The 2W Innolight was turned on.
I have just turned on the PSL Innolight laser. The laser shut down with unknown reason a day ago.
I wanted to make sure Alex's system of Diode laser + laser controller + optical splitter was working fine and then make a manual measurement for AS55 PD. Manasa was supervising my work and helping me with unhooking the fibers and taking power meter readings. I have tuned on the power to REF DET from under the POY table.
I switched on the laser sitting in the 1Y1 rack and turned up the driving current to 240mA. On checking the laser power readings at AS55 (AS table) and REF DET (POY table) simultaneously, we got readings of 1.6mA and 2.4mA respectively. This much difference in readings was not expected and I did not continue taking the readings for transimpedence measurement.
I will rectify if this unequal splitting of power by the 1x16 optical splitter is going to cause any difficulties for the automated PDFR system measurement technique and resolve it if needed.
Summary of work done over the last two days
Immediate next steps:
Summary of work done tonight:
I spent some more time today trying to optimize the modulation frequency and amplitude for the X end PDH, and the alignment/mode-matching of the green to the arm. Some notes:
After the discussion at the meeting today, I decided to try and lock the green by sweeping through PZT dither frequencies in the vicinity of 200kHz without worrying about the AM/PM ratio for now. I was able to lock the PDH loop relatively quickly, at an empirically determined PZT dither frequency of 213.873kHz, 2Vpp (the amplitude was copied from the value at the Y-end). For today's efforts, I borrowed the sum+HPF pomona box from the Y-end, I will make a replica given that we are using Lightwave lasers at both ends now. After adjusting the PZT sliders and lenses on the translational stages at the endtable to maximize the green transmission as best as I could, I was able to get GTRX up to about 0.07 - this is far off from the value of ~0.25-0.3 I seem to remember us having with the old setup, even though we have more green light into the arm cavity. I will take a measurement of the loop transfer function to see what sort of bandwidth we have...
Given that we were seeing green flashes in the arms, I tried to see if I could get the green locked to the arm in a nice mode. For a start, I tried hooking up the PDH box and LO using the same settings as was being used previously. However, this did not work. I suppose we will have to do the whole AM/PM measurement for the Lightwave as well before we can determine what would be a suitable frequency for the LO. The AM measurement was relatively straightforward, I just repeated the same steps as detailed here. The two attachments show the AM response (one from 10kHz to 5MHz, the other for a narrower range of 100kHz to 1MHz, both with an excitation amplitude of 0dBm). To see if I could guess some sweetspot for operation, I tried setting the LO frequency to the two marked notch frequencies but was unsuccessful in getting the PDH lock going. At the moment, the alignment for the optics that picks off the IR after the doubler and routes it to the fiber are ccompletely misaligned, I will align these and do the PM measurement tomorrow and then we should conclusively be able to say what the appropriate frequency is to actuate on the PZT.
Unrelated to this work: the KEPCO high voltage power supply that drives the green steering mirror PZTs was switched off - I suppose this has been the case since the power outage last week. I turned it back on and reset it to the nominal settings: Vout = 100V, and Imax_out = 10mA, the driver board is currently drawing ~7mA which I judged to be consistent with the values labelled on the unit.
Eric and I spent some time yesterday night trying to recover the green in the arm after the laser swap. The problem essentially was that though I was getting ~800uW of green out of the doubling oven, the mode wasn't clean, and hence, the beam profile looked really messed up just before entering the arm cavity.We got to a point where we thought we were getting a good mode out of the doubling oven (as judged by propagating this beam onto the wall with the help of a mirror). But we were only getting ~400uW of green power. I tried tweaking the alignment of the oven on the 4 axis stage for a while, but was not able to improve the situation much. So I decided to start from scratch:
I am beginning to wonder if this ellipticity is inherent from the IR beam from the laser? My beamscan results suggest that the beam is more divergent in the "P direction" as compared to the "S direction", which is borne out by these photographs. And if this is indeed the case, do we need to add cylindrical lenses to correct this?
Unrelated to this work: The ITMX Oplev seems to have wandered off so the X arm won't lock. I am not realigning the Oplev for now, but am turning the ITMX Oplev servo off for the night.
Perhaps related to my work on the endtable: The ETMX oplev MEDM readings seemed to be frozen, though there was red light on the QPD on the endtable. Checking the CDS overview screen, I saw that all models on c1iscex had crashed. I sshed into c1iscex and restarted all the models, but the IOP block remained red. I checked the datetime, and found that this was wrong - so I followed the instructions here, but the "Diag Word" block remains red. I am shutting down the watchdog for ETMX and leaving this as is for now... This seems to have happened before...
The good news: both green beatnotes have now been found. The problem was alignment on the green beat PD on the PSL table which I fixed. They are about -40dBm in amplitude (compare to -25dBm we used to see). But looking at the phase tracker Q output seems to suggest that there is adequate signal...
The bad news: the ALS noise still looks bad (see attachment)- I think the IR beat for the Y was perhaps marginally better. The beat amplitude for the X beat was optimized on the PSL table with the help of the oscilloscope. There may be some headroom for improvement with the Y beat.
I also did the AM/PM measurement for the replaced lightwave, chose an LO frequency based on this, and took the loop OLTF, plots to follow...
After carefully tweaking the mode-matching of the IR into the crystal and the four-axis translation stage on which the doubling oven is mounted, I managed to recover 800uW of green power going into the green Faraday. Considering we have ~225mW of IR power coming out of the IR faraday (and roughly that amount going into the SHG crystal), I'd say this is pretty consistent (if not slightly better) with a recent power budget I had made for the X end. The amount of green power we get out of the doubling crystal is very sensitive to the alignment of the crystal to the beam axis. I suspect we could improve the situation slightly if the mode-matching lenses were mounted on translational stages so we could tweak their position, but the current situation on the X endtable does not provide space for this. In any case, I'd say we are at least as good as we were before, and so this should be an adequate fix until the new end-table is installed (though I don't know why we aren't seeing the predicted SHG conversion efficiency of 3-4% as predicted by Kiwamu's calculations, we are getting more like .36% conversion efficiency)...
Because the alignment of the beam before the doubling oven had changed, I had to adjust the steering mirrors to make the green beam go into the green faraday. I had placed a couple of irides for the green beam as a reference of the old path into the arm, and I used these to adjust some of the green mirrors to center the green beam on these. However, I did not observe any flashes in the arm. I will check if we are still mode-matched to the arm, and if the lenses downstream of the doubling oven need to be moved....
800e-6 / 0.225^2 = 0.016
I thought Kiwamu had roughtly 2%/W.
Next steps in recovering ALS and trying to lock again
Since I could not determine how many volts at the LO input of the pomona box input corresponds to how many volts at the laser PZT, I measured the transfer function between these points using the Agilent network analyzer. The measured TF suggests that for a function generator output of 2Vpp, we get approximately 75mrad of phase modulation, which compares reasonably well with the value of 120mrad reported here. I did not attempt to further increase the LO output signal to push this number closer to 120mrad, as with 2Vpp from the function generator we get +7dBm at the mixer, which is what it wants - so I wanted to avoid any attenuators etc...
Attachments #2 and #3
After ensuring that we have appreciable phase modulation, I set out to measure the PDH OLTFs and adjust the gain on the uPDH boxes accordingly. The X end gain is at 6.0, and the Y end gain is at 4.0. Before measuring the Y-end OLTF, I adjusted the steering mirrors to increase GTRY to ~0.45. GTRX remains a paltry 0.05... But the UGFs seem satisfactory..
Finally, I took the ALS noise spectrum for the green beats. The beat note amplitudes on the network analyzer in the control room are still puny compared to what we had, -40dBm for Y and -45dBm for X. But the phase tracker Q values are ~1000 and ~3000 for X and Y respectively, which are pretty close to what these were if memory serves me right. There may still be some room for optimization of the PDH loop gains etc, and we could perhaps look at lowering the gain of the REFL PD at the X end? I also have yet to do the sweep for the 3 temperatures at which we can find a beatnote and park at the middle one...
These spectra suggest we could even possibly try locking? We are approximately a factor of 3 above the reference for X and on par with the reference for Y....
Unrelated to this work: I also realinged the PMC, PMC transmission is now 0.730V up from ~0.65V.
Why is the transmission of X green so low? Perhaps you can phase lock the IR and then scan the X frequency, using the X arm as the analyzer. i.e. put a slow ramp into MC2 to pull the PSL frquency and thus the green frequency. You can record a movie of the scan using the framegrabber and record the green transmission peaks to see how big the mode match is exactly (which modes are so big)
[Koji, Johannes, gautam]
With Koji's and Johannes' help, I managed to resolve the coupling the pick-off IR beam into the fiber at the X end. I will put up a more detailed elog about how this was done - but in summary, we have about 31% coupling efficiency into the fiber, which isn't stellar, but I felt this was adequate to find a beatnote. Koji also pointed out that the collimation telescope attached to the fiber at the X-end is poorly mounted - this is something to fix when we swap endtables, but this was not addressed right now because if we were to adjust this, we would also have to adjust the mode matching into the fiber.
I then attempted to tune the temperature to find the IR beatnote. While doing so, I noticed some strange features of the controller - there are essentially two display modes relevant to laser crystal temperature, one which allows us to change the setpoint and one which is an actual readback of the temperature (this one can't be adjusted). While tuning the temperature, I noticed that the latter display ("LT") did not change in value. On a hunch, I disconnected the "SLOW" control BNC on the front panel, and voila, I was able to tune the setpoint and observe the measured temperature shift accordingly. I was thus able to find a reasonably strong IR beatnote (-9dBm) at T ~ 44.6 deg C (the beat PD was set to 0dB attenuation, i.e. high gain mode). However, the moment I reconnected the SLOW control BNC, the beatnote vanished (it gradually shifted out of range of the HP network analyzer), and the same thing happens if I terminate the SLOW control BNC connector! I don't understand this behaviour, as the manual says that the range of voltages accepted to this input is +/-10V, so I would assume 0V means do nothing, but clearly this isn't the case, as the beatnote is being shifted in frequency by > 1GHz, and the tuning coefficient is listed as 5GHz/V in the manual. This situation needs further investigation.
Since I had a reasonable IR beatnote setup, I returned the HP analyzer to the control room and tried to see if a green beatnote was present as well - I first ran ASS, then maximized the green transmission using the PZT mirrors, but no beatnote is evident. The contrast isn't great, the ratio of AUX power to PSL power on the green beat PD is something like 5:1, so this probably requires some tuning as well. I will update this elog after today evening's activities...
I've been a little behind on my elogs so here is an update of the end laser situation.
IR beat for X-end recovered
AM/PM characterization of newly installed Lightwave
I continued the hunt for a green beatnote today - I decided to take the output from the RF amplifiers sitting on the PSL table and directly connect it to the analyzer in the control room while I swept the temperature of the end laser 10,000 counts on either side of a temperature at which I had taken this measurement - so I expect the beatnote should be found somewhere in this neighbourhood. But I did not see any peaks throughout the sweep. I re-checked that the mode overlap onto the BBPD is reasonable. We have considerably less transmitted green power from the arm now than we did before the laser swap (by a factor of ~3) but I still expected to see some sort of beat signal.
It would be handy to have the IR beat set up as well for this process, but as mentioned in a previous elog, I was getting only ~0.1 mW of IR power incident on the coupler at the end table last week. As I had suspected, tweaking the alignment of the steering optics for the pick-off IR beam after the doubler improved the situation somewhat, and I am now getting about 1mW of IR power incident on the coupler at the end table. But I've not been able to adjust the alignment into the fiber at the end such that I get any IR light at the PSL table.
Some updates on the laser swap situation:
2. Implementing the new solution:
As I check the manual of the Innolight (pg17) and the datasheet of the Lightwave, I wonder if the Quarter Wave Plate that was placed immediately after the Innolight laser head is even necessary now - I assume the purpose of the combination of QWP+HWP was to turn the elliptically polarized light from the Innolight into linearly polarized light before the Faraday. But the Lightwave already produces linearly polarized light. I will check out what is the configuration on the Y-end table...
After the discussion at the meeting, I decided to go ahead and open the top of the oven so that I could get a visual on where the crystal was located - this helped in the alignment, and I was able to get some green light out of the oven. I had to tweak the position of the Doubling oven a little (with the top open) in order to align the crystal to the beam axis. However - I was only able to get ~140uW of green light going into the Faraday. I had measured the power at various points along the beam path recently with the old setup. We used to have ~860uW of green going into the Faraday there. To see if I could improve the situation a little, I checked that the beam was reasonably centered on both apertures of the IR Faraday, and then removed the irides upstream of the doubling oven. These were preventing me from placing the lenses exactly as per the a la mode solution. Once the irides were removed, I moved the lenses to their optimal positions as best as I could with a tape measure to mark out distances. I then further tweaked the position of the doubling oven using the 4 axis stage, monitoring the green power while doing so. The best I could get was ~200uW. Perhaps the positions of the lenses need to be optimized further. I also checked the IR power before and after the IR Faraday - these numbers are ~260mW and ~230mW respectively (I maximized the transmitted power through the Faraday by rotating the HWP, the QWP that was in the beam path has now been removed as the Lightwave outputs linearly polarized light), and compare favourably to the numbers in the old setup. Doing a naive scaling accounting for the fact that we have less power going into the doubling crystal, I would expect ~700uW of green light coming out, so it looks like the mode matching into the doubling crystal is indeed sub-optimal. However, now that things are roughly aligned, I hope the optimization will go faster...
I tried aligning the green beam, elliptical as it is, to the arm by using the various steering mirrors after the doubling oven. The following was done:
Tonight we embarked on the laser swap. In short, we have gotten ~210mW through the faraday doubler, but no green light is apparent. The laser outputs ~300mW, so it's not exactly a work of art, but I still expected some green. More work remains to be done...
Gautam took numerous photos of the table before anything was touched. One lens was swapped, as per Gautam's plan. The innolight laser and controller are on the work bench by the end table. The lightwave is on the table and on standby, and is not hooked up to the interlock mounted on the table frame, but instead one below the table directly next to the controller. The ETMX oplev laser is turned off.
Steve pointed me to an old elog by Zach where he had measured the waist of the 1W Innolight NPRO. I ran a la mode with these parameters (and the original optics in their original positions prior to last night's activities), and the result is in reasonably good agreement (see Attachment #1) with my initial target waist of 35 um at the center of the doubling oven (which I presume coincides with the center of the doubling crystal). The small discrepancy could be due to errors in position measurement (which I did by eye with a tape measure) or because I did not consider the Faraday in the a la mode calculation. However, I wonder why this value of 35 um was chosen? In this elog, Kiwamu has determined the optimal waist size to be 50um at the center of the doubling crystal. Nevertheless, as per his calculations, the doubling efficiency should be non-zero (about 1% lower than the optimum conversion efficiency) at 35um or 70um, so we should be able to see some green light as long as we are in this fairly large range. So perhaps the fact that we aren't seeing any green light is down to sub-optimal alignment? I don't think there is a threshold power for SHG as such, its just that with lower input power we expect less green light - in any case, 200mW should be producing some green light... From what I could gather from a bunch of old elogs by Aidan, the Raicol PPKPT crystals have dimension 1mm x 1mm x 30mm (long axis along beam propagation), so there isn't a whole lot of room for error perpendicular to the direction of propagation... I wonder if it is possible, for the initial alignment, to have the top cover of the doubling oven open so that we can be sure we are hitting the crystal?
Today is janitor day. It still does not explain why the 2W Innolight tripped off about an hour ago. All back to normal.
.......................................................I asked Keven later, he admitted hitting the emergency shut off next to the chemical storage cabinet.
Our janitor turned off the laser accidentally.
The PMC wasn't locking very happily after this. I tweaked the pointing onto the PMC REFL diode, to make sure it was centered, and touched the alignment into the PMC. I also reset the FSS Slow output to zero. It took a little while for the laser to settle in, for some reason, but the transmission is up at 0.80 now.
Tweaked MC2 pointing to get the MC transmission high enough to let WFS kick in, which nicely got the rest of the MC alignment done. After that, I offloaded the WFS into the MC suspensions.
Lastly, I ran the command that Rana posted in ELOG 10391, to set the FSS input offset (From -0.18 to -0.06)
Didn't you take this opportunity to replace the cooling fan of the innolight controller?
Reflectivity of AR surface of LaserOptik (SN6)
The first step measurements of R for AR surface. I am not convinced with the data....because the power meter is a lame detector for this measurement.
I'm repeating the measurements again with PDs. But below is the log R plot for AR surface.
6000ppm @ 42 deg
3560ppm @ 44 deg
7880ppm @ 46 deg
4690ppm @ 48 deg
We have received the dichroic optics from Laseroptik. The coatings are:
We got two sets with these coatings:
I put them in the "visible optics" drawer of the newish, metal optics cabinet with the thin drawers down the Y arm.
I repeated the transmittance measurements of LaserOptik SN6 @1064nm.
Transmittance for s-polarization
0 deg - 0.524
45 deg - 0.055
Transmittance for p-polarization
0 deg - 0.515
45 deg - 0.1047 0.01047
Raji's measurements are here.
Got confused (even after I talked with Manasa).
The plot shows the number ~0.01 or less at 45deg. But the number is the text does not match with the plot.
Please use the logarithmic scale for the vertical axis.
And more points between 35 to 50 deg please (like ~1deg spacing)
Don't we have the data sheet from the coater? Can we request it?
Please use the logarithmic scale for the vertical axis.
And more points between 35 to 50 deg please (like ~1deg spacing)
I corrected the typo in the text...however, I agree the plot was lame...Will get the data sheet made tomorrow!
I repeated the transmittance measurements of Laseroptik SN6 at 1064nm. The rotation stage could only resolve 2 deg rotation (We should consider buying a better rotation stage).
0.177% 42 deg
0.806% 44 deg
0.57% 46 deg
54.8% 0 deg
1.155% 44 deg
1.159% 46 deg
65.6% 0 deg
The mirror T is completely out of spec. We should find or request the data sheet of the mirror.
> We should consider buying a better rotation stage
I'm already on it
I have two questions:
1) Are we sure that the T measurement is not being compromised by some systematic? i.e. some leakage is making the apparent T appear too high.
2) IF the T is really so high, how should we decide whether or not to use this one rather than the G&H? Is the 532 nm property more important than the high recycling gain?
Here are the German plots. Unfortunately they are not logarithmic.
Ed: Proprietary data removed. Use wiki (Koji)
Steve uploaded data to the 40m wiki / Aux_Optics on 02-07-2013
VEA is now a laser hazard area as usual, several 1064nm lasers in the lab have been turned back on. Apart from this
Over the past week I have been continuing to finalize the daily summary pages, attempting to keep the total run time under half an hour so that they can be run frequently. I have had many hang ups with the spectrograms and am currently using second trends (with this method, the entire script takes 15 minutes to run). I also have a backup method that takes 3 minutes of data for every 12 minutes, but could not implement any interpolation correctly. This might be a future focus, or the summary pages could be configured to run in parallel and full data for the spectrograms can be used. I configured Steve's tab to include one page of images and one page of plots and fixed the scripts so that it corrects for daylight savings time (at the beginning of the running, the program prints 'DST' or 'Not DST').
Right now, I am focusing on making coherence plots in a spectrogram style (similar to the matlab 'coh_carpet' function) and a spectrogram depicting Gaussianity (similar to the plots made by the RayleighMonitor). I have also been working on my final paper and presentation.
On Thursday night (sorry for the late elog) I decided to give the MCL FF one more try.
I first remeasured the actuator transfer function because previous measurements had poor coherence ~0.5 - 0.7 at 3 Hz. I did a sine swept to measure the TF.
Raw transfer function:
The data is attached here: TF.zip
Then I made Wiener filters by fitting the transfer function data with coherence > 0.95 (on the left). Fitting all the data (on the right). Here are the filters:
The offline subtractions (high coh fit on left, all data fit on right). Notice the better IIR performance when all the TF data was fitted.
The online results: (these were aquired by taking five DTT measurements with 15 averages each and then taking the mean of these measurements)
And the subtraction performance:
When making the Wiener filter OFF/ON comparisons, we want to use the median PSD estimates, not the mean (which is what pwelch gives you).
cf. Sujan's note and Evan's follow-up
The median will be less sensitive to the transients / gltiches and will show more improvement I think.
As per Eric's request, here is the code and TF measurement that was used to calculate the MC2 FF filter that is loaded in FM5. This filter module has the filter with the best subtraction performance that was achieved for MCL.
This is being recorded for posterity so we know where to look for the old controls settings.
The last good burt restore that was saved before turning off scipe25 aka c1dcuepics was on September 29, 11:07.
We took the last of the in-vac photos of mirrors today. I'll post in the morning.
Tomorrow, I'll align the DRMI once more to check, and get IPPOS and IPANG out of the vacuum. I'll take a look at POX, POY and POP, but we may just have to cross our fingers and hope for the best on those ones. They were pretty hard to get out of the vac during their initial alignment, since they're so weak.
Also, tomorrow morning Steve is going to try out our new light access connector!!!! I'm so excited!
The goal is to put heavy doors on, on Wed, and start pumping Wed afternoon / Thurs evening.
My hope is that the DRMI flashes will be bright enough to see on the PO beams. IF we get 10 mW through the Faraday, you should get some buildup when the carrier resonates in the DRMI.
If the recycling gain is 10 and the pickoff fraction is 100 ppm you ought to get ~10 uW on PO. How much of the recycling cavity power gets out of POP?
The photos on the OMC table are particularly tricky, since the camera plus the 'bathroom' mirror add a lot of weight....even if the MC locked, the input beam would be completely different, so all of the beams would be wrong.
During some of the work on the BS table, ITMY was realigned to have its beam retro-reflect, since the weight of the camera plus mirror was shifting all of the suspended optics on the BS table. ITMY was restored after that, for subsequent photos.
I'm making a separate entry to go along with this thread of photos...
Putting the camera and 'bathroom' mirror on any table pretty significantly changes the leveling of any table. The mirror especially is very heavy, although the camera is not feather light. We need to come up with a new plan for taking alignment-confirming photos without adding anything to the tables. That, or we have to level the table between each camera shot. Anyone who has ever leveled one of our in-vac tables should shudder in horror at idea #2, so we need to put some thought into idea #1 before our next vent. Vent Czar - can you put this on the list, in addition to the REFL rearrangement stuff?
As a result of this, PZT2 needed to be reverted to the place it was before work began on Saturday (so that the beam goes through the 45 degree target without any extra stuff on the table). This means, unfortunately, that all of the photos / still captures of optics after PZT2 are invalid.
We continued to check the latch logic. Today we found that latch.py didn't catch the change of LSB but did for MSB. We determined that this happens when the slider value is chaged between the polling for LSB and MSB.
SInce these two should always be related to a single gain value, latch.py was modified so.
Now we don't observe any logic error for ~100 gain transisitions (see attached).
[Jenne & Kyung-ha]
We suspended the mirror to one of the main frame with the ECD backplane we finished before. The hard task was to find the right balance for the mirror so that 1) it won't be tilted and 2) it'll be in the right position for the ECD backplanes so that the magnets attached to the mirror holder would be in the very center of each ECD holes. We used optical lever laser (red He/Ne) to check the balance of the mirror. We tried to use the jig for the mirror holder clamps but because of the size difference, we couldn't use it at all. (Since the magnets are very heavy, we thought the wire being not perfectly centered might work better. However, the jig dimension was way too different that the wire ended up in the middle of one of the holes.) Since there was no other clever way to attach the wire in the right position, we just tried to be as center/accurate as possible. After attaching wire to that mirror holder clamps, we hanged it to the frame. Again, we couldn't find any other accurate way to find the center so we held the wire and tried to adjust the mirror height as accurate as possible so that it can be in the right position in respect to ECD backplane and not be tilted at the same time. However, when we hanged the mirror, it was still tilted.. So we adjusted the mirror tilt using the mirror holder clamps. Since the holes on the clamps were ellipse shapes, we could adjust the position of the clamps a little bit. When we adjust the clamps, we started to tighten the screws when the mirror is NOT in the perfect position since the tightening up part changes the mirror angle anyways. Luckily, when we tightened up the last screw, the mirror was in the perfect position! After that, we poked the mirror several times to make sure that it comes back to the same place.
Amazingly, we could finish this whole hanging/adjusting process in about 30 mins! :D (Jan said it's because of his amazing moral support. :P Maybe he'll be there to support us everytime we work on the mirrors?)
I have measured / calculated the latest MICH noise budget. It doesn't really look all that stellar.
As you can see, we are nowhere near being shot noise limited, since there's a huge discrepancy between all of the measured spectra and the teal Shot Noise line.
One possible suspect is that the analog whitening filters weren't on when I took my measurements. I didn't actually check to ensure that they were on, so they might not have been. Right now we're limited by electronics and other boring noises, so I need to make sure we're limited by the noise of the diode itself (we don't have enough light in the IFO to actually be shot noise limited since that takes 2.5mA for AS55 and I only have 1.1mA, but we should be ~within a factor of 2ish).