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Old 05-08-2014, 09:55 PM
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initial test of an SX AO at f4 - calm conditions - windy conditions added

Peter (Poita) very generously let me use his SX AO unit to see if it would work OK for hi res imaging with an f4 Newtonian imager.

Initial testing, designed to study the effects of AO with minimal mount errors, yielded the following:
1. There is slight chromatic aberration from the 13mm corrector plate at f4, but it is not enough to worry about in normal imaging conditions.
2. When active, the AO generally reduced the peak brightness of stars (by up to about 30%) and increased their sizes (by up to about 25% in FWHM) when compared to results with the AO off. This was on a good mount (EQ8) in windless conditions and with average seeing of about 2.5 arc sec FWHM (see attached image for typical un-stretched results)

Discussion:
- An AO would be expected to increase star sizes by a factor of up to 1.4 outside of the isokinetic patch (eg with off axis guide camera). The AO did better than this, so it was presumably also correcting some mount error. However, the AO still degraded the imagery by a noticeable amount.
- The difference between AO and non-AO imagery is detectable at the sampling of 0.93 arcsec/pixel, but with the data resampled to 1.86 arcsec, there is no discernable difference with/without AO.

Opinion:
1. On a good mount under reasonably good seeing, with no wind and with near-Nyquist sampling, there was a perceptible overall drop in resolution, so AO does not appear to be beneficial under these circumstances.
2. However, if seeing is really high quality, it is possible that the beneficial effect of the AO in correcting mount errors may outweigh the loss in resolution. The 2.5 arcsec seeing was nowhere near good enough for this possibility to be tested, so this is conjecture.
3. If the mount is of lower quality and/or the wind is significant, the mount correction by an AO may provide a nett benefit in many other seeing conditions. This is also conjecture, but others have reported that this is the case.
4. If the imaging is undersampled (eg ~2 arcsec is widely used), the effect of an AO may only be noticeable if it corrects significant mount errors.

The next stage will be to test the system on an EQ6 and under windy conditions. For now though, the answers to two of the basic questions would seem to be:
"can an SX AO be used for hi res imaging at f4 ?" - yes
“does an AO increase resolution in average seeing ?” - probably not, likely depends on wind and mount quality, but you may lose resolution in calm conditions

Thanks for reading. Would be very interested to know if these results tally with the experience of others. And once again, thank you Peter for letting me use the AO. Regards ray

Test configuration: SX AO with an off axis Lodestar guide camera at 2x2 binning and 0.1 sec/frame under PHD2. 200f4 Newtonian scope, on an EQ8, with no coma corrector and with a QHY5 L2 as the imaging camera (at 10 second frames to minimise the mount error while still allowing the effects of AO to be assessed in isolation). Imaging sampling was 0.93 arcsec. Test sequences were 10 frames with AO active, followed immediately by 10 frames with it off.
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Last edited by Shiraz; 13-08-2014 at 07:04 PM.
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Old 06-08-2014, 07:45 AM
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Ray,

I did some experiments and got a reasonable FWHM improvement on a GSO RC10 at f/8 and an image scale of around 0.56 arcsec/pixel. IIRC, it was windy the night I was testing.

Martin Pugh is also getting some fantastic results from AO but that's on a big scope (17", I think?) at SRO with very good seeing.

Not surprisingly, it seems that AO is most beneficial when you're pushing the limits of your mount.

Cheers,
Rick.
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Old 06-08-2014, 08:26 AM
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Quote:
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Ray,

I did some experiments and got a reasonable FWHM improvement on a GSO RC10 at f/8 and an image scale of around 0.56 arcsec/pixel. IIRC, it was windy the night I was testing.

Martin Pugh is also getting some fantastic results from AO but that's on a big scope (17", I think?) at SRO with very good seeing.

Not surprisingly, it seems that AO is most beneficial when you're pushing the limits of your mount.

Cheers,
Rick.
Thanks Rick.

Seems to be a consistent pattern emerging - if the mount error is dominant (eg either the mount is lower quality, the seeing is exceptional or it is windy) then AO can be beneficial. If seeing is dominant, AO can make matters worse.

My test was designed so that seeing dominated and the limitations of off axis guiding, with uncorrelated noise in the guide channel, were evident. It showed quite clearly that AO as implemented can actually exacerbate seeing errors. Next stage will be to show what it can do to fix mount problems.

I wonder if we ever get good enough seeing in Aus for AO to be worthwhile on a good quality mount in still conditions?

regards Ray

Last edited by Shiraz; 06-08-2014 at 08:50 AM.
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Old 06-08-2014, 09:09 AM
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Have you read this article, Ray? https://www.sbig.com/about-us/blog/differential/
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Old 06-08-2014, 11:05 AM
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I haven't used it much, but when I had a CGEM it was a massive improvement in image quality on that mount.

I think AO works like a champion with a big scope and an imperfect mount. I bought it more to fix tracking errors than for seeing issues.
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Old 06-08-2014, 03:30 PM
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Quote:
Originally Posted by RickS View Post
Have you read this article, Ray? https://www.sbig.com/about-us/blog/differential/
Quote:
Originally Posted by Poita View Post
I haven't used it much, but when I had a CGEM it was a massive improvement in image quality on that mount.

I think AO works like a champion with a big scope and an imperfect mount. I bought it more to fix tracking errors than for seeing issues.
Yes thanks Rick - Peter Ward pointed it out. It's worth a (very careful) read. Nice to see tech stuff peeking through the clouds of marketing hype. The main thing I took from it was that SBIG are aware that the currrent method of guiding on a single off axis star is not too good - hence their apparent intention to move to multiple stars, which will overcome the problem of noise injection into the guide channel.

Hi Peter. I agree that mount correction is where these things shine, not seeing correction, which is how they have been sold. Maybe there is a marketing problem - ie how do you sell something that fixes mount problems to people who believe that their mounts are perfect?

regards Ray

Last edited by Shiraz; 06-08-2014 at 04:18 PM.
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Old 06-08-2014, 04:13 PM
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Stars are noticeably rounder with AO on. What's the guide rate you managed you say? 10Hz? Must have been a very bright guide star...
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Old 06-08-2014, 04:23 PM
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Stars are noticeably rounder with AO on. What's the guide rate you managed you say? 10Hz? Must have been a very bright guide star...
yep, it was bright. The lodestar exposure was 0.1s, but the actual update rate was less than 10Hz. I was doing testing, so chose a bright guide star by just scanning around to find one with a high SNR and then imaged whatever happened to be in the field at the time. I imagine that finding a bright enough guidestar would be a major practical issue.

The unguided EQ8 was producing nice round stars, so the AO could not improve on them. Have you ever found that the stars can be rounder, but a bit bigger, with AO on?
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Old 06-08-2014, 04:27 PM
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yep, it was bright. I was doing testing, so chose a bright guide star by just scanning around and then imaged whatever happened to be in the field at the time.
I imagine that finding a bright enough guidestar would be a major practical issue.
In practice I think the best guiding rates I've achieved were around the 4Hz mark. Good enough to correct for wind buffeting and mount errors. But I doubt it did any improvement with seeing. Although at long FL 3m and over at an image scale of say 0.7 second of arc per pixel your stars are definitely not as fat. So good for any typical SCT with a large aperture.
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Old 06-08-2014, 08:31 PM
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Interesting result.

I'd question whether all AO's are created equal.

The groundwork to SBIG's devices was done by Brad Wallis (Awarded a NASA prize his programming skills during the Cassini mission) and Benoit Shillings, ( BeOS creator and Software engineer with few equals). SBIG went through a very thorough evaluation process to determine what benefits an AO might offer, some time prior to offering it to the amateur market. ( I recall being asked to provide data for this early assessment some 15 years ago)

If the system hysteresis, acceleration and dampening is not well understood, modelled and controlled, it would not surprise me in the slightest to see tip/tilt systems making images worse.
(BTW AO acceleration and displacement is user/software adjustable with SBIG's systems)

But this has not been my experience using AO7, AO8, AOL and AOX units. I have always found stellar footprints were as small and intense with AO enabled as without. When the seeing has been good, I have constantly seen 30-50% improvements in both. As has been pointed out earlier, one need only look at Martin Pugh's recent AO data and compare that to his earlier, non-AO, images to see the benefits.
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Old 06-08-2014, 08:49 PM
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Would an AO unit be of any practical significance imaging at 530-850mm focal length?

H
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Old 06-08-2014, 08:54 PM
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Would an AO unit be of any practical significance imaging at 530-850mm focal length?

H
I'd say no, but frankly have never tested such short focal lengths with an AO.
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Old 06-08-2014, 10:26 PM
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Quote:
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Interesting result.

I'd question whether all AO's are created equal.

The groundwork to SBIG's devices was done by Brad Wallis (Awarded a NASA prize his programming skills during the Cassini mission) and Benoit Shillings, ( BeOS creator and Software engineer with few equals). SBIG went through a very thorough evaluation process to determine what benefits an AO might offer, some time prior to offering it to the amateur market. ( I recall being asked to provide data for this early assessment some 15 years ago)

If the system hysteresis, acceleration and dampening is not well understood, modelled and controlled, it would not surprise me in the slightest to see tip/tilt systems making images worse.
(BTW AO acceleration and displacement is user/software adjustable with SBIG's systems)

But this has not been my experience using AO7, AO8, AOL and AOX units. I have always found stellar footprints were as small and intense with AO enabled as without. When the seeing has been good, I have constantly seen 30-50% improvements in both. As has been pointed out earlier, one need only look at Martin Pugh's recent AO data and compare that to his earlier, non-AO, images to see the benefits.
I rather think that you have completely missed the point Peter. The testing was done specifically to show that the concept of guiding on a star outside of the isokinetic patch can in some circumstances lead to negative results - AO's cannot be expected to "fix" sub-standard seeing because they suffer from this problem, although it is often claimed that they can. If I had used an SBIG AO I would expect the same result - unless they have somehow managed to bypass the physics of the atmosphere. As an SBIG distributor, you will naturally favour that product, but it seems a bit offside that you view this evaluation in terms of a shootout between competing AOs, when only one is being tested.

As was pointed out, if an AO is used under good seeing conditions, it should be able to fix residual mount/wind errors - I presume that is what Martin Pugh is finding, although I doubt that I will ever get seeing that is anything like good enough to confirm that idea.

Quote:
Originally Posted by Octane View Post
Would an AO unit be of any practical significance imaging at 530-850mm focal length?

H
Should be useful for correcting major mount errors (eg wind) in good seeing H. My testing was done at 800mm fl and the AO did what any AO would have done under the circumstances. The fl is immaterial (provided there is enough back focus) - what matters is the angular pixel scale, since the job of an AO is to modify the angles of the light rays. If you use small pixels at these focal lengths, AO should be advantageous under some circumstances - if you use big pixels and undersample, AO will be of less use.

Last edited by Shiraz; 07-08-2014 at 01:59 PM.
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Old 07-08-2014, 01:34 AM
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Perhaps it would help to set out a mudmap of when (I think) single star guided AO probably makes sense and when it is counterproductive. The graphics will not win any awards, but show the general idea - it is a best guess distillation of the little that I know about how AO works and what others have noted. The shaded area has been tested as reported on in this thread. Other areas will be tested if possible when conditions allow, in an attempt to validate that which is currently based on anecdote and surmise. Maybe this is a bit of a Quixotic approach, but hey, this is a fun hobby, so why not.

The results of the current test should be read in the context of the attached diagram - this is not intended to be a blanket condemnation of AO, nor of the SX unit, just a demonstration of what happens under one of the sets of conditions in which AO is not beneficial.
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Last edited by Shiraz; 07-08-2014 at 12:43 PM.
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Old 07-08-2014, 07:54 AM
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I like that Graph. Simple and concise. Bookmarked.
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Old 07-08-2014, 02:09 PM
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Disregarding a post I'm about to make re my problems with Protrack, I've been wondering if it makes any sense to consider a combination of Protrack and AO to avoid sending any guide corrections to the mount whatsoever?

Expectations are that Protrack on a Paramount can yield 5-10 min unguided images with perfect stars. Some users here report that they are able to do this but I have never succeeded. But I would guess that the small amount of egg shaped stars could easily (and possibly more effectively) be dealt with using AO.

How much drift can AO account for before it sends a mount guiding correction?

Thanks,

Peter
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Old 07-08-2014, 02:23 PM
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I suspect a mount such as a PME with protrack would not benefit from any SX AO. But SBIG AO tilt mirror is far superior and might make a difference because of the update rate. It's chalk and cheese.
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Old 07-08-2014, 02:33 PM
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I suspect a mount such as a PME with protrack would not benefit from any SX AO. But SBIG AO tilt mirror is far superior and might make a difference because of the update rate. It's chalk and cheese.
Hi Marc,

I don't doubt what you said re SBIG vs SX AO. But, my idea wasn't necessarily to improve on any mount errors per se, but to avoid using the mount motors to provide guiding corrections. Wouldn't AO offer superior and much faster and smoother corrections regardless of brand? If the goal was to say avoid drifting a few pix over 10-15 min wouldn't AO have a better chance of doing this more smoothly than the mount?

Peter
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Old 07-08-2014, 02:36 PM
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Hi Marc,

I don't doubt what you said re SBIG vs SX AO. But, my idea wasn't necessarily to improve on any mount errors per se, but to avoid using the mount motors to provide guiding corrections. Wouldn't AO offer superior and much faster and smoother corrections regardless of brand? If the goal was to say avoid drifting a few pix over 10-15 min wouldn't AO have a better chance of doing this more smoothly than the mount?

Peter
AO is another component in the imaging train interfering with the light path. In the case of the SX AO a 10mm thick piece of glass. Better straight light than some other surface to go through if you have a top mount and can help it. That's where I'm coming from. Less is more.

The AO will do very fast corrections but if you have a consistent drift you should sort that out first. When the star goes out of range of the AO the mount is bumped to re-center the system.
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Old 07-08-2014, 08:26 PM
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Happy to loan it to you Peter if you want to see if it will help with your setup.
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