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  #1  
Old 09-01-2012, 10:24 PM
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RickS (Rick)
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Active Optics vs Guiding - an experiment

I recently purchased a Starlight Xpress AO-LF active optics unit and thought it would be interesting to see how the results of using it compared to auto-guiding. So, I took advantage of clear(ish) brightly moonlit skies last night to do an experiment...

Equipment:
  • Mount: Astro-Physics AP900
  • Scope: GSO RC10, FL 1984mm (according to Pinpoint plate solve)
  • Imaging train: Atlas focuser, SX AO-LF, SX Filterwheel, OAG/Lodestar, SXVR-H18 camera (0.56 arcsec/pixel image scale)
The scope was in an exposed position and there were strong gusty winds, so I was unable to make long guided exposures without wind gusts ruining every one of the subs. I ended up using 60 second exposures.

The experiment: I ran a series of 20 auto-guided and 20 AO-guided Luminance subs, each 60 seconds long. I actually interleaved groups of 5 subs alternately auto-guiding and AO-guiding to try to keep conditions approximately the same for both in case variations in temperature or seeing affected the results. Auto-guiding was done using 5 seconds subs and AO-guiding was done at a 4Hz rate (approximately).

The results:
  • the big difference was how well wind gusts were handled. Of the 20 auto-guided subs, 4 were badly ruined by wind. Only one of the 20 AO-guided subs was noticeably affected (in that case the wind gust was strong enough to knock the guide star out of the tracking box completely).
  • AO-guiding produced consistently rounder stars. I threw out the 4 bad auto-guiding subs and the 1 bad AO-guiding sub and then measured the remaining ones with CCDInspector. The average aspect ratio for the auto-guided subs was 13% (standard deviation of ~4.6) compared to 6.5% (standard deviation ~1.72) for the AO-guided subs (lower percentages are better). After integrating the auto-guided and AO-guided subs separately I also did a DynamicPSF analysis in PI which gave a roundness of 0.926 for the auto-guided image and 0.952 for the AO-guided image.
  • Auto-guiding produced stars with a FWHM a few percent smaller than AO-guiding but the variation in FWHM between subs was bigger with auto-guiding. Once again considering the good quality subs only, CCDInspector gave an average FWHM of 2.22 arcsec/pixel (standard deviation ~0.115) for auto-guiding compared to 2.34 (standard deviation ~0.084) for AO-guiding. The FWHM for the integrated images was 2.29 for auto-guiding and 2.37 for AO-guiding which is a little closer.
  • visual inspection of the integrated subs showed that the AO-guided image contains some very dim stars that the auto-guided image does not, but the effect was very subtle.
I don't understand why auto-guiding produced better FWHMs than AO-guiding. It certainly wasn't what I expected. I will probably do some further experiments on a still night when I can make much longer guided subs to see if I get the same result.

I was using a very good mount and had good polar alignment. I expect that AO-guiding would show greater benefits on a poorer mount!

I did cheat a little bit and didn't remove the AO unit from the imaging train to do the auto-guided subs - it was still present but not operational. The extra optical element may have had some effect on the auto-guided subs, but I don't think it would have been significant.

Overall, I am very pleased with the performance of the AO unit. Last night after running these tests I was able to take 15 minute Ha subs despite the wind. This would have been completely impossible with auto-guiding. If only the AO unit was also able to deal with the clouds that appeared around 11pm...

Cheers,
Rick.
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  #2  
Old 09-01-2012, 10:27 PM
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Fascinating experiment Rick. Thanks for writing up so well.
Any chance of gawking on some example pics?
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Old 09-01-2012, 10:32 PM
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Thanks, Rob. I'll put a couple of images up tomorrow. As I said, the differences are pretty subtle. Hopefully, they would be more noticeable with longer exposures and more total exposure time... or a crappier mount
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Old 10-01-2012, 12:43 AM
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Quote:
Originally Posted by RickS View Post
If only the AO unit was also able to deal with the clouds that appeared around 11pm...

Cheers,
Rick.
For that u'll need the laser addon unit Z1A2P-EM, i'm sure that will be developed soon
Interesting read, thanks for that.

Marcus.
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  #5  
Old 10-01-2012, 01:42 AM
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Quote:
Originally Posted by RickS View Post
Thanks, Rob. I'll put a couple of images up tomorrow. As I said, the differences are pretty subtle. Hopefully, they would be more noticeable with longer exposures and more total exposure time... or a crappier mount
Easily fixed, I'll swap you for my EQ6
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Old 10-01-2012, 07:22 AM
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Great writeup Rick.

There is a lack of comparison data on the net between AO and non AO performance.

Thanks.

Greg.
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Old 10-01-2012, 08:25 AM
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Originally Posted by Mighty_oz View Post
For that u'll need the laser addon unit Z1A2P-EM, i'm sure that will be developed soon
Interesting read, thanks for that.
Thanks, Marcus. I'm first on the list for the laser addon!

Quote:
Originally Posted by Poita View Post
Easily fixed, I'll swap you for my EQ6
No thanks, Peter

Quote:
Originally Posted by gregbradley View Post
Great writeup Rick.

There is a lack of comparison data on the net between AO and non AO performance.
Thanks, Greg. Yes, it's hard to find info on AO performance at all apart from the odd anecdotal comment. There don't seem to be that many people using them, although there are a few producing excellent results like Leonardo Orazi.

I asked about the increase in FWHM on the Starlight Xpress group and got a response that suggested that it was due to chasing poor seeing. That's certainly plausible, but I wanted to be correcting at a fast rate to deal with the wind. I will have to play with the parameters and learn how best to make use of the AO unit. It may require different strategies for different conditions. No surprise there!

Cheers,
Rick.
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Old 10-01-2012, 09:00 AM
cfranks (Charles)
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Excellent write up Rick and very timely as I am about to start the process of learning to use my AO-LF. I assume your AO-LF has the Lodestar attached whereas my Lodestar is attached to a QSI 683wsg and the AO doesn't have the Guider attachment.
What is your Imaging/Guiding software?

Charles
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Old 10-01-2012, 09:29 AM
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Rick,

Very interesting - thanks for sharing.

Quote:
I don't understand why auto-guiding produced better FWHMs than AO-guiding. It certainly wasn't what I expected.
The sign of a good experiment!

James
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Old 10-01-2012, 09:39 AM
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I looked around the net some time ago for AO write ups and found it to be a bit thin too. I know that Martin Pugh was using one on his gear and the results were very good. I have considered getting one for the RC at some point myself and will be watching this thread keenly for anything you might come up with Ric.
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  #11  
Old 10-01-2012, 10:10 AM
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Quote:
Originally Posted by cfranks View Post
Excellent write up Rick and very timely as I am about to start the process of learning to use my AO-LF. I assume your AO-LF has the Lodestar attached whereas my Lodestar is attached to a QSI 683wsg and the AO doesn't have the Guider attachment.
What is your Imaging/Guiding software?

Charles
Charles,

I'm using the OAG which is part of the filterwheel/camera assembly rather than the one that came with the AO-LF. I think your configuration should work fine. I'm using MaximDL for guiding and image acquisition.

If you have problems communicating with the AO-LF over the serial connection try plugging in to the guide port instead. On my unit the labels were the wrong way around. I was pulling out my hair for a couple of hours until I tried that as a last resort!

Cheers,
Rick.
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Old 10-01-2012, 10:11 PM
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I asked about the increase in FWHM on the Starlight Xpress group and got a response that suggested that it was due to chasing poor seeing. That's certainly plausible, but I wanted to be correcting at a fast rate to deal with the wind. I will have to play with the parameters and learn how best to make use of the AO unit. It may require different strategies for different conditions. No surprise there!

Cheers,
Rick.[/QUOTE]

That doesn't make sense as the whole point of an AO unit is to overcome the effects of the seeing.

Perhaps it is more how many hertz you can get the thing running at.

For example I can get an ST402ME or a Lodestar or a Starfish autoguider running at .5 second exposure times. But by the time the download is done and the corrections are done it is more like 1 second or a tad more.

So the AO would have to beat that lag. I thought that was the whole theory of how they work. The mirror can shift faster than a correction in the mount motors can be executed.

So the whole game would be to try to achieve the highest hertz (number of times per second) you can and that would mean getting the brightest guide star.

Perhaps the ST402 might be better for it as it is more sensitive and cleaner and therefore able to produce brighter guide stars that can be guided off.

One thing I am not sure of - I assume the hertz means the guide camera takes that many exposures per second? Like 3 hertz means 3 camera exposures per second. That then would requite a fast guide camera and fast download time.

Perhaps an SBIG STi may be better as it is a CCD plus it has a reducer lens built in lowering the F ratio making brigher stars.

Greg.
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Old 10-01-2012, 10:13 PM
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Quote:
Originally Posted by RobF View Post
Any chance of gawking on some example pics?
Here's a couple, Rob. They are small crops of IC2220, very lightly processed (DBE, slight MMT, and stretch). Don't expect too much from fewer than 20 x 1 minute exposures at f/8 The first was AO-guided and the second was auto-guided.

I hope it's not my imagination, but I reckon the loop of nebulosity at 10 o'clock is better defined in the AO version and that most of the dimmer stars look slightly better defined in the AO version too.

Cheers,
Rick.
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Old 10-01-2012, 10:23 PM
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Thanks Rick. See what you mean about FHWM and fainter stars. Certainly seems to be lots of interest in your AO exploits.

Actually, I'm drooling over your stars in both images....
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Old 10-01-2012, 10:30 PM
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Thanks Rick. See what you mean about FHWM and fainter stars. Certainly seems to be lots of interest in your AO exploits.

Actually, I'm drooling over your stars in both images....
My next experiment is to determine the effect of drool on FWHM
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Old 10-01-2012, 10:39 PM
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Dear Greg
My AO unit can sample up to about 8hz. It does this by only downloading a small box around the guide star- about 20 pixels square. The problem is getting a bright enough star to sample at that rate. I find that even 1/2sec exposures give very good results and this gives many more choices for guide stars
Quote:
Originally Posted by gregbradley View Post
I asked about the increase in FWHM on the Starlight Xpress group and got a response that suggested that it was due to chasing poor seeing. That's certainly plausible, but I wanted to be correcting at a fast rate to deal with the wind. I will have to play with the parameters and learn how best to make use of the AO unit. It may require different strategies for different conditions. No surprise there!

Cheers,
Rick.
That doesn't make sense as the whole point of an AO unit is to overcome the effects of the seeing.

Perhaps it is more how many hertz you can get the thing running at.

For example I can get an ST402ME or a Lodestar or a Starfish autoguider running at .5 second exposure times. But by the time the download is done and the corrections are done it is more like 1 second or a tad more.

So the AO would have to beat that lag. I thought that was the whole theory of how they work. The mirror can shift faster than a correction in the mount motors can be executed.

So the whole game would be to try to achieve the highest hertz (number of times per second) you can and that would mean getting the brightest guide star.

Perhaps the ST402 might be better for it as it is more sensitive and cleaner and therefore able to produce brighter guide stars that can be guided off.

One thing I am not sure of - I assume the hertz means the guide camera takes that many exposures per second? Like 3 hertz means 3 camera exposures per second. That then would requite a fast guide camera and fast download time.

Perhaps an SBIG STi may be better as it is a CCD plus it has a reducer lens built in lowering the F ratio making brigher stars.

Greg.[/QUOTE]
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  #17  
Old 10-01-2012, 11:13 PM
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That doesn't make sense as the whole point of an AO unit is to overcome the effects of the seeing.
Terry,

My opinion is that an active optical system like the AO-LF (which is much simpler than observatory grade adaptive optical systems) probably corrects for polar alignment errors, mount periodic error and other physical stuff like wind pretty well. I suspect they can help to some extent with seeing, but only a limited amount. That's for two reasons: seeing can almost certainly change faster than the system can react at times and also seeing is unlikely to be consistent over the whole FOV so a simple tilt mechanism can't correct over the whole FOV (adaptive optical systems that I've read about can actually make localized changes to the shape of the primary mirror to make different corrections in different regions of the FOV).

And yes, the "Hertz" is the frequency of guide exposures and tilt adjustments per second.

Cheers,
Rick.
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Old 11-01-2012, 06:25 AM
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Dumb question - how can you have 8 cycles per second with 0.5s guide exposures plus download tmes?
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Old 11-01-2012, 06:59 AM
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Dumb question - how can you have 8 cycles per second with 0.5s guide exposures plus download tmes?
You can't. With 0.5s guide exposures you'd be getting a little under 2Hz on my setup. In my tests I had 0.25s guide exposures producing an AO guide frequency around 3.8Hz.
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Old 11-01-2012, 07:38 AM
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Sorry my previous post was a little garbled. I only wrote the top bit. My fruit based not so smart phone garbled the quote.
I find that the AO corrects tracking problems well but won't help much when it is windy enough to rock the scope.
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