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Old 20-05-2015, 06:13 PM
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Looking for advice on AO

I normally prefer to make a decision of buying equipment based on previous experience but in this instance I don't have any base line to work from.

I am looking at buying an AOX for my STXL 11002 and it will be used on my RC12.

I get fairly good seeing generally and that does currently contribute to how sharp my images appear. However as pointed out to me last night (that I am a bit more than obsessive compulsive about image perfection), I am wondering if an AO will in fact provide greater results over and above what I get now. I really want to produce sharper images with tighter stars and as I understand it good seeing really makes these units achieve good results.

I am looking for opinions from both people who use these pieces of kit and from those that have done the research and have an opinion either way.

So please let me know if it is worth the not inconsiderable cost of an AOX.
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Old 21-05-2015, 01:30 AM
ericwbenson (Eric)
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Hi Paul,

I used to use an AO-8 and ST-8 (and I wrote the original Maxim driver for the AO7 which we tested with an ST-7). For the really fast AO-7 (40Hz+) with the very small KAF402 the improvement was evident. For the slower AO8 (20Hz max) plus ST8 the device still had an impact (especially when not using a PME!).

I am more skeptical now of the results with larger chips. A couple of years ago I was gung-ho to make a large AO unit, or somehow fit the SX unit into my image train. But having read Allan Holmes last whitepaper carefully, the correlation between guide star wander and largish main chip image wander is not great. In fact I think he acknowledges that multi-star guiding may do just as well as tip-tilt AO for large chips, since only the low-frequency disturbances are correlated over many arc-minutes. Multi-star guiding averages out the high-frequency wander and you're left with corrections based on slow moving waves. In fact demonstrations that multi-star guiding works better than regular guiding is also the evidence that AO won't do much, if anything, for large CCDs.

But hey there is nothing stopping you from trying...what's two more adapters anyways

Best,
EB
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Old 21-05-2015, 07:54 AM
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I think the fact that Martin uses one to such great advantage with his STXL11 and CDK17 is all the evidence you need. He swears by it.

Typical consensus is that it works best on long focal length scopes like your RC as per Erics post.

I see the effects of seeing in my CDK17 very very clearly and those nights where the seeing is good the difference in the quality of the subs is amazing.

I have an SX AO unit I plan to use with the Trius on the RHA. Even though the RHA is only 1159mm focal length the small chip acts as a digital zoom and its a small field of view so still a galaxy imaging machine. It may not be immediately as I need to sort adapters and probably a new filter set for an SX filter wheel. But I will know in about 2 months.

Leo's images with a 10 inch GSO RC show definite extra sharpness from his AO unit.

Greg
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Old 21-05-2015, 12:10 PM
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Isn't Martin still set up at Sierra Remote Observatories that is already subject to 1 arcsec seeing year round. An AO unit will only build upon that. Depends on how far you want to take it, but I can tell you that purchasing an AO unit is chicken feed compared to the cost of hosting your telescope on a mountain top that experiences great seeing. If you want to get the best you can from your site, then I would go for it Paul. There are so many factors to getting sharper images and tighter stars. Seeing is just one of them.
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Old 21-05-2015, 01:04 PM
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I use an AO8 on my ST10XME which isn't a big chip now.
I still think it is worth while using the AO though even at slow guiding rates. If your mount is reasonably good the mount never needs to have any guiding corrections. All of the guiding corrections are made by the AO unit. This means moving a tiny piece of glass rather than 18kg of scopes.
If it corrects some seeing effects as well then great but it is more the ease of guiding corrections. If it could also use multi star guiding then great but I haven't seen any software to do this yet.
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Old 21-05-2015, 02:36 PM
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Thanks guys.

Yes Martin is still up at SRO with his 17 STXL11002 and AOX. And; the results have been nothing short of stunning. His images have been my baseline of making my decision and asking the questions I have here. My seeing is no where near as good as that, but compared to what seems to be most of the east coast gets my seeing is very good.

So it seems my decision is clear now. I want to get the best out of the setup and that seems to be getting an AOX.

Yes Eric, it is just another adapter (fortunately not two ) That will make it around 12 or 13 I have had made for this system. Several were made to find out about back focus.
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Old 21-05-2015, 02:53 PM
SpaceNoob (Chris)
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Hi Paul,

I use an AO8 but my sampling is on the silly side (0.44"), if there is any improvement to seeing impacts, I would say it is quite minimal. The biggest benefit I see is as Terry has mentioned, moving glass as opposed to moving a mount to perform corrections. I have observed a major disadvantage though, it can amplify micro-lens issues by washing out the very tiny diffraction spikes at every angle. On small pixels, the wash quickly fills the wells, not to saturation, but high enough to be very obvious. At least this is what I have noticed and I have since toned down to around 2Hz. Probably totally different game for bigger pixels though. But if you're already seeing flat guide logs, and tight stars, I'm not sure if you'll see an actual improvement to the image subs, apart from slightly better FWHM, but still the same seeing impacts, with a reduced likelihood of trashed subs due to the guider/mount stuffing up for some unknown reason. AO can get a mind of its own though sometimes, especially if you haven't found the right star. It can be a little more hands on than an OAG, depending on what automation you're using, from a tuning perspective (each target is different).

Just don't expect 10Hz+ to work for every target in an image run as the AO will spend most of its time being inefficient and offering nothing over an OAG, probably worse in some instances.
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Old 22-05-2015, 10:08 PM
jase (Jason)
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Quote:
Originally Posted by ericwbenson View Post
In fact I think he acknowledges that multi-star guiding may do just as well as tip-tilt AO for large chips, since only the low-frequency disturbances are correlated over many arc-minutes. Multi-star guiding averages out the high-frequency wander and you're left with corrections based on slow moving waves. In fact demonstrations that multi-star guiding works better than regular guiding is also the evidence that AO won't do much, if anything, for large CCDs.
Eric, I have used multi star guide with MaximDL using John Winfield's plugin. Now MaximDL 6 supports it natively. I know your an ACP user (as am I), do you know if ACP works fine when multi-guide star is enabled in MaximDL? I'm still running MaximDL5 but have the license to upgrade to 6, just haven't had the interest.

I have reservations about multi star guiding at long focal lengths however... It would work and work well but your OAG pick off and guider need to be a fair size. As you've no doubt experienced with your long focal length set up, sometimes you're lucky to get two stars on the chip, but often its only one.
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Old 22-05-2015, 10:52 PM
ericwbenson (Eric)
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Eric, I have used multi star guide with MaximDL using John Winfield's plugin. Now MaximDL 6 supports it natively. I know your an ACP user (as am I), do you know if ACP works fine when multi-guide star is enabled in MaximDL? I'm still running MaximDL5 but have the license to upgrade to 6, just haven't had the interest.

I have reservations about multi star guiding at long focal lengths however... It would work and work well but your OAG pick off and guider need to be a fair size. As you've no doubt experienced with your long focal length set up, sometimes you're lucky to get two stars on the chip, but often its only one.
Hi Jase,
Funny, I have the Maxim 6 license too, but like you I have not updated the imaging computer with it (it wasn't broke!). I did recently read a thread in the ACP comm center about the v6 multi guide star + ACP setup, eventually they did get it all to work, so I was going to try it, eventually...but I did want to get Scheduler working first.

Even at 3.5m fl some parts of the sky I can see a dozen guide stars with the SX Lodestar, other parts just a few, but almost always more than one. That is with the rotator though chasing the best spot. I have also set my minimum guide exposure length to 1 sec since the tracking on the A200HR is reaaally good and any small corrections faster than that are often just seeing/wind/noise, and likely better ignored. I'd make that even longer but then dithering becomes a pain since after the dither the guider needs to lock on again causing a few more guide cycles. I think ~5sec is longest exposures I'd use for that reason.

With a rotator + >1 sec exposures I think multiple stars is feasible most of the time. Worst case is multi-star turns into single-star.

Best,
EB
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Old 23-05-2015, 07:10 AM
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Paul,

I assume you have followed this thread: http://www.iceinspace.com.au/forum/s...d.php?t=124461

Yes, different AO system, but the discussion is interesting. From my reading it would seem that amateur AO is not going to help a very good mount when it comes to seeing, unless there is perhaps already excellent seeing. In average seeing it might just make images worse. Anyway, the whole subject seems to invite differing opinions. Perhaps AO adds a degree of complexity without a clear advantage which is why I have so far stayed away even though quite tempted at times.

Good luck with the decision.

Peter
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Old 23-05-2015, 08:08 AM
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Quote:
Originally Posted by Paul Haese View Post
I normally prefer to make a decision of buying equipment based on previous experience but in this instance I don't have any base line to work from.

I am looking at buying an AOX for my STXL 11002 and it will be used on my RC12.

I get fairly good seeing generally and that does currently contribute to how sharp my images appear. However as pointed out to me last night (that I am a bit more than obsessive compulsive about image perfection), I am wondering if an AO will in fact provide greater results over and above what I get now. I really want to produce sharper images with tighter stars and as I understand it good seeing really makes these units achieve good results.

I am looking for opinions from both people who use these pieces of kit and from those that have done the research and have an opinion either way.

So please let me know if it is worth the not inconsiderable cost of an AOX.

Hi Paul,
Let me chime in with what I know:

I would suggest you try it out if at all possible.
I have over 20 years experience with industrial closed loop control systems -
which is exactly what a telescope mount with an OAG is -
in fact - it's a dual axis closed loop control system RA & DEC.

I have found that all the best industrial systems use what is
known as a nested loop:
this means that there is one closed loop operating at one frequency -
let's say 1 Hz and another one nested inside it operating at say 10 Hz.
The ratio is normally 10 to 1.
This gives much greater accuracy than a single closed loop axis.

Notice that the normal ratio is 10 to 1.
That is important otherwise the 2 loops fight against each other & produce poor results.
Therefore - if your OAG system is operating at 1 Hz & your AO is at 2 Hz there would not be an advantage.
You would need a bright guide star to get 10 Hz for Adaptive Optics.

cheers
Allan
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Old 23-05-2015, 10:01 AM
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Interesting post Allan. I notice SBIG promote the AOX as being able to do 10Mhz. Perhaps that is why its getting such good results.

I must say overall though having looked at thousands of AP images that I can usually tell the ones that use AO units. They are almost always sharper. Why that is to me is not important. They just are. If its seeing or high frequency mount corrections they seem to add something to the images. I see a similar effect when my own imaging is on a night of good seeing with excellent guiding. The images have quite a bit more detail and pop.

So I don't really see a downside here Paul. Except for the cost but to me I would consider it low risk.

Greg.
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Old 23-05-2015, 01:42 PM
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Originally Posted by gregbradley View Post
Interesting post Allan. I notice SBIG promote the AOX as being able to do 10Mhz. Perhaps that is why its getting such good results.

I must say overall though having looked at thousands of AP images that I can usually tell the ones that use AO units. They are almost always sharper. Why that is to me is not important. They just are. If its seeing or high frequency mount corrections they seem to add something to the images. I see a similar effect when my own imaging is on a night of good seeing with excellent guiding. The images have quite a bit more detail and pop.

So I don't really see a downside here Paul. Except for the cost but to me I would consider it low risk.

Greg.

Hi Greg,
bear in mind that the normal AO units are not operating as a nested loop.
They normally have a flip mirror & a guide star.
If the mount error takes the mirror to a certain preset angle - then
a signal is sent to the mount to correct & take the flip mirror back
to a right angle & then the AO process starts again.

The advantage of the AO is that it can correct at a much higher frequency than a mount.
I would see the real advantage is if you get a guide star that allows
for say a 10 Hz capture rate of the guide camera as most
mounts are really only able to correct at about 1 Hz &
even then they would possibly lag in performance by about a second.

If your guide star only allows for a 1 Hz capture rate then I doubt
you would see much difference in performance but I could be wrong.
There are many variables in this e.g.
what about a worm drive that causes a sudden 0.5 second spike?
- your mount wouldn't correct it but the AO would.


cheers
Allan
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Old 23-05-2015, 02:55 PM
jase (Jason)
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Hi Jase,
Funny, I have the Maxim 6 license too, but like you I have not updated the imaging computer with it (it wasn't broke!). I did recently read a thread in the ACP comm center about the v6 multi guide star + ACP setup, eventually they did get it all to work, so I was going to try it, eventually...but I did want to get Scheduler working first.

Even at 3.5m fl some parts of the sky I can see a dozen guide stars with the SX Lodestar, other parts just a few, but almost always more than one. That is with the rotator though chasing the best spot. I have also set my minimum guide exposure length to 1 sec since the tracking on the A200HR is reaaally good and any small corrections faster than that are often just seeing/wind/noise, and likely better ignored. I'd make that even longer but then dithering becomes a pain since after the dither the guider needs to lock on again causing a few more guide cycles. I think ~5sec is longest exposures I'd use for that reason.

With a rotator + >1 sec exposures I think multiple stars is feasible most of the time. Worst case is multi-star turns into single-star.

Best,
EB
Eric, thanks for the detailed response. I think I may bite the bullet and give it a try. I'll do some reading up on it. I think my ACP and Scheduler license is about to expire so had better get cracking. I'll let you know how I get on.

Cheers
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Old 23-05-2015, 03:04 PM
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Bassnut (Fred)
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The elephant in the room, as Allan says, is you need a very bright guide star for AO at 10hz (mag 3?). Simple test, take 1/10th of a second guide exposures on what you think is a normal brightesh guide star and see if you think you can guide on that!.

I dont remember if you have a rotator Paul. A rotator is an absolute necessity with AO (at 10hz) IMO, forget it without one. The chance of getting a sufficiently bright star for 10hz OAG AO with a given object well composed is just about nil without a rotator.
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Old 23-05-2015, 04:55 PM
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Thanks everyone for the interesting points. I am going to go through a bit of a learning curve with getting on of these units, but I have ordered an AOX and it should be here by mid week next week. I am not expecting miracles but a significant improvement might be what I would like to see.


Quote:
Originally Posted by PRejto View Post
Paul,

I assume you have followed this thread: http://www.iceinspace.com.au/forum/s...d.php?t=124461

Yes, different AO system, but the discussion is interesting. From my reading it would seem that amateur AO is not going to help a very good mount when it comes to seeing, unless there is perhaps already excellent seeing. In average seeing it might just make images worse. Anyway, the whole subject seems to invite differing opinions. Perhaps AO adds a degree of complexity without a clear advantage which is why I have so far stayed away even though quite tempted at times.

Good luck with the decision.

Peter
Yes Peter, I read that thread with a lot of interest at the time. Time will tell if it is a good purchase or not, but everyone I know that has one has said you will not know the difference. Even if the AOX is correcting for mount discrepancies only, as small as they are, then I would be happy to have that little bit extra. The pursuit of perfection can lead to trying to find an extra edge.

Quote:
Originally Posted by Bassnut View Post
The elephant in the room, as Allan says, is you need a very bright guide star for AO at 10hz (mag 3?). Simple test, take 1/10th of a second guide exposures on what you think is a normal brightesh guide star and see if you think you can guide on that!.

I dont remember if you have a rotator Paul. A rotator is an absolute necessity with AO (at 10hz) IMO, forget it without one. The chance of getting a sufficiently bright star for 10hz OAG AO with a given object well composed is just about nil without a rotator.
Yep rotator on board Fred. I have found for the last three or so galaxy images I have been imaging at 0.5 sec guide exposures and the stars have been there to image with. The guide camera in the STXL filter wheel is pretty good at picking up stars. Though I do agree with you that a rotator is a must and was part of my thinking all along, not only for composition but also with the notion that guide stars at long focal lengths can be hard to find.
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Old 23-05-2015, 05:20 PM
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Hi Paul,

I don't think anyone can dispute the fact that your images are already extremely good and in my opinion are something of a benchmark.

I agree that even a small potential improvement in image quality is an improvement worth having, but I do not believe that AO would be of any quantifiable benefit to you, primarily because of your mount.

Consumer AO units (whether it be SBIG or SX, at whatever operating frequency) simply cannot compensate for high-order atmospheric inhomogeneities, irrespective of what anecdotal or marketing sources might claim.

The effectiveness of corrections made by a tip/tilt system are limited to the isoplanatic patch occupied by the target guide star. Even at the best sites, this equates to an area of around 2-3 arcseconds at visible wavelengths and 5-6 arcseconds in the near infrared.

This is why AO can cause a degradation in image quality in less-than-ideal seeing conditions -- the tip-tilt adjustments being made for the area of the image local to the guide star are incompatible with the turbulence occurring elsewhere in the field of view. On a small sensor (like a KAF407 as someone mentioned) at long focal lengths, this would not necessarily be the case, especially if the imaging sensor field of view and the guide star were situated within the same isoplanatic patch.

I feel that since you're not trying to counter wind or mechanical imperfections in the mount -- two situations where high-frequency active optics can assist -- you'd be more likely to see improvements by addressing the issue of atmospheric turbulence up-front and guiding in the near-infrared.
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Old 23-05-2015, 11:24 PM
ericwbenson (Eric)
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Originally Posted by Bassnut View Post
The elephant in the room, as Allan says, is you need a very bright guide star for AO at 10hz (mag 3?). Simple test, take 1/10th of a second guide exposures on what you think is a normal brightesh guide star and see if you think you can guide on that!.
Actually the original rule of thumb for the AO-7 was 10/10/10. 10th mag star on a 10 inch scope could correct at 10Hz. And remember this was in 1997-8 with the TC-211 guide chip, hardly a QE barn burner. What a giant leap when the TC-237 came along!

In my notes from 2007, my tests with a C11f/10 + ST8XME guide chip (the TC-237): exp time=0.1 sec, SNR=16 for 10.1 mag star using 3x3 binning and autodark.
So this is close to the 10/10/10 rule, although I would say the rule was a bit optimistic since you would likely be using a guide star with SNR~3. Below what I would consider a robust choice (I set ACP's minimum automatic guide star finder SNR to at least 7). But then again 10th mag guide stars are not falling on to your small guide chip all the time!

As mentioned by Eden and others previously, I think the actual elephant in the room is the isoplanatic path. While I believe it is larger than a few arcseconds (IIRC on the order of arcminutes depending on the type of deformation), even the most serious pro system today cannot overcome it, no matter how many kHz or higher orders are corrected. The input information, the guide star motion/deformation, is not correlated to the main image motion/deformation. The only way around this is an artificial guide star (e.g. a laser), or a bright star as the target.

One thing that always bothered me with the AO implementation: the bump algorithm. When the AO mirror/plate thingy gets past a predetermined point in its range of travel (say 50%), the software activates the normal guider relays to push the star back towards the middle of the guider chip, so that the AO deflection can be reduced below the threshold. Sounds ok at first but...the original SBIG implementation, and the MaxIm one I did, did not bump smartly.

The bump would activate the one relay (of N/S/E/W) that would push the star in the direction closest to back in the middle, and the relay activation time was a configurable, but constant number of seconds, 100 msec the default IIRC? With the AO7 running at high speed this actually worked ok since you would get multiple AO corrections while the guide star was slewing from the mount motors, hence the guide star sorta holding steady on the chip while the deflection percent was being reduced. So you wouldn't notice it too much.

With the somewhat slower AO8/AOL, or with fainter guider stars, where you are stuck at <2Hz update rate, the bump is noticeable. Plus it happened in the same direction for most of the exposure (DEC drift or RA drift is fairly constant for <1hr exposures). Additionally if the drift was at 45 deg from the RA or DEC axis, you would get two bumps in short order.

If you picked a larger bump time, you would get fewer bigger bumps. A small bump time meant lots of little ones as the deflection percentage straddled the threshold. What was better? I never really figured that one out...

So ideally the bump would be in the exact direction and the correct magnitude to get to deflection 0%, and slow enough for the AO to compensate for the motion, call it a drift correction slew. Or to be really smart the AO software would know the bump is happening and correct simultaneously without waiting for guide star exposures - this last one might be too smart and make things worse due to backlash etc but it looks good on paper

Best,
EB
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Old 23-05-2015, 11:35 PM
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Happy to report I just enabled Multistar Guiding in Maxim 6 tonight after reading this thread - all seemed to work fine.

Haven't really had a chance to analyse the results, but the guiding graph was certainly smooth, albeit on a fairly high end mount at only 625mm focal length. I will endeavour to report back after trying it with my 10inch RC at 1800mm.

Eden - how does one guide in the near IR?

DT
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Old 24-05-2015, 03:04 AM
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Happy to report I just enabled Multistar Guiding in Maxim 6 tonight after reading this thread - all seemed to work fine.

Haven't really had a chance to analyse the results, but the guiding graph was certainly smooth, albeit on a fairly high end mount at only 625mm focal length. I will endeavour to report back after trying it with my 10inch RC at 1800mm.

Eden - how does one guide in the near IR?

DT
Hi David,

Near-IR guiding is done using an On-Axis Guider (ONAG). There are a few threads floating about on the forum here which discuss it in detail.

Multi-star guiding looks to be another promising addition to the guiding toolbox. I have been testing a developer build of OpenPHD with multi-star guiding support. It's still a bit raw, but there is a improvement over single star guiding albeit with a couple of caveats.
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