We moved the MC-trans pick-off mirror (= the beam splitter between the input of the Faraday and the steering mirror located right after MC3). Now the beam goes through the Farady without getting clipped.
This is the list of the things that have to be done next:
To get a feel for the Capacitive Bridge problems, we setup a simple bridge using fixed (1 nF) caps on a breadboard. We used an SR830 Lock-In amplifier to drive it and readout the noise.
The measurement setup for the Capacitor Bridge Test is still sitting on one of the work benches.
Unless the experiment is supposed to continue today, the equipment shouldn't have been left on the bench. It should have been taken back to the lab.
Also the cart with HP network analyzer used for the test was left in the desk area. That shouldn't have left floating around in the desk area anyway.
The people responsible for that, are kindly invited to clean up after themselves.
BTW, latex launched this new thing for writing pdfs. doesnot require any installations. check http://docs.latexlab.org
I could not dare to share my google doc with this site...
Just in case, granted access to Google docs can be revoked any time from here:
I wrote down the settings according to which I tuned the optickle model of the 40m Upgrade.
Basically I set it so that:
In this way when the carrier becomes resonant in the arms we have:
The DARM offset for DC readout is optional, and doesn't change those conditions.
I also plotted the carrier and the sideband's circulating power for both recycling cavities.
I'm attaching a file containing more detailed explanations of what I said above. It also contains the plots of field powers, and transfer functions from DARM to the dark port. I think they don't look quite right. There seems to be something wrong.
Valera thought of fixing the problem, removing the 180 degree offset on the SRM, which is what makes the sideband rather than the carrier resonant in SRC. In his model the carrier becomes resonant and the sideband anti-resonant. I don't think that is correct.
The resonant-carrier case is also included in the attachment (the plots with SRMoff=0 deg). In the plots the DARM offset is always zero.
I'm not sure why the settings are not producing the expected transfer functions.
Today I started writing the IFO modeling wiki page.
The idea is to make it a reference place where to share our modeling tools for the 40m.
i added my laptop's mac address to teh martian at port 13 today.
No personal laptop is allowed to the martian network. Only access to the General Computing Side is permitted.
Please disconnect it.
I calculated the phase shifts that the sidebands would pick up in the arms in the case we changed the arm length to 38.4m as proposed. I obtained the following values (in degrees):
phi(-f2) = 0.66; phi(-f1) = -0.71; phi(f1) = 0.71; phi(+f2) = -0.66
These are the plots with the results as I obtained from an Optickle simulation (the second zooms in around 38.4m).
These values agree with what Koji had already estimated (see elog entry 3023).
Since we can't make the arm longer than that, to increase the distance from the resonance, we would like to adjust the length of the short cavities to compensate for that. For f2 (=55MHz), 0.7 degrees correspond to about 5cm. That is about the length change that we expect to make to the design.
I simulated with Optickle the effect of changing the length of either the SRC or the PRC. The best way I found to do that, was to measure the cavity circulating power when the macroscopic lengths change.
The following plots show the effect of changing either the PRC or SRC length (left or right figure), on the circulating power of both cavities at the same time (top and bottom plots).
You can compare these with the case of perfect antiresonance as in the following plots:
It seems that the design length for the short cavities are not too bad. f1 is not optimized in the PRC, but changing the length of the cavity wold just make f2 worse in SRC.
These simulations seem to support the choice of not changing the design cavity lengths for PRC and SRC.
Of course these are only an "open loop" simulations. At the moment we don't know what would be the effect of closing the control loops. That is something I'm going to do later. It'll be part of my studies on the effects of cavity absolute length on the whole IFO.
You should have been in my lecture yesterday!
Power in the cavity is not a good index (=error signal) to judge the optimal length.
You should look at the phases of the length signals. (i.e. demodulation phase which gives you the maximum amplitude for CARM, PRC, SRC, etc)
You must move the SRC and PRC lengths at the same time.
The resonance of f1 (mostly) depends on the PRC length, but that of f2 depends on both the PRC and SRC lengths.
Right. Ultimately the phase gain inside the cavity is what we look at. Calculating that for the SBs inside PRC and SRC is actually the first thing I did.
But I kept getting very small angles. Too small, I thought. Maybe there was some problem in the way I calculated it.
Then I made a power analysis to check if the SBs were getting affected at all by that 0.7degree phase shift they're picking up in the arms.
I wanted to show the point where I am, before leaving. But, I keep working on it.
I also removed two of the AM stabilizers from the 1Y2 rack. The other one, which is currently running th MC modulations, is still in the rack, and there it is going to remain together with its distribution box.
I stored both AM stabilizers and the Stochmon box inside the RF cabinet down the East arm.
Lately I've been trying to calculate the corrections to the recycling cavity lengths that would compensate for the phase that the sidebands will pick up from the arms in the upgraded interferometer.
To do that calculation , I tried two quite different ways, although equivalent in principle. They both use the optickle model of the 40m, but the calculation is made differently.
In the first way, I looked directly at the phases of the field: phase of [input field] / [reflected field], phase of [input field at PRM] / [transmitted field at SRM].
In the second way I looked at the demodulation phases of the LSC signals.
The first way is much simpler, especially from a computational point of view. It is the first I tried several weeks ago, but then I had abandoned because back then I thought it wasn't the correct way.
Anyway, both ways gave me the same results for the PRC length.
For the SRC length, the first way has given me a clear outcome. On the other hand, the second way has produced a less clear result.
According to these results, these would be the proposed adjustements to the cavity lengths:
dl(PRC) = -0.0266 m; dl(SRC) = 0.0612 m
I) 1st Way
a) case of arms ideal length (33.86 m)
b) case arm length = 38.40 m
II) 2nd Way
a) case of arms ideal length (33.86 m)
Tell me whether it is correct or not. Otherwise I won't be able to sleep tonight.
According to these results, these would be the proposed adjustements to the cavity lengths:
dl(PRC) = -0.0266 m; dl(SRC) = 0.612 m
Sorry. I was in a rush to go to the LIGO "all hands" meetings when I posted that elog entry, that I forgot a zero in the SRC length value. The correct values are:
dl(PRC) = -0.0266 m; dl(SRC) = 0.0612 m
The cavity absolute lengths are then:
L(PRC) = 0.5/2/f1*c - 0.0266 = 6.7466 m
L(SRC) = c/f2 + 0.0612 = 5.4798 m
where c is the speed of light; f1 = 11065399 Hz; f2 = 55326995 Hz
Today I noticed that the FE SYNC counters of c1susvme1/2 on the RFM network screen were stuck at 16384. I tried to reboot the machines to fix the problem but it didn't work.
The BS watchdog tripped off when I did that, because I had forgotten to disable it. I had to wait for a few minutes before it settled down again.
Later I also re-locked the mode cleaner. But before I could do it, Rana had to reduce the MC_L offset for me.
"They (shellfish) shall be an abomination to you; you shall not eat their flesh, but you shall regard their carcasses as an abomination." (Leviticus 11:11)
It looks like something wrong happened around the PSL front end. One of the PSL channel, C1:PSL-PMC_LOCALC, got crazy.
One of the PSL channel, C1:PSL-PMC_LOCALC, got crazy.
We found it by the donkey alarm 10 minutes ago.
The attached picture is a screen shot of the PMC medm screen.
The value of C1:PSL-PMC_LOCALC ( middle left on the picture ) shows wired characters. It returns "nan" when we do ezcaread.
Joe went to the rack and powered off / on the crate, but it still remains the same. It might be an analog issue (?)
The problem seems to be a software one.
In any case, Kiwamu and I looked at the at the PMC crystal board and demod board, in search of a possible bad connection. We found a weak connection of the RG cable going into the PD input of the demod board. The cable was bent and almost broken.
I replaced the SMA connector of the cable with a new one that I soldered in situ. Then I made sure that the connection was good and didn't have any short due to the soldering.
By looking at the reference pictures of the rack in the wiki, it turned out that the Sorensen which provides the 10V to the 1Y1 rack was on halt (red light on). It had been like that since 1.30pm today. It might have probably got disabled by a short somewhere or inadvertently by someone working nearby it.
Turning it off and on reset it. The crazy LO calibrated amplitude on the PMC screen got fixed.
Then it was again possible to lock PMC and FSS.
We also had to burtrestore the PSL computer becasue of the several reboots done on it today.
Today we measured the phase noise of the oscillator used for the FSS.
The source is a Wenzel crystal at about 21.5MHz that Peter Kalmus built some time ago.
We basically used the same technique that Frank and Megan have been using lately to measure the Marconi's phase noise.
Today we just did a quick measurement but today next week we are going to repeat it more carefully.
Attached is a plot that shows the measurement calibrated for a UGF at about 60 Hz. The noise is compared to that specified by Wenzel for their crystal.
The noise is bigger than that of the MArconi alone locked to the Rubidium standard (see elog entry). We don't know the reason for sure yet.
We'll get back to this problem next week.
I uploaded an updated optickle model of the upgrade to the SVN directory with the optickle models (here).
[Koji, Steve, Kiwamu, Alberto]
- This afternoon we installed a few new optics on the BS table: GR_PBS, GRY_SM2, GRY_SM1.
- We pulled up the cables so that we had more freedom to move one of the cable towers farther South.
- Then we re-leveled the table. PRM OSEMs were adjusted to be nominal insertions.
- Koji released the earthquake stops on BS but the readout of the OSEMs was apparently frozen on the MEDM screens.
Initially we thought it was a software problem. a nuclear reboot didn't solve it. We spent the following three hours investigating the cause.
Eventually it turned out that the earthquake stops on BS weren't actually fully released.
We opened the tank and accessed to BS. Releasing the earthquake stops in full solved the issue. The OSEMs readout went back to normal.
I just restarted the elog after I found it down a few minutes ago.
I finished assembling the frequency generation unit for the upgrade. I tested it through to check that the power levels are as expected at the various connection (see attached png, showing in black the design power values, and in red the measured ones).
Because of some modifications made on the design along the construction, I have to recalculate the SNR along the lines.
I can now start to measure phase noise and distortion harmonics.
A document with a description of the design and the results of the characterization measurements will be available in the end.
A few weeks ago, on Jul 24, Rana and I measured the phase noise of the FSS frequency box (aka the 'Kalmus Box'). See elog entry 3286.
That time, for some reason, we measured a phase noise higher than we expected; higher than that of the Marconi.
I repeated the measurement today using the SR785 spectrum analyzer. Here is the result:
(The measurement of July 24 on the plot was not corrected for the loop gain. The UGF was at about 30 Hz)
To make sure that my measurement procedure was correct, I also measured the combined phase noise of two Marconis. I then confirmed the consistency of that with what already measured by other people in the past (i.e. Rana elog entry 823 in the ATF elog).
This time the noise seemed reasonable; closer to the Marconi's phase noise, as we would expect. I don't know why it was so bad on July 24.
The shoulder in the Marconi-to-Marconi measurement between 80Hz and 800Hz is probably due to the phase noise of the other Marconi, the one used as LO.
I'm going to repeat the measurement connecting the setup to the DAQ, and locking the Marconi to the Rubidium standard.
Ultimately, the goal is to measure the phase noise of the new Sideband Frequency Generation Box of the 40m Upgrade.
Today I put the FSS frequency box back into the 1Y1 rack.
To power it on, I turned on the 24V and 15V Sorensen switches in the same rack.
The PMC crystal board in the same rack should not be affected (it runs with 10V), but, to make sure it was not powered, I disconnected it from its crate. Since the board was disconnected from the EOM for the PSL table's upgrade, I wanted to avoid having the RF output floating.
We just have to remember to plug it back in, when we need it again.
I just turned on the other Sorensen's too in 1Y1.
I measured the phase noise of the LO output of the FSS box from the DAQ. I'm attaching the results.
As we expected, the measurement is limited by the internal phase noise of the Marconi.
The measurement was done as shown in this diagram.
The differences between this setup and the one used previously is the lack of the 50 Ohm terminator in the mixer output and
that the SR560 readout with the G=100 should come before the first SR560 via T, so as not to be spoiled by the high noise of the G=1 SR560.
I removed the 50 Ohm in-line terminator when I did the measurement with the SR785. The for some reason I was getting more noise, so I removed it.
Now I put it back in and I did the measurement with the DAQ. I also moved the SR560 that amplifies the signal for the DAQ, Tee'ing it with the input of the in-loop SR560.
Now the setup looks like this:
And the phase noise that I measure is this:
Comparing it with the phase noise measured with the previous setup (see entry 3506), you can see that the noise effectively is reduced by about a factor of 2 above 10 Hz.
I found this very interesting German maker of cool cable cutting tools. It's called Jokari.
We should keep it as a reference for the future if we want to buy something like that, ie RF coax cable cutting knives.
The Netgear Network Switch in the top shelf of Nodus' rack has a broken fan. It is the one interfaced to the Martian network.
The fan must have broken and it is has now started to produce a loud noise. It's like a truck was parked in the room with the engine running.
Also the other network switch, just below the Netgear, has one of its two fans broken. It is the one interfaced with the General Computer Side.
I tried to knock them to make the noise stop, but nothing happened.
We should consider trying to fix them. Although that would mean disconnecting all the computers.
Last week I noticed that the high power amplifiers in the Frequency Generation Box became hot after 2 hours of continuous operation with the lid of the box closed. When I measured their temperature it was 57C, and it was still slowly increasing (~< 1K/hr).
According to the data sheet, their maximum recommended temperature is 65C. Above that their performances are not guaranteed anymore.
These amplifiers aren't properly dissipating the heat they produce since they sit on a plastic surface (Teflon), and also because their wing heat dissipator can't do much when the box is closed. I had to come up with some way to take out their heat.
The solution that I used for the voltage regulators (installing them on the back panel, guaranteeing thermal conduction but electrical isolation at the same time) wouldn't be applicable to the amplifiers.
I discussed the problem with Steve and Koji and we thought of building a heat sink that would put the amplifier in direct contact with the metal walls of the box.
After that, on Friday I've got Mike of the machine shop next door to make me this kind of L-shaped copper heat sink:
On Saturday, I completely removed the wing heat dissipator, and I only installed the copper heat sink on top of the amplifier. I used thermal paste at the interface.
I turned on the power, left the lid open and monitored the temperature again. After 2 hours the temperature of the amplifier had stabilized at 47C.
Today I added the wing dissipator too, and monitored again the temperature with the lid open. then, after a few hours, I closed the the box.
I tracked the temperature of the amplifier using the temperature sensors that I installed in the box and which I have attached to the heat sink.
I connected the box temperature output to C1:IOO-MC_DRUM1. With the calibration of the channel (32250 Counts/Volt), and Caryn's calibration of the temperature sensor (~110F/Volt - see LIGO DOC # T0900287-00-R), the trend that I measured was this:
The heat sink is avoiding the amplifier to overheat. The temperature is now compatible with that of the other component in the box (i.e., crystal oscilaltors, frequency multiplier).
Even with the lid closed the temperature is not too high.
Two things remain untested yet:
1) effect of adding a MICA interface sheet between the heat sink and the wall of the chassis. (necessary for gorund isolation)
2) effect of having all 3 amplifiers on at the same time
I am considering opening air circulation "gills" on the side and bottom of the chassis.
Also we might leave the box open and who ever wants can re- engineer the heat sink.
- Ideally we would like that the heat sink had the largest section area. A brick of metal on top the amplifier would be more effective. Although it would have added several pounds to the weight of the box.
- We need these amplifiers in order to have the capability to change the modulation depth up to 0.2, at least. The Mini-Circuit ZHL-2X-S are the only one available off-the-shelf, with a sufficiently low noise figure, and sufficiently high output power.
Here are the results of my phase noise measurements on the 7 outputs of the Frequency Generation Box. (BIN=95L applied by DTT). See attached pdf for a higher definition picture.
The plot shows that the phase noise of the 11 MHz outputs (Source, EOM modulation signal, Demodulation signal) is as low as that of the Marconi. The Marconi is limiting my measurement's resolution.
The mode cleaner signal's oscillator (29.5 MHz output, blue trace) is higher than the 11MHz above 1KHz.
The 55MHz signals have all the same phase noise (traces overlapped), and that is higher than the 11 MHz ones from about 100Hz up. i don't know what's going on.
I need to use the spare 11MHz Wenzel crsytal to have a better reference source for the measurement.
We need a distribution unit in the LSC rack to: 1) collect the demod signals coming from the Frequency Generation Box 2) adjust the power level 3) generate 2nd harmonics (for POP) 4) distribute the demod signals to the single demodulation boards.
The base line plan is the following:
The box can be build up gradually, but the priority goes to these parts:
I need help for this work. I know exactly how to do it, I just don't have the time to do it all by myself.
Besides the Distribution Box, the demodulation part of the upgrade would still require two steps:
1) upgrade the Band Pass Filters of the demodulation boards (I have all the parts)
2) cabling from the distribution box to the demod board (one-afternoon kind of job)
The Nodus connection to the Martian network stopped working after someone switched cables on the Netgear router. Apparently that router doesn't like to have the 23 and 24 ports connected at the same time.
Joe fixed the connection just freeing either the 23 or the 24 port.
I measured the amplitude noise of the source outputs and the EOM outputs of the Frequency Generation box.
the setup I used is shown in this diagram:
(NB It's important that the cables from the splitter to the RF and LO inputs of the mixers are the same length).
The results of the measurements are shown in the following plot:
1) both Crystals (29.5MHz and 11MHz) have the same noise
2) the 55MHz source's noise is bigger than the 11 MHz (~2x): the frequency multiplication and amplification that happen before it, add extra noise
3) the noise at EOM outputs is ~2x bigger than that of the relative sources
When I have the chance, I'll plot the results of my calculations of expected noise and compare them with the measurements.
I stored the Busby Box, the Rai's Box and the SR554 preamp in the RF cabinet down the Y arm.
I uploaded all the material about the RF frequency Generation Box into the SVN under the path:
I structured the directory as shown in this tree:
I'm quickly describing in a section of the Rf system upgrade document with LIGO # T1000461.
I completed a LIGO document describing design, construction and characterization of the RF System for the 40m upgrade.
It is available on the SVN under https://nodus.ligo.caltech.edu:30889/svn/trunk/docs/upgrade08/RFsystem/RFsystemDocument/
It can also be found on the 40m wiki (http://lhocds.ligo-wa.caltech.edu:8000/40m/Upgrade_09/RF_System#preview), and DCC under the number T1000461.
As the suspension work winds down (we'll be completely done once the ETMs arrive, are suspended, and then are placed in the chambers), I'm going to start working on the RF system.
Step 1: Figure out what Alberto has been up to the last few months.
Step 2: Figure out what still needs doing.
Step 3: Complete all the items listed out in step 2.
Step 4: Make sure it all works.
Right now I'm just starting steps 1 & 2. I've made myself a handy-dandy wiki checklist: RF Checklist. Hopefully all of the bits and pieces that need doing will be put here, and then I can start checking them off. Suggestions and additions to the list are welcome.
There's also a page dedicated to the progress in the PD upgrade process:
There you can find a pdf document with my notes on that.
ELOG reverted to 2.7.5 due to editing difficulties
- /cvs/cds/caltech/elog/start-elog.csh reconfigured to launch 2.7.5
- /cvs/cds/caltech/elog/elog is linked to ./elog-2.7.5
- logbook dir of 2.8.0 was copied in the dir of 2.7.5. The old and obsolete 2.7.5 was discarded.
I think I had the same problem when I switched to 2.75 from 2.65.
Then the problem was FCKeditor.
We should try the solution I put in the elog page of the wiki.
[Koji and Kevin]
I was trying to characterize the REFL11 photodiode by shining a flashlight on the photodiode and measuring the DC voltage with an oscilloscope and the RF voltage with a spectrum analyzer. At first, I had the photodiode voltage supplied incorrectly with 15V between the +15 and -15 terminals. After correcting this error, and checking that the power was supplied correctly to the board, no voltage could be seen when light was incident on the photodiode.
We looked at the REFL55 photodiode and could see ~200 mV of DC voltage when shining a light on it but could not see any signal at 55 MHz. If the value of 50 ohm DC transimpedance is correct, this should be enough to see an RF signal. Tomorrow, we will look into fixing the REFL11 photodiode.
I just wanted to remind you that the most up to date resource about the RF system upgrade, including photodiodes, is the SVN.
Because I was doing new things all the time, the wiki is not up to date. But the SVN has all I've got.
For those of you who spend annoying amounts of time looking for tools, fear no more. Toolboxes for each optical table are coming!
They will probably have:
IR Viewer (a few optical tables will have IR viewers, these specific tables will be labeled in the diagram coming out later)
Ball screw drivers (3/16 in.) 6-8 in. handle
Various Connectors (I'll find out what's needed at some point)
Small flat screwdrivers (for adjusting camera gains)
Please suggest what else may be needed in these boxes.
The boxes will be held to the side of the tables, either by magnets or screws. A diagram of where they will be placed on each optical table in order to minimize obstruction of walkways will be distributed soon. Any objections can then be noted.
A heavy duty plastic box is the likeliest candidate for the optical table toolbox. It measures 5 9/16 in. x 11 5/8 in. x 4 5/8 in. and fits all the tools comfortably. ( http://www.mcmaster.com/#plastic-bin-boxes/=m4yh4m , under Heavy Duty Plastic Bin Boxes)
The list of tools has been updated to include a pen and a wire cutter as well as everything previously stated.
In addition, Steve has recommended that boxes should be secured to the walls or surfaces near the optical tables as opposed to the optical tables themselves, as to keep the tables from wobbling when tools are being exchanged.
A diagram of tentative box placements will go out soon.
I also took every allen key I can find so they can be sorted. They will be back in the appropriate drawer locations soon.
No, the small boxes must be attached to the optical tables. They won't be heavy enough to change the table tilt.
Also, all tools must be color coded according to the optical table using the 3M Vinyl table color code:
So the new tentative plan on the boxes is to bolt them (magnetic strips were proposed but overruled on the grounds that they're not strong enough to withstand being knocked down by accidents).
The boxes are going to be a mix of the Thorlabs Benchtop Organizer (http://www.thorlabs.com/thorProduct.cfm?partNumber=BT17) and the original box. The box will have a region covered in mesh, so tools can be placed and held there. The box will also have a spacer at the bottom, with another mesh right above it, lined up. However, this double-mesh will only cover half of the box. The other half of the box will be compartmentalized to hold things such as screws, connectors, etc. I will talk to Steve about building the boxes.
Also, using nail-polish to coat the Allen wrenches is not going to work. Nail polish does not stick easily enough. The tentative new plan is oil paint, but this is to be reviewed.
Finally, the diagram with the placement of the boxes relative to the optical tables has been put on paper, but needs to be computerized so it's easier to read. This will be done as soon as possible.
There are some tips for how to appy nail polish on YouTube from MKNails and MissJenFABULOUS. Their tips on how to prepare the site for a strong bonding strength are probably helpful for our gold/nickel coated tools. For chrome tools we may need to abrade the surface with a stone or fine sandpaper for it to take the layer better. IF the YouTube videos don't do it for you, then I suggest contacting Tom Evans at LLO to find out what kind of nail polish he uses.
This is the tentative box placement per optical table. The toolboxes are going to be color-coded by a combination of two colors (the order won't matter). The side of each toolbox will have a little panel to let you know which box corresponds to which set of colors.
On the diagram, the set of colors is simply the color of the box border and the color of the text.
If anyone has a problem with any of the colors or the box placement let me know before they are installed and become an annoyance:
ETMY: Box will be attached to the underside of the table by magnets. The box will be on the north side of the optical table.
POY: Box will be attached to the side of the optical table by magnets. The box will be on the west side of the optical table.
BSPRM: Box will be attached to the side of the optical table by magnets. The box will be on the west side of the optical table.
AS: Box will be attached to the side of the optical table by magnets. The box will be on the north side of the optical table.
PSL1: Box will be inside the optical table, in the northeast corner.
PSL2: Box will be inside the optical table, in the southwest corner.
POX: Box will be attached to the side of the optical table by magnets. The box will be on the south side of the optical table.
MC2: Box will be attached to the side of the optical table by magnets. The box will be on the south side of the optical table.
ETMX: Box will be attached to the side of the optical table by magnets. The box will be on the east side of the optical table.
I decided to go see what the electrical tape looks like on the other tools.
These are the tools I felt were necessary to label with tape: (the others don't seem to be terribly important in terms of not interchanging between boxes)
On another note I'm not sure why electrical tape can't be used on the Allen Wrenches too.
I also plan on ordering smaller flash lights for each table (this one is bulky and unwieldy), and filling in the gaps of the Allen Wrench sets as soon as I get the go-ahead.
Albert, our new undergrad work force received 40m specific- basic safety training last week. Please read and sign 40m procedures booklet.
These are the tentative box placements. Roughly. I don't actually have the box finalized yet, but the box should be around that size.
How do I perspective ._.
Thanks for the loan guys. I do have a lot of warm weather clothing at home that is not so necessary in the California climate. I will find some suitable attire for the Rb clocks there.