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Old 13-08-2014, 11:14 PM
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Does AO work.....

While gathering some data tonight, I took the time to collect some AO vs normal guiding data.

The end result?

AO improved peak stellar intensities by about 12%
Improved resolution by about 30%

You can see the data here
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Old 14-08-2014, 06:12 AM
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Hi Peter, thanks for the comparison data.

I have never used/owned an AO so if I am understanding this correctly the AO allows you to get tighter stars during each exposure with smaller FWHM readings?

You mentioned you had good seeing what is the expected effect as the quality of the seeing goes down? Would you expect even bigger gains from the AO in comparsion to the same rig without the AO in poor seeing?

I imagine as the focal length of the scope increases the benefits of the AO also increase?

At what focal length scope do you think the benefits of the AO would make it worthwhile having one? I mean at what point would the results be noticeable in a sub image.

Cheers
Brett
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Old 14-08-2014, 06:27 AM
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AO ? Adaptive Optics ?
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Old 14-08-2014, 07:42 AM
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Very significant improvement. I wouldn't have thought so. Not by that much anyway.
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Old 14-08-2014, 10:21 AM
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AO is all very well and good if there's a bright guide star, but so often my ST8-XME simply doesn't have a 500ADU+ guide star in the guider chip for anything less than a 15 second exposure. As a result I struggle to see how I'd get value for money out of AO, as much as I'd love to have it
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Old 14-08-2014, 10:40 AM
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Did you see similar results for multiple stars and multiple subs, Peter?

Cheers,
Rick.
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Old 14-08-2014, 11:49 AM
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Quote:
Originally Posted by rogerg View Post
AO is all very well and good if there's a bright guide star, but so often my ST8-XME simply doesn't have a 500ADU+ guide star in the guider chip for anything less than a 15 second exposure. As a result I struggle to see how I'd get value for money out of AO, as much as I'd love to have it
Obviously you would not be able to benefit from AO with your current equipment. That's like saying that you use a guidescope and want to know the benefits of using it. There aren't any. If your camera guided in front of the filters, with a more sensitive chip then it would be a different story.
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Old 14-08-2014, 01:55 PM
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Quote:
Originally Posted by RickS View Post
Did you see similar results for multiple stars and multiple subs, Peter?

Cheers,
Rick.
Yes

Quote:
Originally Posted by rogerg View Post
AO is all very well and good if there's a bright guide star, but so often my ST8-XME simply doesn't have a 500ADU+ guide star in the guider chip for anything less than a 15 second exposure. As a result I struggle to see how I'd get value for money out of AO, as much as I'd love to have it
Filter attenuation is indeed a problem with the legacy cameras. That said, I've not found any issues with current systems that place the guide chip ahead of the filters.

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Originally Posted by multiweb View Post
Very significant improvement. I wouldn't have thought so. Not by that much anyway.
Well, not whole lot in intensity, but the FWHM reduction makes it worthwhile.

Quote:
Originally Posted by ZeroID View Post
AO ? Adaptive Optics ?
Yes

Quote:
Originally Posted by Spookyer View Post
Hi Peter, thanks for the comparison data.

I have never used/owned an AO so if I am understanding this correctly the AO allows you to get tighter stars during each exposure with smaller FWHM readings?

You mentioned you had good seeing what is the expected effect as the quality of the seeing goes down? Would you expect even bigger gains from the AO in comparsion to the same rig without the AO in poor seeing?

I imagine as the focal length of the scope increases the benefits of the AO also increase?

At what focal length scope do you think the benefits of the AO would make it worthwhile having one? I mean at what point would the results be noticeable in a sub image.

Cheers
Brett
To use a marine wave analogy, I find AO works really well when there is a smooth swell, with no chop. i.e stars don't look like fuzz-balls, and indeed results improve with focal length

While I've not seen AO make things better in "fuzz-ball" or seeing, or to put it another way, when the higher frequency atmospheric disturbances dominate the image, I've not seen AO make things worse.

I've not used AO's on focal lengths less that about 1000mm, so can't give you any insight on what the cut-off would be, but I'd wager when your pixel sky coverage approaches 3-4 arc sec per pixel, unless the seeing or guiding errors were absolutely tragic, light will fall on a single pixel anyway, hence no benefits to be had.
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Old 14-08-2014, 02:23 PM
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Quote:
Originally Posted by Peter Ward View Post
Filter attenuation is indeed a problem with the legacy cameras. That said, I've not found any issues with current systems that place the guide chip ahead of the filters.
I don't use filters other than Red often, mostly use clear. The real problem even in this situation of no filters is FOV and sensitivity of the guide chip. I guess in the end a better solution than dual chip is dual camera where a more appropriate (larger and more sensitive) guide chip can be used.

I suppose in the end my comment was taking aim at the usefulness of AO in the most requiring situations - long focal length. Like, in my case at 0.84"/pixel I'd really love to have AO and don't mind paying for it, but as focal length increases, FOV decreases, and guide stars become less available, so I'd be paying for something I can only use a third of the time. It's sort of the opposite of what is required. At shorter focal lengths it's pretty much guaranteed to have a suitable guidestar but the AO is less required. What someone with my situation really needs is a guide chip of similar size and sensitivity to my ST8 primary chip, but that's not really practical to do.
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Old 14-08-2014, 05:37 PM
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Quote:
Originally Posted by rogerg View Post
I don't use filters other than Red often, mostly use clear. The real problem even in this situation of no filters is FOV and sensitivity of the guide chip. .....
A little more acreage on the main chip also helps, as you can usually find a good guide star, then crop for a more pleasing image.

I've also found using the FOV markers (guide and main sensor )with Sky-X gives great position and rotation options should a guide star not be immediately obvious.
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Old 14-08-2014, 05:44 PM
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A rotator for AO is pretty much de rigueur IMO too.
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Old 16-08-2014, 10:26 AM
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looks like a useful performance gain Peter.

Did a quick analysis of some of your unsaturated stars using Pixinsight (assuming the image data is not stretched). If I have the right pixel scale, the FWHM for the best fit Moffat PSFs went from about 6.2x5.5 arcsec down to 5.7x5.3 arcsec (ie AO mainly tidied up the star roundness). The peak signal went up by ~10%, as you found, so I guess no stretching.

But 5+ arcsec seems to be really bad seeing - I assumed 1.6 arcsec pixel scale, is that right?

Last edited by Shiraz; 16-08-2014 at 10:21 PM.
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Old 20-08-2014, 08:18 PM
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Quote:
Originally Posted by Shiraz View Post
looks like a useful performance gain Peter.

Did a quick analysis of some of your unsaturated stars using Pixinsight (assuming the image data is not stretched). If I have the right pixel scale, the FWHM for the best fit Moffat PSFs went from about 6.2x5.5 arcsec down to 5.7x5.3 arcsec (ie AO mainly tidied up the star roundness). The peak signal went up by ~10%, as you found, so I guess no stretching.

But 5+ arcsec seems to be really bad seeing - I assumed 1.6 arcsec pixel scale, is that right?
Sorry, that's not correct. (I've since uploaded CCDinspector's FWHM analysis of over 130 stars in each)

The web (visible) star field data is 8 bit .jpg.

I took all measurements using raw 16 bit (flat/dark corrected) .fits files.

Symmetry certainly was improved a tad, but the raw intensity curve is quite different to the 8 bit data.

This is my take on the physics as to why it works:

Waves of air are always passing by the telescope aperture. Sometimes they are slow and smooth, fast but still smooth, slow but rough and lastly fast and rough.

If the dominant wavelength slowly moving and is more than the telescope aperture, AO tip-tilt systems can make a correction for the angle which the wave it is causing the image to shift (up/down/left/right). It is here where amateur AO works.

Nights of bad seeing make the frequency of the wave increases.
Add in higher order harmonics to make things worse.

Amateur AO (read tip/tilt) is unlikely to have either the correction rate, or will likely follow a higher-order harmonic of the wave in the wrong direction.

But on nights where the atmosphere is rolling along at a smooth 5-10Hz, as was likely on the night I grabbed the data here, both resolution and stellar intensities show significant improvement with AO than without.

Last edited by Peter Ward; 20-08-2014 at 09:56 PM. Reason: additional data
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Old 20-08-2014, 08:30 PM
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A very interesting post. I know you are the SBIG dealer but would you wager the SBIG AO unit is more workable than the Starlight Express?

Greg.
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Old 20-08-2014, 08:51 PM
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Perhaps we can do a test at Clayton. My seeing is quite good, we can see what sort of results I get over say a loan period of 3 years.

This is quite an interesting discussion. The improvement in stellar profiles does require good seeing as I understand it, but is the real improvement over the mount discrepancies? Even a mount of Bisque quality would have minor defects that an AO could improve upon? I have seen results in SN that seem to indicate an AO can produce better results.
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Old 20-08-2014, 09:02 PM
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Originally Posted by gregbradley View Post
A very interesting post. I know you are the SBIG dealer but would you wager the SBIG AO unit is more workable than the Starlight Express?

Greg.
Having never used a SX unit, I really couldn't say.

That said, SBIG's first (reflective optic) AO's than ran under DOS (the Jurassic version of windows 8 to our younger readers ), while difficult to set up, were simply amazing and have not been bettered. 30Hz guiding was easily obtainable, the downside was, bright guide stars (to take advantage of that amazing guide rate) were often not.

On a more pragmatic side, if you want to use a KAF16803 chip (or KAF11002), SBIG's AO-X is the only AO on the market, period, that has the aperture to do the job.
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Old 20-08-2014, 09:16 PM
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Perhaps we can do a test at Clayton. My seeing is quite good, we can see what sort of results I get over say a loan period of 3 years.
Now a reading from "The castle": "tell 'em the're dreamin'

Quote:
Originally Posted by Paul Haese View Post
This is quite an interesting discussion. The improvement in stellar profiles does require good seeing as I understand it, but is the real improvement over the mount discrepancies? Even a mount of Bisque quality would have minor defects that an AO could improve upon? I have seen results in SN that seem to indicate an AO can produce better results.
That's why I published the mount RMS tracking data. About a quarter arc-sec in *both* RA and Dec. Well below the seeing, plus you'd expect the RA to dominate in any mount-error effect. That was not observed/supported by the data.
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