are the quantum noise models for those SRC detunings, assuming DC readout. I fudged the power into the IFO to make my modelled quantum noise curve at RSE line up with the high frequency part of the "Measured DARM" curve. To measure Ponderomotive Squeezing unambiguously, we need the quantum noise curve to "dip" as is seen around 40 Hz for an SRC tuning of 80 degrees, and that to be the dominant noise source. Evidently, this is not the case.
The case for balanced homodyne readout:
I haven't analyzed it in detail yet - but it may be possible that if we can access other quadratures, we might benefit from rotating away from the DARM quadrature - the strength of the optomechanical coupling would decrease, as demonstrated in Attachments #1 and #2, but the coupling of classical noise would be reduced as well, so we may be able to win overall. I'll briefly investigate whether a robust measurement can be made at the site once the BHD is implemented.
Last week and this week I've been working on the characterization of the Q3000 QPDs. The QPDs were named 81, 82, 83, and 94.
My recommendation is to use #81 and #84 as they have similar dark current characteristics between the segments. But basically, all the QPDs look fine.
The actual junction capacitance and the RF dark noise should be characterized by the actual WFS head circuit.
The QPD packages were labeled and returned to Gautam to be implemented in the WFS heads.
gautam: S/N #84 was installed as the AS WFS QPD. The remaining 3 are stored in the clean cabinet at EX (where the rest of the RF photodiodes are).
Five Agilent pressure gauges were delivered to the 40m. It is stored with the controller and cables in the office area. This completes the inventory for the gauge replacement - we have all the ordered parts in hand (though. not necessarily all the adaptor flanges etc). I'll see if I can find some cabinet space in the VEA to store these, the clutter is getting out of hand again...
in addition, the spare gate valve from LHO was also delivered today to the 40m. It is stored at EX with the other spare valves.
It is stored along with the cables that arrived a few weeks ago, awaiting the gauges which are now expected next week sometime.
Basically, they repeated our specs and showed the coating performances for HR/AR for 10deg P and PR/AR for 45deg P. There is no RoC measurement by the vendor.
Nevertheless, their RoC (paper) specs should be compared with our request.
Optics --> Cabinet at south end (Attachment #1)
Scanned datasheets--> wiki. It would be good if someone can check the specs against what was ordered.
Attachment #1 - Proposed mods for 40m RF freqs.
Attachment #2 - Modelled TFs for the case where all the notches are stuffed, and where only the 2f notch is stuffed.
Attachment #3 - Modelled TFs for the case where all the notches are stuffed, and where only the 2f notch is stuffed.
Any other red flags anyone sees before I finish stuffing the board?
WFS head and housing. Need to finalize the RF transimpedance gain (i.e. the LC resonant part), and also decide which notches we want to stuff.
Where do we want to install the interface and readout electronics for the AS port WFS? Options are:
There isn't much difference in terms of cable length that will be required - I believe the AS WFS is going to go on the AP table even in the new optical layout and not on the ITMY in-air oplev table?
The project requires a large number of new electronics modules. Here is a short update and some questions I had:
Approximately half of the assembly of the various electronics is now complete. The basic electrical testing of the interface chassis and demod chassis are also done (i.e. they get power, the LEDs light up, and are stable for a few minutes). Detailed noise and TF characterization will have to be done.
Ordered 11/16 from CDW, on PO# S492940, the high voltage Tripp Lite SMART5000XFMRXL for TP-1. Should be arriving in about a week.
yes, both problems can be fixed. Usually we just order some spare lead-acid batteries from SRS (Steve may have some spare ones somewhere). The DC offset often comes from a busted FET input. I bought 50 of those at one point - they're obsolete. Its also possible to replace the input stage with any old FET pair.
I'll handle the one with the offset if you leave it on my desk.
Shruti picked it up @4pm.
true. also try to choose a cap with a goow high frequency response. In the Electronics Noise book by Ott there's some graph about this. I bet you good do a Bing search and also find something more modern. Basically we want to make sure that the self resonance is not happening at low frequencies. Might be tought to find one with a good HF response, a high voltage rating, and > 1uF.
Yes. The datasheet has a recommendation circuit with 10uF caps. Companies are careful to show reproducible, reliably functional circuit examples on datasheets. So, if the caps are there you should try to replicate the design.
Additional bypass capacitors? I use 0.1 uF, 700V DC ceramic capacitors as bypass capacitors close to the leads of the PA95, as is recommended in the datasheet. Can adding a 10uF capacitor in parallel provide better filtering? I'm not sure if one with compatible footprint and voltage rating is readily available, I'll look around.
I now think the excess noise in this circuit could be coming from the KEPCO switching power supply (in fact, the supplies are linear, and specd for a voltage ripple at the level of <0.002% of the output - this is pretty good I think, hard to find much better).
All component references are w.r.t. the schematic. For this test, I decided to stuff a fresh channel on the board, with new components, just to rule out some funky behavior of the channel I had already stuffed. I decoupled the HV amplifier stage and the Acromag DAC noise filtering stages by leaving R3 open. Then, I shorted the non-inverting input of the PA95 (i.e. TP3) to GND, with a jumper cable. Then I measured the noise at TP5, using the AC coupling pomona box (although in principle, there is no need for this as the DC voltage should be zero, but I opted to use it just in case). The characteristic bump in the spectra at ~100Hz-1kHz was still evident, see the bottom row of Attachment #1. The expected voltage noise in this configuration, according to my SPICE model, is ~10 nV/rtHz, see the analysis note.
As a second test, I decided to measure the voltage noise of the power supply - there isn't a convenient monitor point on the circuit to directly probe the +/- HV supply rails (I didn't want any exposed HV conductors on the PCB) - so I measured the voltage noise at the 3-pin connector supplying power to the 2U chassis (i.e. the circuit itself was disconnected for this measurement, I'm measuring the noise of the supply itself). The output is supposedly differential - so I used the SR785 input "Float" mode, and used the Pomona AC coupling box once again to block the large DC voltage and avoid damage to the SR785. The results are summarized in the top row of Attachment #1.
The shape of the spectra suggests to me that the power supply noise is polluting the output noise - Koji suggested measuring the coherence between the channels, I'll try and do this in a safe way but I'm hesitant to use hacky clips for the High Voltage. The PA95 datasheet says nothing about its PSRR, and seems like the Spice model doesn't include it either. It would seem that a PSRR of <60dB at 100 Hz would explain the excess noise seen in the output. Typically, for other Op-Amps, the PSRR falls off as 1/f. The CMRR (which is distinct from the PSRR) is spec'd at 98 dB at DC, and for other OpAmps, I've seen that the CMRR is typically higher than the PSRR. I'm trying to make a case here that it's not unreasonable if the PA95 has a PSRR <= 60dB @100 Hz.
So what are the possible coupling mechanisms and how can we mitigate it?
What do the analog electronics experts think? I may be completely off the rails and imagining things here.
Update 2130: I measured the coherence between the positive supply rail and the output, under the same conditions (i.e. HV stage isolated, input shorted to ground). See Attachment #2 - the coherence does mirror the "bump" seen in the output voltage noise - but the coherence is. only 0.1, even with 100 averages, suggesting the coupling is not directly linear - anyways, I think it's worth it to try adding some extra decoupling, I'm sourcing the HV 10uF capacitors now.
I had to go through five SR560s in the lab yesterday evening to find one that had the expected 4 nV/rtHz input noise and worked on battery power. To confirm that the batteries were charged, I left 4 of them plugged in overnight. Today, I confirmed that the little indicator light on the back is in "Maintain" and not "Charge". However, when I unplug the power cord, they immediately turn off.
One of the units has a large DC output offset voltage even when the input is terminated (though it is not present with the input itself set to "GND" rather than DC/AC). Do we want to send this in for repair? Can we replace the batteries ourselves?
The results from the ringdown are attached - in summary:
I want to test out an AS port WFS now that I have all the parts in hand - I guess the Michelson / PRMI will suffice until I make the ALS noise good again, and anyways, there is much assembly work to be done. Overnight, I'm repeating the suspension eigenmode measurement.
For the input matrix diagonalization, it seemed to me that when we had a significant seismic event or a re-alignment of the optic with the bias sliders, the input matrix also changes.
Meaning that our half-light voltage may not correspond to the half point inside the LED beam, but that rather we may be putting the magnet into a partially occluding state. It would be good to check this out by moving the bias to another setting and doing the ringdown there.
While proceeding with the interferometer recovery, I noticed that there appeared to be no light on WFS2. I confirmed on the AP table that the beam was indeed hitting the QPD, but the DC quadrants are all returning 0. Looking back, it appears that the failure happened on Monday 26 October at ~6pm local time. For now, I hand-aligned the IMC and centered the beams on the WFS1 and MC2T QPDs - MCT is ~15000 cts and MC REFL DC is ~0.1, all consistent with the best numbers I've been able to obtain in the past. I don't think the servo will work without 1 sensor without some retuning of the output matrix.
It would appear that both the DC and RF outputs of WFS2 are affected - I dithered the MC2 optic in pitch (with the WFS loop disabled) at 3.33 Hz, the transmission and WFS1 sensors see the dither but not WFS2. It could be that I'm just not well centerd on the PD, but by eye, I am, so it would appear that the problem is present in both the DC and RF signal paths. I am not going into the PD head debugging today.
Looking back through the elog, 1mtorr is the pressure at which it is deemed safe to send the full power beam into the IMC. After replacing the HR mirror in the MCREFL path with a 10% reflective BS, I just cranked the power back up. IMC is locked. With the increased exposure on the MC2T camera, lots of new scattered light has become visible.
I've uploaded some more photos here. I believe the problem is a worn out thread where the main rotary handle attaches to the shaft that operates the valve.
This morning, I changed the valve config such that TP2 backs TP1 and that combo continues to pump on the main volume through the partially open RV2. TP3 was reconfigured to pump the annuli - initially, I backed it with the AUX drypump but since the load has decreased now, I am turning the AUX drypump off. At some point, if we want to try it, we can try pumping the main volume via the RGA line using TP2/TP3 and see if that allows us to get to a lower pressure, but for now, I think this is a suitable configuration to continue the IFO work.
There was a suggestion at the meeting that the saturation of the main volume pressure at 1mtorr could be due to a leak - to test, I closed V1 for ~5 hours and saw the pressure increased by 1.5 mtorr, which is in line with our estimates from the past. So I think we can discount that possibility.
Main volume pressure as of 11:30AM 2020/11/10
[koji, rana, gautam]
1100 - EY chamber inspected, no issues were found --> EY heavy door on
1200 - OMC chamber was inspected. OM6 was marginally tweaked to bring the beam down a little in pitch, and also a little northwards in Yaw. --> Heavy door on.
1230 - Pumpdown started. Initially, the annuli volume was pumped down. The procedure calls for doing this with the small turbopumps. However, V7 was left open, and hence, in the process, the TP1 foreline pressure (=P2) hit ~30 torr. This caused TP1 to shutdown. We were able to restart it without issue. This case was not caught by the interlock code, which was running at the time. It should be recitified.
1330 - OMC breadboard clean optics and DCPD hardware were wrapped up and packed into tupperware boxes and stored along the south arm. OMC cavity itself, the OMMT, and the breadboard the OMC was sitting on are wrapped in foil/Ameristat and stored in cabinet S13, lower 2 shelves.
1915 - P1a = 0.5 torr pressure reached. Switched over to pumping the main volume with TP1, backed by TP2 and TP3, which themselves are backed by their respective dry pumps and also the AUX drypump for some extra oomph. All cooling fans available in the area were turned on and directed at the turbo pumps. RV2 was used to throttle the flow suitably.
It was at this point that we hit a snag - RV2 has gotten stuck in a partially open position, see Attachment #1. We can see that the thread doesn't move in response to turning the rotary dial. Fortunately, the valve is partially open, so the main volume continues to be pumped - see Attachment #2 for the full history of today's pumping. We are leaving the main volume pumped in this configuration overnight (TP1 pumping main volume backed by TPs 2 and 3, which are in turn backed by their respective drypumps and also the AUX dry pump). I think there is little to no risk of any damage to the turbo pumps, the interlocks should catch any anomalies. The roughing pumps RP1 and RP3 were turned off and that line was disconnected and capped.
What are our options?
We need some vacuum experts to comment. Why did this happen? Is this an acceptable failure mode of the valve?
2230 - P1a = 0.025 torr. The pressure is coming down with log-linear scale. x0.1 per 2.5 hours or so.
> I didn't bother to align the green beams to the arm cavities or re-center the Oplevs - is this necessary? It is a step in the pre-close up checklist, so maybe we should do it... The green transmission does reach the PSL table...
I don't think so. The beam is reaching the PSL, so we have no motivation to change the green alignment. Regarding the oplev, the green refl should come back to the PDH PD and this gives us additional beam reference. As soon as we find the green resonance after the pumping, we can tweak the green axis so that the spots on the mirrors become reasonable (as well as the green trans CCD on the PSL table).
Basic IFO alignment checks were done.
Tomorrow, we should do some visual checks of the chambers / EQ stops on ETMY etc but I don't see any major problems at the moment...
Barring any catastrophic failures and provided all required personnel are available, we will do the final pre-close-up checks, put the heavy doors back on, and pump down starting 10 am Monday, 9 Nov 2020.
I was able to boot one of the 3 new Supermicro machines, which I christened c1bhd, in a diskless way (with the boot image hosted on fb, as is the case for all the other realtime FEs in the lab). This is just a first test, but it is reassuring that we can get this custom linux kernel to boot on the new hardware. Some errors about dolphin drivers are thrown at startup but this is to be expected since the server isn't connected to the dolphin network yet. We have the Dolphin adaptor card in hand, but since we have to get another PCIe card (supposedly from LLO according to the BHD spreadsheet), I defer installing this in the server chassis until we have all the necessary hardware on hand.
I also have to figure out the correct BIOS settings for this to really run effectively as a FE (we have to disable all the "un-necessary" system level services) - these machines have BIOS v3.2 as opposed to the older vintages for which there are instructions from K.T. et al.
There may yet be issues with drivers, but this is all the testing that can be done without getting an expansion chassis. After the vent and recovering the IFO, I may try experimenting with the c1ioo chassis, but I'd much prefer if we can do the testing offline on a subnet that doesn't mess with the regular IFO operation (until we need to test the IPC).
I am working on the setup of a CDS FE, so please do not attempt any remote login to the IPMI interface of c1bhd until I'm done.
The PMC servo railed and so I re-locked it at ~half range. I've been noticing that the diurnal drift of the PZT control voltage has been larger than usual - not sure if it's entirely correlated with temperature on the PSL table. Anyway the cavity is locked again so all is good.
Attachment #1 shows the main result - there are 4 peaks. The frequencies are a little different from what I have on file for ETMY and the Qs are a factor of 3-4 lower (except SIDE) than what they are in vacuum, which is not unreasonable I hypothesize. The fits suggest that the peak shape isn't really Lorentzian, the true shape seems to have narrower tails than a Lorentzian, but around the actual peak, the fit is pretty good. More detailed diagnostic plots (e.g. coil-to-coil TFs) are in the compressed Attachment #2. The condition number of the matrix to diagonalize the sensing matrix (i.e. what we multiply the "naive" OSEM 2 Euler basis matrix by) is ~40, which is large, but I wouldn't read too much into it at this point.
I see no red flags here - the PIT peak is a little less prominent than the others, but looking back through the elog, this kind of variation in peak heights doesn't seem unreasonable to me. If anyone wants to look at the data, the suspension was kicked every ~1100seconds from 1288673974, 15 times.
I'm measuring the free-swinging spectra of ETMY overnight.
So all the primary vent objectives have been achieved 🙌 . The light doors are on the chamber right now. I'm measuring the free-swinging spectra of ETMY overnight. Barring any catastrophic failures and provided all required personnel are available, we will do the final pre-close-up checks, put the heavy doors back on, and pump down starting 10 am Monday, 9 Nov 2020. Some photos here.
The IMC isn't resonant for a TEM00 mode at the time of writing - we are waiting for the stack to relax, at which point if the IMC isn't resonant for a TEM00 mode, we will tweak the input pointing into the IMC (we want to use the suspended cavity as the reference, since it is presumably more reliable than the table from which we removed ~50 kgs of weight and shifted the balance.
I got a call from Calum ~830am today saying some facilities people entered the lab, opened the south entrance door, and tripped the alarm in the process. I came to the lab shortly after and was able to reset the alarm by flipping the switch on the alarm box at the south end entrance to "Alarm OFF". Then, I double checked that the door is closed, and re-enabled the alarm. The particle count at the SP table is not unusually high and the lasers (Oplev HeNe and AUX X) were still on, so doesn't look like any lasting damage was done. The facilities people were apparently wearing laser safety goggles.
Good point - looking back, I also see that I already removed the mirror at the SW corner of the table in 2016. Revised photo in Attachment #1. There is an optic on the east edge of this table whose purpose I'm not sure of, but I'm pretty sure it isn't essential to the main functionality and so can be removed.
I believe the mirror next to IM1 is for the green beams to be delivered to the PSL table. I think we still want to keep it. Otherwise, the plan looks fine.
To be a bit more clear about what we are going to do in the OMC chamber, I marked-up some photos, see Attachments #1 and #2.
I anticipate that after this work, the only components on the table will be
Are we in agreement with this plan?
See #15656 for the updated photo
This morning, we did the following;
The OSEMs remain in the EY vacuum chamber. The next set of steps are:
We will most likely work on this tomorrow. At ~1615, I briefly opened the PSL shutter and tweaked the IMC alignment. We will almost certainly change the pointing into the IMC when we remove the old OMC and rebalance that table, so care should be taken when working on that...
We are now ready to take the doors off. I've already done the basic prep work (loosened bolts, cleaned chamber, carts for tools, fresh ameristat on portable HEPAs etc).
If everything else looks good, I'll start letting the dry N2 into the main volume after lunch.
Now the green transmissions are visible by the green PDs. Attachment 9 shows the trans and ref of each green beams with and without locking to TEM00. The questionable green TRY was ~0.3. If we compare this with the histrical data (Attachment 10), it is about 1/4 of the value in the past. It's not too crazy but still quite low.
BTW, nice video! @ Koji, How difficult was it to edit it into this form?
I went to 40m yesterday at around 2:30 pm and Koji showed me how to acquire lock in different arms and for different lasers. Finally, we took a preliminary measurement of shaking the ETMX at some discrete frequencies and looking at the beatnote frequency spectrum of X-end laser's fiber-coupled IR and Main laser's IR pick-off.
We verified that we can send discrete frequency excitation signals to ETMX actuators directly and see a corresponding peak in the spectrum of beatnote frequency between fiber-coupled X-end IR laser and main laser IR pickoff.
If full interferometer had been locked, we could have used the DARM error signal output to calibrate it against this measurement.
At this point, I'm leaving the lab. All the suspensions (incl SRM) are aligned. PSL/GRX/GRY shutters were left open.
Ok I was using the PD in the black mount because Rana recommended it a few weeks ago.
Regarding the M2ISS, I acquired the hardware from QIL some months ago, including a circuit board, and 2 PDs. These had LEMO outputs though (not BNC), and the mounts are not 4". These photodiodes are what I'm using as the airBHD DCPDs right now, and some photos are here - are these the photodiodes you mentioned? Or are there yet more M2ISS photodiodes? I remember Johannes had some custom mounts extruded to make them 4" high, do you mean those? Can I retrieve them his Cryo setup?
BTW, my elog scraping shows only one spectra from Stefan in the ATF elog, and the performance there is more like 1e-7/rtHz @ 100 Hz, and that’s using a dedicated high BW servo circuit, not the SR560. Am I just missing the measurement of 2e-8/rtHz?
that little PD in the black mount was never very good. The AD829 is not a good opamp for transimpedance and especially not good for low frequencies. Stefan Ballmer and I were able to get 2e-8 out of these (@100 Hz) many years ago.
I wonder if we have some of Zach's M2ISS photodetectors around, perhaps in QIL or Cryo. I doubt that any of them are in use now. Those had good performance nad BNC output.