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| What's your general perspective and experiences with adaptive optics units? |
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I do not own an AO unit but I am also very interested in the the view of somebody who use it. I am very worried about the back focus you need with AO, OAG and FW when you use a field flattener or coma corrector. It wwould be also interesting for me to know how is the guiding quality you can get with an AO unit and a not-very-stable mount. |
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| From what I've read consumer AO are best suited to fix under-performing mounts. I have one on a PME with the thinking that it would help not the mount, but seeing with my 3,340mm FL. I've also read that seeing is 'extremely local' meaning it varies between the center of the chip and the OAG!!! See Innovations Foresight discussion on seeing. |
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The so called adaptive optics devices for amator are in fact active optics. Meaning it corrects for everything but seeing. Only in special cases it would. Atmospheric turbulence is local (as you said) and fast. The seeing is defined by three parameters wich are site dependents: - the coherence length (from which you can deduce the seeing value in arcsec). This is the strengh of the turbulence that everyone knows (and suffer!) -the coherence time: the time life of the turbulence (in fact of a speckle). Basically this is the time for which a perturbation is valid. Typically the coherence time is of orders of few milliseconds. It means that you have to correct a few times faster than this value to be really efficient. Slower correction leads to poorer, marginal improvement. -the anisoplanetic angle: Basically the distance from your guide star for which the perturbation is strongly degraded. This is of the order of some arcseconds! Which means that if your guide star is arcminutes away (it is the case for off axis guiders), you correct for... nothing (but seeing). In other words if your AO is working at few Hz and looking at a guide star some arcminutes away from your imaged object, your are not doing adaptive optics. It is a lie to call these devices adaptive optics. I am targeting sbig in this case. Starlight is offering the same device and they call it active optics, which is the right term. Now whatever it is called, these devices are efficient for correcting poor mount performances and wind gusts. You can indeed use it if your not satisfied with your mount. I used to put 16kg on a gm8 to do astrophotography at 1500mm focal length. Without the sbig AO it would not have been possible. |
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Thaks so much bingocrepuscule. That's just the kind of information I would like to get. Best regards, Javier. |
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| I've seen a few reports that say ONAG on NIR stars near the actual target will help tame seeing better than AO with OAG. I'm building such a system and will have first-hand experience to report on in the not too distant future. Again reference Innovations Foresight for an insightful discussion. I have an ONAG, but have not yet put it into practice. |
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But ONAG isn't AO right? I think they use the system for measuring and correcting focus. But it is not the atmospheric focus. It will help to have the guide star on-axis. If your field is large, your correction will degrade from centre to edges. You'll need also a guide star bright enough right on your object to correct fast. The chances to have one is limited and increases with the telescope aperture. For planetary imaging it has chances to work, cause you can use the bright object for guiding. It is quite difficult to foresee what kind of improvement you will get for a given seeing, frequency of correction and telescope aperture. One would need to make simulations that professionals do. It is also very difficult to assess correctly the improvement that you'll do experimentally. This is because on a few minutes the seeing can vary a lot, and turning your AO on and off and making the fwhm comparison isn't a correct way. Simply because you'll not be able to tell if the seeing has changed or if you have corrected for mount errors. In my opinion the only way is to have a beam splitter (or dichroic) sharing the light between corrected and uncorrected path. In addition one need to have a good mount and compare low frequency and high frequency corrections. It is a rather complicated set-up. If you don't want to face criticism or scepticism, the experiment and the way you present it, is of great importance. |
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| ONAG is not AO. It is exactly as you described: a dichroic splitter sending NIR to the guider and visible light to the main imager. "FocusLock" takes advantage of the star shapes caused by the splitter and is able to distinguish between in and out of focus by looking at the guide star shape. I've yet to see this for myself, but hopefully will have that system up and running soon. |
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I use SBIG AO on both my CDK14/MEII and FSQ106/MYT. In general, I find it an easier way to guide. I don't think it provides any real improvement past the seeing, but moving the mass of the little piece of glass is much more efficient than the full mass of the OTA and camera. Adjustments can be made in less time. Practically, I see small improvements in guiding error. For example, if I use tradition guiding and I'm seeing .5 a-s errors, I might get .35-.40 a-s errors switching to AO. With the shorter scope in less than perfect conditions, I've seen the AO allow shooting at wind levels that traditional guiding would not. Mind you, this isn't going to get you 1 or 2 a-s FWHMs in those conditions but it has saved my trip to the desert. Kevin |
