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  #21  
Old 07-08-2014, 08:47 PM
clive milne
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Ray there may be some benefit orienting the off axis guider such that it is pointing to the sky in the upstream direction (of the air flow) relative to the imaging chip. (typically west)


I could elaborate, but I am sure you can understand why this would be preferable.
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  #22  
Old 07-08-2014, 08:59 PM
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I am surprised by these results. I thought the whole point of an AO unit was to help get tight stars in weak seeing. If it doesn't do that and adds some CA then that is pretty pathetic.

Martin Pugh is getting amazing results from an SBIG AOX unit at Sierra Remote. Peter Ward's recent Eagle shows AO benefits even at moderate focal length.

So is this more the SX unit is not very good?

Greg.
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  #23  
Old 07-08-2014, 09:57 PM
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Originally Posted by clive milne View Post
Ray there may be some benefit orienting the off axis guider such that it is pointing to the sky in the upstream direction (of the air flow) relative to the imaging chip. (typically west)


I could elaborate, but I am sure you can understand why this would be preferable.
Hi Clive. Now that is lateral thinking - will try it if I have time.

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Originally Posted by gregbradley View Post
I am surprised by these results. I thought the whole point of an AO unit was to help get tight stars in weak seeing. If it doesn't do that and adds some CA then that is pretty pathetic.

Martin Pugh is getting amazing results from an SBIG AOX unit at Sierra Remote. Peter Ward's recent Eagle shows AO benefits even at moderate focal length.

So is this more the SX unit is not very good?

Greg.
Hi Greg.

all single element refractive AO devices add CA. However, it appears that it is not a major problem even at f4 and with the relatively thick plate of the SX, so it is not an issue for your system.

As far as I can tell, the main benefit of an AO is to give even better results in good seeing by tidying up residual mount errors - surely that is not too "pathetic". Rick hit the nail on the head - AO helps most when you are pushing against the mount limits (eg the seeing is <1.5arcsec and you are imaging at much less than an arcsec/pixel).

The penalty of sampling the guide signal from a star that is a long way from the image means that the total seeing error increases. All amateur AO systems do this - even the SBIG paper referenced earlier shows that to be the case. So current AO does not really fix poor seeing - because the geometry is wrong and the update rates are too slow. "Fixes the seeing" still seems to be used for some marketing purposes though. At least SX only claim "Overcomes rapid gear errors to stabilise even difficult mounts".

AO can tidy up mount errors due to lower build quality or wind induced motion, even in poor seeing - lots of evidence that it can provide a nett gain, even with the inherent reduction in resolution.

I would certainly not say that the SX AO is "not very good" - it works as expected. I have no idea how it compares to the SBIG alternative.

As an aside, the errors introduced by using a single off axis guide star are apparently well understood in SBIG. According to the paper referenced by Rick, they are working on a way to use multiple guide stars and an on-axis guide scope, which will get around the problem of decreased resolution - but I don't think they are there yet. If they can pull it off, they probably won't be able to improve resolution by much in poor seeing, but at least they will not make it worse.

Last edited by Shiraz; 07-08-2014 at 10:55 PM.
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  #24  
Old 08-08-2014, 01:55 AM
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I remain puzzled as to why you are seeing better performance with the SX AO turned off, as opposed to it being active. My intention was not to bag SX, but your data does not reflect the results Alan Holmes has published, nor my own qualitative results or that seen by many other (SBIG) AO users.

Alan's data shows a clear improvement & correlation, in both Dec and RA that slowly decreases with angular distance from the guide star ( if only mount errors were being corrected, that would not expect a Dec correlation).

I think it incorrect to say, the physics predicts a first order AO will make things worse.

Quite the contrary.

If the atmosphere's isoplanic patches on the night are at least as large as the telescope aperture, tip-tilt or first-order corrections, provided they are fast enough, will be beneficial....this is exactly how professional systems work prior to applying complex wavefront corrections for higher order components via deformable optics.

On nights when higher order turbulence dominates the image, sure, I'd expect zero improvement...but imaging during tragic seeing, regardless of how good your optics/mount/etc. are, is a futile exercise IMHO.

Suggesting residual mount errors/wind buffeting is all that is being corrected by the AO doesn't offer much insight ( with a system similar to mine )as my mount's PE is around 1-2 arc sec and, being in a dome, there is zero buffeting. Begging the question: if not isoplanic waves, what is being corrected ? That said, I am now curious.

It will be interesting to gather some data, with and without high-speed first-order corrections ( in my case being applied by an AO-8 and AO-X) and similarly quantify the results. I'll happily upload the results when done.
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  #25  
Old 08-08-2014, 05:56 AM
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I would be interested in that Peter. There are really only 2 AO systems on the market SBIG and SX. I was always somewhat suspicious of the SX unit seeing as the company is more of a middle ground type product maker, but willing to have it be otherwise.

I have often noticed Leo from Italy's images are very sharp with tiny stars and he uses an SX AO unit but on a GSO RC with relatively long focal length.

The SBIG units have many great examples of tight images.

By the way the term is Isoplanetic (also spelt Isoplanatic) Patch: which means the angular section of atmosphere where electromagnetic waves are parallel.

I think Ray referred to it as Isokinetic Patch which is another term for the same thing.
Greg.

Last edited by gregbradley; 08-08-2014 at 06:06 AM.
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  #26  
Old 08-08-2014, 09:47 AM
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Originally Posted by gregbradley View Post
By the way the term is Isoplanetic (also spelt Isoplanatic) Patch: which means the angular section of atmosphere where electromagnetic waves are parallel.

I think Ray referred to it as Isokinetic Patch which is another term for the same thing.
Greg.
Isoplanatic

quite correct! I typed that while offline and my ipad autocorrect didn't like either
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  #27  
Old 08-08-2014, 12:21 PM
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Originally Posted by Peter Ward View Post
I remain puzzled as to why you are seeing better performance with the SX AO turned off, as opposed to it being active. My intention was not to bag SX, but your data does not reflect the results Alan Holmes has published, nor my own qualitative results or that seen by many other (SBIG) AO users.

Alan's data shows a clear improvement & correlation, in both Dec and RA that slowly decreases with angular distance from the guide star ( if only mount errors were being corrected, that would not expect a Dec correlation).

I think it incorrect to say, the physics predicts a first order AO will make things worse.

Quite the contrary.

If the atmosphere's isoplanic patches on the night are at least as large as the telescope aperture, tip-tilt or first-order corrections, provided they are fast enough, will be beneficial....this is exactly how professional systems work prior to applying complex wavefront corrections for higher order components via deformable optics.

On nights when higher order turbulence dominates the image, sure, I'd expect zero improvement...but imaging during tragic seeing, regardless of how good your optics/mount/etc. are, is a futile exercise IMHO.

Suggesting residual mount errors/wind buffeting is all that is being corrected by the AO doesn't offer much insight ( with a system similar to mine )as my mount's PE is around 1-2 arc sec and, being in a dome, there is zero buffeting. Begging the question: if not isoplanic waves, what is being corrected ? That said, I am now curious.

It will be interesting to gather some data, with and without high-speed first-order corrections ( in my case being applied by an AO-8 and AO-X) and similarly quantify the results. I'll happily upload the results when done.
Maybe Alan's own words can convince you: "Figure Six shows the star image that would result if the errors were only due to stellar motion. If I didn’t guide at all, but had a perfect drive, I would obtain the result given by the "Slow Guide curve. The higher the peak value, the better the star image. With Two Star guiding, the result actually got a little worse. This is what I mentioned might happen before – the guide system is chasing the seeing, but one frame behind. " ie Alan's published results match very well with what I observed - the star images are "a little worse" with an AO than without. And they were taken in similar conditions of: "These measurements were made on a night with only a slight wind." Alan then goes on show how reducing the framerate can actually improve performance and then to explain how much better multi-star guiding would be, but that discussion has nothing to do with current systems.

As for mount errors, Alan's analysis method specifically excludes them (he uses differential measures). I excluded them by measuring on a windless night and with short image exposures (10sec) on a pretty good mount. And we came up with the same answer - AO (of any brand) is quite capable of reducing resolution. If you accept this, then the next question is "well why do they help at all?" - what you get probably depends on the spectrum of the seeing error, but I suspect that an AO can tidy up a myriad of mount wobbles, producing a nett benefit only when the improvement from the tidy up is more significant than the increase in seeing noise (ie the seeing must be good to start with). This is pure conjecture though and the complete answer is probably nothing like that simple.

Agree that pro systems must use tip-tilt in conjunction with deformable optics, but I am pretty sure that they would never even dream of trying it with a guide star outside of the isokinetic patch and at bandwidths of 10Hz.


In any case, would really look forward to seeing some hi res FWHM measurements . I also hope to get some more measurements soon in conditions that are more suited to showing that improvements are possible with AO - not just degradation. In the end, a theory is only of value if it is backed up by measurements - however, if we can get a clearer understanding of what exactly is going on, it may be possible to push the limits a bit harder. In particular, it may be possible to choose an optimum update rate, since faster is not always better.

On isokinetic and isoplanatic patch, I think that they are generally regarded as being slightly different: the isoplanatic patch is that region over which all turbulent effects are correlated. The isokinetic patch is that over which only the low order tip-tilt stuff is correlated - it is bigger than the isoplanatic patch.

Ref: https://www.sbig.com/about-us/blog/differential/

Last edited by Shiraz; 10-08-2014 at 01:30 AM.
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  #28  
Old 13-08-2014, 07:14 PM
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Last night was windy with over 20knots of gusting sea breeze. Set up the AO system (SX AO on f4/200 Newtonian) in the open on an EQ6, a situation that would normally rule out any imaging due to the uncontrolled wobbling of the mount with wind loading.

In the worst conditions of the night, neither AO nor unguided imaging was possible – there was just too much movement. However, after a few hours, the wind abated slightly and the AO could manage the jitter - results at:
http://www.astrobin.com/full/113137/0/?real=&mod=
The top row of 10 heavily cropped sub images shows what happened unguided in these conditions – I would only class one of the subs as usable. The lower row of subs shows how well the AO corrected mount movement – over half of the subs were usable. The AO did a good job, since the better quality subs had FWHM of around 2.5 arc sec and reasonable star roundness – I would regard these as high enough quality to be useful for either colour or luminance. Very bright stars showed some smearing caused by uncontrolled motion (before the AO reacted), but these images could still be usable with median stacking.

As an additional test, I tried everything with the RCC1 coma corrector in place – there is just enough back focus to fit the AO, a thin OAG and a thin filter wheel. There did not appear to be any hidden "gotcha"s.

Although the scope is an f4 with 800mm fl, it is being used for high resolution imaging – the small pixels yield a system with sampling of 0.93 arcsec, which is about equivalent to a 2m fl scope with 9micron pixels. From this perspective, the AO did a good job in windy conditions and the results validate the suggestion in the earlier diagram that an AO can be useful in wind. The other suggestions that AO works well with a lower capability mount and also in very good seeing, and that it is not worthwhile in poor seeing, are all borne out by the observations of other users.

And finally, thanks again to Poita(Peter) for letting me use the AO.
Thanks for reading. Regards Ray

Last edited by Shiraz; 14-08-2014 at 12:55 AM.
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  #29  
Old 13-08-2014, 09:40 PM
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hi Ray,

Interesting results, I have the older Orion Steadystar AO where the OAG prism can actually be tilted to point to a different area in the FOV, would this help at all if tilted to point toward the centre?

Secondly, do you think using the lodestar in 2x2 binning had any effect on your results? the lodestar has fairly large pixels at 8.2micron. when you bin, don't you lower the resolution of the guiding component and would this affect centroid calculations especially if it is a bright star?

I had a brief chance to test the AO on my 10inchF4 with the stf8300 and it showed very promising results, this was with an NEQ6pro, and not very windy conditions. I normally use an OAG.
did you do any tests with 1x1 bin of the lodestar?

I got a lodestar X2 and had no issues with getting a guidestar at 100ms from a very LP backyard.
cloudy as hell, but I'll post some results as well when it clears up.

Looks like the benefits are more evident in a system with a higher resolution, typically less than 0.7 arcsec/pixel.

Cheers
Alistair
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  #30  
Old 14-08-2014, 08:48 AM
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hi Ray,

Interesting results, I have the older Orion Steadystar AO where the OAG prism can actually be tilted to point to a different area in the FOV, would this help at all if tilted to point toward the centre?

Secondly, do you think using the lodestar in 2x2 binning had any effect on your results? the lodestar has fairly large pixels at 8.2micron. when you bin, don't you lower the resolution of the guiding component and would this affect centroid calculations especially if it is a bright star?

I had a brief chance to test the AO on my 10inchF4 with the stf8300 and it showed very promising results, this was with an NEQ6pro, and not very windy conditions. I normally use an OAG.
did you do any tests with 1x1 bin of the lodestar?

I got a lodestar X2 and had no issues with getting a guidestar at 100ms from a very LP backyard.
cloudy as hell, but I'll post some results as well when it clears up.

Looks like the benefits are more evident in a system with a higher resolution, typically less than 0.7 arcsec/pixel.

Cheers
Alistair
Hi Alistair.

Don't know if angling the prism will move the aimpoint much - have you tried it?

I tried the Lodestar at 1x1 and 2x2. The only perceptible difference was that the update was slower at 1x1, so I used 2x2 (as recommended by SX) for all testing. The centroid finder in phd is apparently able to resolve to much better than a pixel (below 1/10 pixel with good SNR). I used stars with >30 SNR, so expect that the centroids were accurate to a small fraction of an arcsec.

My impression is that AO has most benefit when using a mid quality mount (eg EQ6) with a fairly heavy load under most seeing conditions - and when using a high quality system under very good seeing conditions. Agree that pixel scale is important - there is probably not much point in improving the guiding by 1/2 an arcsec if you are imaging with a pixel scale of say 2 arcsec - you won't be able to see the difference.

What sort of improvement did you see when using AO on your system - better resolution or rounder stars?

Regards Ray

Last edited by Shiraz; 14-08-2014 at 09:09 AM.
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  #31  
Old 14-08-2014, 11:03 AM
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Originally Posted by gregbradley View Post
I am surprised by these results. I thought the whole point of an AO unit was to help get tight stars in weak seeing. If it doesn't do that and adds some CA then that is pretty pathetic.

Martin Pugh is getting amazing results from an SBIG AOX unit at Sierra Remote. Peter Ward's recent Eagle shows AO benefits even at moderate focal length.

So is this more the SX unit is not very good?

Greg.
I don't think any of the Active Optics systems (SBIG or SX) out there truly correct for poor seeing, true Adaptive Optics systems that deform the mirror to cope with poor seeing do a great job, but we won't be seeing those in amatuer astronomy any time soon.

What they can do is correct way faster, with better accuracy and with less momentum shift than mount-motor corrections. This makes them very useful for slightly windy conditions when you couldn't otherwise image, and for where your mount is at the limits of its capability. Both situations are common in our hobby, and AO gives a real benefit.

With crappy seeing however, I just go back inside and catch up on IIS

AO works best when seeing is good, and when seeing is good, that is when you really want to be imaging, so it is a win-win to me.
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  #32  
Old 14-08-2014, 01:04 PM
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Interesting thread - thanks Ray. I won't dispute your findings because I've never done any scientific-like testing!! However, I have used an AO-L quite a bit (only at F11.7 on my Tak) and never experienced a degradation unless the AO guide parameters weren't correctly tuned (aggressiveness and, to a lesser extent, slew rate).

I set aggressiveness depending on guide star brightness (guide frequency) and seeing. The higher the guide frequency, the lower the aggressiveness and I moderate this based on seeing. The point being I never try to correct any one guide error with a single correction because that likely means you're chasing seeing (regardless of how good that might be). The objective is to minimize "wander". Wander is a measurement that indicates the amount of position error that has not been corrected (and it can never consistently be zero). Using TheSkyX and an AO-L it's possible to monitor Wander and adjust the AO settings to ensure wander is minimised and also diminished from the values observed without the AO switched on. The intent is to ensure there is at least some benefit to switching the AO on.

Slew rate just limits the maximum correction that the AO is allowed to apply (arcsecs/sec). The maximum correction is the slew rate divided by the guide frequency (Hz). Ie a setting of 10 at 10Hz limits any correction to less than 1 arcsec. This will help avoid over correction when the seeing is bad.

In your case, I'd be interested in seeing your guide error graphs with and without the AO.

Cheers, Marcus
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  #33  
Old 16-08-2014, 11:10 AM
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Interesting thread - thanks Ray. I won't dispute your findings because I've never done any scientific-like testing!! However, I have used an AO-L quite a bit (only at F11.7 on my Tak) and never experienced a degradation unless the AO guide parameters weren't correctly tuned (aggressiveness and, to a lesser extent, slew rate).

I set aggressiveness depending on guide star brightness (guide frequency) and seeing. The higher the guide frequency, the lower the aggressiveness and I moderate this based on seeing. The point being I never try to correct any one guide error with a single correction because that likely means you're chasing seeing (regardless of how good that might be). The objective is to minimize "wander". Wander is a measurement that indicates the amount of position error that has not been corrected (and it can never consistently be zero). Using TheSkyX and an AO-L it's possible to monitor Wander and adjust the AO settings to ensure wander is minimised and also diminished from the values observed without the AO switched on. The intent is to ensure there is at least some benefit to switching the AO on.

Slew rate just limits the maximum correction that the AO is allowed to apply (arcsecs/sec). The maximum correction is the slew rate divided by the guide frequency (Hz). Ie a setting of 10 at 10Hz limits any correction to less than 1 arcsec. This will help avoid over correction when the seeing is bad.

In your case, I'd be interested in seeing your guide error graphs with and without the AO.

Cheers, Marcus
Thanks Marcus - great post and a breath of fresh air.

I had been concerned that AO is being sold as the panacea that can "fix the seeing" - something it certainly does on pro systems - but I could not see how it could do that in the way we use it.

Amateur AO seems to be a bit like putting a generic shock absorber on your car - it may help a bit but it can also be worse than nothing. My test in calm seeing was with default settings for the AO and was simply to show that AO certainly can make things worse in some circumstances by introducing uncorrelated seeing noise into the tracking loop. It was never intended as a blanket bucketing of AO, although it seems to have been interpreted that way.

You provide a sound strategy for optimising AO parameters to get a balance between fixing the slow (mount?) errors without introducing excessive seeing noise. My attempts to optimise the AO were not as systematic, being limited to trying different update rates and setting the minimum error at which correction was attempted. I did not keep the error logs - did not realise that they would interest anyone.

Your strategy seems to suggest that you view seeing as the enemy and you attempt to keep it from doing too much damage while still getting some guiding gain. That raises the question of what the AO may actually be correcting for, if it isn't the seeing. Do you have any thoughts on that? Could it just be mount wobbles or could it be dome seeing or tube current effects?

regards ray

Last edited by Shiraz; 16-08-2014 at 01:28 PM.
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  #34  
Old 16-08-2014, 12:05 PM
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I don't think any of the Active Optics systems (SBIG or SX) out there truly correct for poor seeing, true Adaptive Optics systems that deform the mirror to cope with poor seeing do a great job, but we won't be seeing those in amatuer astronomy any time soon.

What they can do is correct way faster, with better accuracy and with less momentum shift than mount-motor corrections. This makes them very useful for slightly windy conditions when you couldn't otherwise image, and for where your mount is at the limits of its capability. Both situations are common in our hobby, and AO gives a real benefit.

With crappy seeing however, I just go back inside and catch up on IIS

AO works best when seeing is good, and when seeing is good, that is when you really want to be imaging, so it is a win-win to me.
Good summary Peter, should have just asked you in the first place
regards Ray

Last edited by Shiraz; 16-08-2014 at 01:29 PM.
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  #35  
Old 17-08-2014, 08:55 PM
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...
Your strategy seems to suggest that you view seeing as the enemy and you attempt to keep it from doing too much damage while still getting some guiding gain. That raises the question of what the AO may actually be correcting for, if it isn't the seeing. Do you have any thoughts on that? Could it just be mount wobbles or could it be dome seeing or tube current effects?

regards ray
Well, I'm an empiricist . I know (because I've watched it) that seeing contains both high (>10Hz) and low frequency (around 1Hz and longer?) components.

If I have a bright guide star that allows 10Hz corrections then a properly tuned & calibrated AO unit will reduce the amplitude of seeing induced guide errors of less than 10Hz. I know it won't remove the errors (especially the near 10Hz components) because the guiding is reactive, not predictive, but it has to attenuate them. You can monitor this to some extent by watching wander and guide error graphs. The limitation with this monitoring is the sampling frequency of these graphs.

If I can only get 1Hz corrections (a fainter guide star) then at least I know that the low frequency seeing components will be attenuated and I don't have to rely on mount corrections for any lower frequency wander. There's no way an ME can correct faster than about 0.5Hz anyway and even that is pushing it with a heavy scope and probably meaningless from an imaging perspective since most seeing is much faster than this.

The only way to verify an AO's performance (which I don't feel compelled to do because I've seen it working ) is to compare residual error graphs for multiple runs with and without 10hz guiding. If a guide error graph has increased amplitude after turning on an AO then it's likely the AO was not properly tuned.

Cheers, Marcus

PS: Naturally, seeing is not just about guide star displacement (ie residual error or wander). Bad seeing is usually accompanied by "boiling" in which case, I always switch off, pack up & go to bed! AO will only reduce wander.

PPS: I make no claim about the source of the wander I'm trying to minimise - there's no way for me to know that - although at frequencies greater than 1Hz it's unlikely to be from the PME.

Last edited by marc4darkskies; 17-08-2014 at 10:35 PM.
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  #36  
Old 20-08-2014, 04:39 PM
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Now I am wondering even more if this is really the difference between a Starlight Express AO unit and an SBIG one. From what little I gather about the SX unit it seems 10hz corrections are unlikely although they do have the new Lodestar which is 2X more sensitive than the old one which was about the most sensitive guide camera around.

Greg.
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Old 20-08-2014, 04:49 PM
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Originally Posted by gregbradley View Post
Now I am wondering even more if this is really the difference between a Starlight Express AO unit and an SBIG one. From what little I gather about the SX unit it seems 10hz corrections are unlikely although they do have the new Lodestar which is 2X more sensitive than the old one which was about the most sensitive guide camera around.

Greg.
I have the Lodestar X2 and its a lot more sensitive than the lodestar which I had previously.
with the X2 and my Orion AO, I get atleast 5 stars with good SNR almost anywhere in the sky with 100 to 250ms exposures. with bin 2x2, SNR improves. but its very clean and hassle free.

Here's a quick 3min test of my Orion AO, at 3Hz. Stars are elongated due to bad collimation.
https://www.dropbox.com/s/6pyiuhiq4u...-fnl1.png?dl=0
and this was without a single mount correction.

my OTA has a lot of flexure so the AO certainly helped reducing the size of the stars and dropped my fwhm, primary benefit that I can see are improvements of an average mount and bad OTA.
Hoping to do some more tests and exposures this week.

the Orion has a Maxim plugin so everything is done from within Maxim which helps a lot.

Cheers
Alistair
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  #38  
Old 20-08-2014, 06:31 PM
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Originally Posted by gregbradley View Post
Now I am wondering even more if this is really the difference between a Starlight Express AO unit and an SBIG one. From what little I gather about the SX unit it seems 10hz corrections are unlikely although they do have the new Lodestar which is 2X more sensitive than the old one which was about the most sensitive guide camera around.

Greg.
Hi Greg.

I only did two tests of the SX AO, so I have no basis at all to compare it with an SBIG one. However, it worked well and behaved exactly as expected under two conditions: it made the stars slightly worse in short exposures, on a good mount in still conditions: it did a good job of cleaning up stars in bad wind. Others have found that AO is generally worth having in more usual conditions, particularly with lesser quality mounts, so I did not test under those conditions.

I used the SX with manufacturer-recommended settings (with minor tweaks). Marcus has pointed out that some software provides mechanisms for optimising AO performance to match the mount and conditions, but I did not have any such software. It may well be that the SX AO could have performed better with alternative software.

I ran the Lodestar at 0.1 sec exposure, but the actual update rate (of any AO) was less than 10Hz since it depends on a wide range of factors:
guide exposure +
USB download+
centroid and offset calculation+
serial comms to AO+
step AO to demanded position (variable depending on offset)+
serial handshake to PC+
begin next guide exposure

Plus there is unpredictable Win8 scheduling before it does anything new.



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Originally Posted by marc4darkskies View Post
Well, I'm an empiricist . I know (because I've watched it) that seeing contains both high (>10Hz) and low frequency (around 1Hz and longer?) components.

If I have a bright guide star that allows 10Hz corrections then a properly tuned & calibrated AO unit will reduce the amplitude of seeing induced guide errors of less than 10Hz. I know it won't remove the errors (especially the near 10Hz components) because the guiding is reactive, not predictive, but it has to attenuate them. You can monitor this to some extent by watching wander and guide error graphs. The limitation with this monitoring is the sampling frequency of these graphs.

If I can only get 1Hz corrections (a fainter guide star) then at least I know that the low frequency seeing components will be attenuated and I don't have to rely on mount corrections for any lower frequency wander. There's no way an ME can correct faster than about 0.5Hz anyway and even that is pushing it with a heavy scope and probably meaningless from an imaging perspective since most seeing is much faster than this.

The only way to verify an AO's performance (which I don't feel compelled to do because I've seen it working ) is to compare residual error graphs for multiple runs with and without 10hz guiding. If a guide error graph has increased amplitude after turning on an AO then it's likely the AO was not properly tuned.

Cheers, Marcus

PS: Naturally, seeing is not just about guide star displacement (ie residual error or wander). Bad seeing is usually accompanied by "boiling" in which case, I always switch off, pack up & go to bed! AO will only reduce wander.

PPS: I make no claim about the source of the wander I'm trying to minimise - there's no way for me to know that - although at frequencies greater than 1Hz it's unlikely to be from the PME.
Thanks Marcus. Your empirical approach of "use it because it works" is eminently sensible. However, I have a different approach - I want to know what it is actually doing, not for academic interest, but because that may give some insight into how to improve the system in other ways, as well as (or even in place of) AO. Amateurs use AO in ways that appear to be in conflict with the way that the pro's use them - I want to know why they work as well as they seem to and where the limits are.

Quote:
Originally Posted by alistairsam View Post
I have the Lodestar X2 and its a lot more sensitive than the lodestar which I had previously.
with the X2 and my Orion AO, I get atleast 5 stars with good SNR almost anywhere in the sky with 100 to 250ms exposures. with bin 2x2, SNR improves. but its very clean and hassle free.

Here's a quick 3min test of my Orion AO, at 3Hz. Stars are elongated due to bad collimation.
https://www.dropbox.com/s/6pyiuhiq4u...-fnl1.png?dl=0
and this was without a single mount correction.

my OTA has a lot of flexure so the AO certainly helped reducing the size of the stars and dropped my fwhm, primary benefit that I can see are improvements of an average mount and bad OTA.
Hoping to do some more tests and exposures this week.

the Orion has a Maxim plugin so everything is done from within Maxim which helps a lot. Maxim appears to give you a fair bit of help in optimising the AO - what procedure do you use?

Cheers
Alistair
Hi Alistair. Good results and look forward to seeing more. Looks like AO is definitely worth doing with your system and conditions. Looks like Maxim helps with optimising the AO - what procedure do you use?

Regards Ray

Last edited by Shiraz; 20-08-2014 at 06:54 PM.
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Old 20-08-2014, 08:49 PM
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bert (Brett)
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Just for the record I have used an Sbig ao @32hz on a core 2 duo machine... Which is hardly a powerhouse.

Currently I'm using an aol with an onag xt at f11, guided only in ir. That seems to work pretty well. I have read anecdotal evidence that guiding in ir may have some benefits... But still tuning.
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Old 20-08-2014, 09:59 PM
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Quote:
Originally Posted by Shiraz View Post
Thanks Marcus. Your empirical approach of "use it because it works" is eminently sensible. However, I have a different approach - I want to know what it is actually doing, not for academic interest, but because that may give some insight into how to improve the system in other ways, as well as (or even in place of) AO.
I actually don't understand what you mean Ray. I'm correcting two dimensional guide star position error by tip/tilting a piece of glass in two dimensions - that's what it's doing. I'm not trying to be flippant here. It's simple feedback control, essentially, to cancel position noise. There is no point trying to understand anything else IMO and certainly no "need" to understand the physics of seeing - that's for the pros who are deforming huge mirrors to compensate for wavefront error. If you accept seeing can cause say low frequency (1Hz) to high frequency (>10Hz) guide star position errors then cancelling out some of that error with simple feedback control will be increasingly effective as the tip/tilt frequency increases.

Quote:
Originally Posted by Shiraz View Post
... Amateurs use AO in ways that appear to be in conflict with the way that the pro's use them - I want to know why they work as well as they seem to and where the limits are.
Pros have large deformable segmented mirrors and while I think they do use tip/tilt on mirror segments (??) for coarse position error corrections, the really cool AO happens because they can analyse the wave front distortions (generally using laser generated reference stars) and correct for them with very clever computation and fast actuators. Pros may also be doing feed forward control.

Amateur AO is limited to correcting position errors only and then only at a relatively low frequency. The effectiveness is completely dependent on the frequency of correction (guide star brightness) and tuning. The main assumption is that the guide star is experiencing approximately the same positional abberations as the rest of the field - a pretty safe assumption given the apertures we use.

Cheers, Marcus
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