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Adaptive Optics, Tracking the SOTA for ground-based observation
JRehling
post Nov 16 2016, 06:23 PM
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Here, as mentioned in the Neptune thread, is a link to an article about the PALM-3000 system working with the 5m Hale Telescope. The attached photo of Ganymede is one of the best ground-based images I've ever seen, and easily blows away HST resolution.

http://inspirehep.net/record/1252803/plots


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Bjorn Jonsson
post Nov 16 2016, 08:35 PM
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Wow!!!!

Somehow the fact that groundbased images of the Galileans have become as good as the image above had escaped my attention. Keeping in mind what the best images so far of e.g. Uranus look like (they show lots of belts and spots), the next several years are going to be interesting for everyone interested in the outer solar system.
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Phil Stooke
post Nov 17 2016, 02:14 AM
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Wow indeed. Amazing. OK, now I want to see... the list is never-ending!

This is the same side of Ganymede seen by Galileo:

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(lousy pic pulled off the web quickly, as I am otherwise occupied right now)

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Decepticon
post Nov 17 2016, 07:57 AM
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The features don't seem to line up?
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AndyG
post Nov 17 2016, 08:25 AM
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Rotate Phil's 90° clockwise.

Andy
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JRehling
post Nov 17 2016, 03:01 PM
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It certainly is impressive. Not quite Voyager level, and we don't get to choose the perspective, but the increased resolution of ground-based observations is something of an escape valve on the pressure to launch certain outer solar system missions, and is bound to improve with time.

Roughly speaking, resolution is a linear function of aperture, and that 5 meter telescope is delivering resolutions about 40 times better than a telescope with about a 40th the aperture. This would not have been the case in the past, with "seeing" limiting the resolution of a massive telescope. But that's now changed. When we have 30-to-40 meter telescopes served by the next decade's technologies, we may get pictures of, say, the Uranian satellites that exceed Voyager coverage, and imaging other latitudes as the seasons to change will just be a matter of waiting.
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Phil Stooke
post Nov 17 2016, 03:18 PM
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"Rotate Phil's 90° clockwise."

No, rotate the other one! Mine has north at the top. Call me old-fashioned but that's the way I like it.

Phil


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algorimancer
post Nov 17 2016, 05:56 PM
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Thanks for posting. That is crazy impressive resolution smile.gif

What ever happened to the promise of optical interferometry, with multiple telescopes separated by large distances to emulate the performance of a larger scope? We had Keck II, which did some impressive work, but that was more of a pilot project for the larger plans.
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JRehling
post Nov 17 2016, 06:42 PM
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QUOTE (algorimancer @ Nov 17 2016, 10:56 AM) *
What ever happened to the promise of optical interferometry, with multiple telescopes separated by large distances to emulate the performance of a larger scope? We had Keck II, which did some impressive work, but that was more of a pilot project for the larger plans.


Definitely the preeminent telescope of the future is the E-ELT, which will have a 39-meter composite mirror. You could think of that as multiple telescopes separated by a distance, but with extra telescopes filling in all the space between them! If that can deliver an 8x improvement on the resolution in the Ganymede photo that I posted, as it should, it'll as big of an advance over existing telescopes than the HST was over what came before it.

I'm not sure why the architecture that you mention has not been advanced, but there was a great article in SiAm last year…

https://www.scientificamerican.com/article/...from-yesterday/

…about the very acrimonious and political battle to develop the world's next biggest telescopes that was a bitter fight over resources and ended up not with one big winner, nor two, but three roughly equal rivals. As the article mentions, a unified community would probably have given the world a single massive telescope many years earlier. That's the downside. The upside is that we'll probably have two of them a decade from now, and that doubles the number of observations. (The third, TMT, is in limbo due to even different political reasons, but we may have three massive telescopes 15 or 20 years from now???)

Given the intense struggle and animosity, it's likely that many worthwhile projects have been stalled by the fierce competition for resources.

The world's biggest radio telescope has also been built, in China. There's no doubt we're on the cusp of a new golden age of telescopes, and many breakthroughs are inevitably to follow.

Personally, I'm most excited about the visual observation of exoplanets within 50-100 light years, but it's also sure to mean breakthrough observations of objects as close as the asteroid belt and as far as the cosmological scale.
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algorimancer
post Nov 17 2016, 07:54 PM
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I see that there is the ESO's VLTI, which is a large telescope system which uses interferometry:

https://www.eso.org/sci/facilities/paranal/...copes/vlti.html

However, I can only find extra-solar results for the VLTI. They did do some neat adaptive optics imaging of Jupiter though, but they don't seem to have used interferometry to do it:

https://www.eso.org/public/images/eso0833a/
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Phil Stooke
post Nov 17 2016, 08:15 PM
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I don't know a lot about this, but my impression is that interferometry is really good at separating very close point sources (close binaries, whether stars or asteroids, don't know if it's so good with s star plus planet pair because of the huge difference in brightness) but has not been so successful with imaging of extended objects. Adaptive optics seems to be much better at producing images.

Phil


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Decepticon
post Nov 17 2016, 08:19 PM
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biggrin.gif

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algorimancer
post Nov 18 2016, 03:21 PM
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It's interesting to compare this adaptive optics image of Ganymede to the one taken by the Pioneer 10 flyby:

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We've come a long way.
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Gerald
post Nov 18 2016, 08:55 PM
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QUOTE (algorimancer @ Nov 17 2016, 09:54 PM) *
I see that there is the ESO's VLTI, which is a large telescope system which uses interferometry:

https://www.eso.org/sci/facilities/paranal/...copes/vlti.html

However, I can only find extra-solar results for the VLTI. They did do some neat adaptive optics imaging of Jupiter though, but they don't seem to have used interferometry to do it:

https://www.eso.org/public/images/eso0833a/

According to Wikipedia, VLTI works only for very bright objects, i.e. for nearby stars:
QUOTE
Because of the many mirrors involved in the optical train, about 95 percent of the light is lost before reaching the instruments at a wavelength of 1 µm, 90 percent at 2 µm and 75 percent at 10 µm.[40] This refers to reflection off 32 surfaces including the Coudé train, the star separator, the main delay line, beam compressor and feeding optics. Additionally, the interferometric technique is such that it is very efficient only for objects that are small enough that all their light is concentrated. For instance, an object with a relatively low surface brightness such as the moon cannot be observed, because its light is too diluted.

They reference the document "Puech, F.; Gitton, P. (2006). "Interface Control Document between VLTI and its instruments". VLT-ICD-ESO-15000-1826", which I didn't find online, possibly exists only as print version.
Wikipedia also says:
QUOTE
When fringe tracking is introduced, the limiting magnitude of the VLTI is expected to improve by a factor of almost 1000, reaching a magnitude of about 14.

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algorimancer
post Nov 18 2016, 10:10 PM
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QUOTE (Gerald @ Nov 18 2016, 02:55 PM) *
...VLTI works only for very bright objects, i.e. for nearby stars:


I guess I've been overly optimistic with regard to optical interferometry. After seeing what radio telescopes do with it, I keep hoping to see optical methods catch up.
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