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Neil Russell
2016-May-28, 09:30 PM
Maybye not now but soon our technology will allow us to build super large space telescopes. So what sort of resolution could we expect from a space telescope 100km in diameter? compared to Hubble telescope? Would we be able to see the planets (if any?) orbiting Alpha Centauri.

danscope
2016-May-29, 03:11 AM
You'd probably cook the focusing sensor.

antoniseb
2016-May-29, 03:20 AM
All other things being equal (detector sensitivity etc), such a telescope would have 40,000 times the resolution as Hubble, so could observe down to a little worse than 1 microarcsecond resolution. This would observe with a resolution of about 250 kilometers at the distance of Alpha Centauri. You would need to find a way to mask the glare of the stars, but resolution-wise, it could even get approximate shapes of continents and large weather patterns on an Earth-like planet if there was one.

publiusr
2016-May-29, 09:04 PM
I wanted OWL in the worst way: https://en.wikipedia.org/wiki/Overwhelmingly_Large_Telescope

"It has been estimated that a telescope with a diameter of 80 meters would be able to spectroscopically analyse Earth-size planets around the forty nearest sun-like stars.[4] As such, this telescope could help in the exploration of exoplanets and extraterrestrial life (because the spectrum from the planets could indicate the presence of molecules indicative of life)."

Neil Russell
2016-May-30, 09:30 PM
Great information, thank you, now if I can only fix to live longer so I can appreciate the view!!!

George
2016-May-31, 02:37 PM
A colossal mirror would not likely be built but two or more separate mirrors working as an interferometer. The greater the separation the greater the resolution. But it won't be easy.

AFJ
2016-Jun-01, 09:34 PM
What would it take to image individual stars in galaxies (just) outside the Local Group?

Hypmotoad
2016-Jun-02, 04:51 AM
I'd love to see funding for a couple of Webb style telescopes to be placed in opposing orbits, lets say, one or 2 A.U.'s out. Not sure I said that right but I think you get the idea. Knowing zilch about optics, I've read where we could read an alien's newspaper from many light years out at the low, low cost of TWICE the budget of the original.

Understanding economics as I do (only slightly more than I do optics) I would say we could easily do away with funding education, as it seems to be doing little good in my beloved state of Louisiana, judging by the amount of drivers with a cell phone to their ear I see all the time...

Of course we could always tax tobacco again so that the six smokers left would either quit or benefit humanity.

Joking aside, why not shoot for something like this rather than waste money in an ever increasing game of, "See what we could have done with mo money?"

George
2016-Jun-02, 06:09 PM
What would it take to image individual stars in galaxies (just) outside the Local Group? Here (https://en.wikipedia.org/wiki/Optical_telescope#Surface_resolvability)shows is a rough way to calculate the aperture size for a given arcsecond angle of resolution:

D = 138/alpha

D (diameter) is in millimeters and alpha (angle) is in arcseconds.

So, to see something as small as, say, 10000 km, one million light years from here, you would need about 400,000 km in effective diameter. [You would not need a mirror disk this size only pieces of the disk with this separation and a whole lot of science and engineering to make it stable.]

The equation is linear, so to see 1/10th the size you would need 10x the aperture diameter.

Amber Robot
2016-Jun-02, 07:28 PM
Here (https://en.wikipedia.org/wiki/Optical_telescope#Surface_resolvability)shows is a rough way to calculate the aperture size for a given arcsecond angle of resolution:

D = 138/alpha

D (diameter) is in millimeters and alpha (angle) is in arcseconds.

So, to see something as small as, say, 10000 km, one million light years from here, you would need about 400,000 km in effective diameter. [You would not need a mirror disk this size only pieces of the disk with this separation and a whole lot of science and engineering to make it stable.]

The equation is linear, so to see 1/10th the size you would need 10x the aperture diameter.

But to study individual stars in external galaxies you don't need to image them, just isolate their light from nearby stars. Therefore, the size to be resolved is not the size of a star, but the interstellar separation distance, which is much, much greater.

PetersCreek
2016-Jun-02, 08:16 PM
Hypmotoad,

Joking or not, you've been warned about inappropriate, off-topic posts in forums like Q&A. That you've taken it in a political direction just makes it worse. This forum is for mainstream answers to questions and not for jokes or general commentary.

AFJ
2016-Jun-02, 09:15 PM
Here (https://en.wikipedia.org/wiki/Optical_telescope#Surface_resolvability)shows is a rough way to calculate the aperture size for a given arcsecond angle of resolution:

D = 138/alpha

D (diameter) is in millimeters and alpha (angle) is in arcseconds.

So, to see something as small as, say, 10000 km, one million light years from here, you would need about 400,000 km in effective diameter. [You would not need a mirror disk this size only pieces of the disk with this separation and a whole lot of science and engineering to make it stable.]

The equation is linear, so to see 1/10th the size you would need 10x the aperture diameter.

That is really cool thanks! but also a bit disappointing in being rather unrealistic for many many years to come, if ever.

@ Amber Robot: no i actually do mean individual stars. Suppose we could do that, and image a few stars in another galaxy from which we are 99% certain they are exactly equal to our sun. Then we would have a physical size + apparent size; extrapolate this ruler to that whole galaxy and get it's true size + true distance. A bit of sf story but i guess not impossible in physics / mainstream i reckon (?).

George
2016-Jun-02, 10:05 PM
But to study individual stars in external galaxies you don't need to image them, just isolate their light from nearby stars. Therefore, the size to be resolved is not the size of a star, but the interstellar separation distance, which is much, much greater.To see surface features directly on the star does require use of the aperture size, but your point about separation is also very important since glare can be overwhelming. I wonder too how much of a problem gas and dust would be given that the greater the distance the greater the encounter with these atmospheric-like elements.

Neil Russell
2016-Jun-02, 10:51 PM
I admit to being a bit sneaky here! If we can get such a telescope we should then be able to see if there was indeed a big bang? ie an edge?? or an infinite universe??

antoniseb
2016-Jun-03, 12:32 AM
I admit to being a bit sneaky here! If we can get such a telescope we should then be able to see if there was indeed a big bang? ie an edge?? or an infinite universe??
Ummm. You could have an optical telescope 100 light years in diameter and still not be able to see light from before the first stars. The edge (if there is one) can't be observed in such a way.

Amber Robot
2016-Jun-03, 04:43 PM
To see surface features directly on the star does require use of the aperture size

Fair enough, but when an astrophysicist thinks about studying individual stars in another galaxy, imaging their surfaces is not what comes to mind.


I wonder too how much of a problem gas and dust would be given that the greater the distance the greater the encounter with these atmospheric-like elements.

Well, that will depend on the galaxy, but we see so many galaxies it's hard to believe that there are many out there totally obscured by dust.

korjik
2016-Jun-03, 05:32 PM
I admit to being a bit sneaky here! If we can get such a telescope we should then be able to see if there was indeed a big bang? ie an edge?? or an infinite universe??

We can already see all the way back as far as is possible with electromagnetism. Its called the Cosmic Microwave Background.

publiusr
2016-Jun-03, 09:37 PM
You would not need a mirror disk this size only pieces of the disk with this separation and a whole lot of science and engineering to make it stable.

The equation is linear, so to see 1/10th the size you would need 10x the aperture diameter.

There was some talk about large structures made of bubbles in space: http://nextbigfuture.com/2007/04/possibly-last-niac-studies-are-being.html

I can see a bubble silvered on the inside--cut in half--and the two halves used as an nterferometer. The best of both worlds.
That or as giant bino's
http://www.space.com/33121-giant-balloon-technology-near-space-telescopes.html

George
2016-Jun-03, 09:40 PM
Fair enough, but when an astrophysicist thinks about studying individual stars in another galaxy, imaging their surfaces is not what comes to mind. Yes and by a huge extent. Only the disk of a few number of stars have been seen. Betelgeuse being one example. I felt that direct imaging of their disk was the question being asked.


Well, that will depend on the galaxy, but we see so many galaxies it's hard to believe that there are many out there totally obscured by dust.I was curious if we could have a few "exo air mass" values for regions outside our atmosphere. In other words, given the average (probably not an abstract term) particle count in the case for viewing within the galaxy (avoiding clouds), outside the galaxy (where gas and dust is far less dense), what distance would approximately equate to one air mass (the mass of air directly above us)? Any given direction can have such a varying change that I doubt such a distance is stated, but it wouldn't be too hard to determine since there are some average densities stated here an there, and we know the mass in one air mass.

AFJ
2016-Jun-04, 09:59 AM
Yes and by a huge extent. Only the disk of a few number of stars have been seen. Betelgeuse being one example. I felt that direct imaging of their disk was the question being asked.

As said i was thinking more about imaging individual stars that are equal to our Sun, and comparing size with apparent size to get host galaxy size / distance.

Given we had the necessary technology, could this be another cosmic yard stick, or are there other physics obstacles that would make such a method unusable?

George
2016-Jun-04, 12:00 PM
Given we had the necessary technology, could this be another cosmic yard stick, or are there other physics obstacles that would make such a method unusable? Measuring disk size, I would bet, would help refine distance, especially for binaries since mass can be determined. Knowing mass and estimating age would allow an actual size to be calculated, so what is observed would yield distance.

Of course, super resolution would also improve parallax measurements, which yields distance.

AFJ
2016-Jun-04, 05:26 PM
Measuring disk size, I would bet, would help refine distance, especially for binaries since mass can be determined. Knowing mass and estimating age would allow an actual size to be calculated, so what is observed would yield distance.

Of course, super resolution would also improve parallax measurements, which yields distance.

Great! Now all we need is a telescope to be able to do this, but that's the easy part ;)

Not sure, but wouldn't the usefullness of parallax decrease faster with distance than measuring disk size?

Hornblower
2016-Jun-04, 05:39 PM
Great! Now all we need is a telescope to be able to do this, but that's the easy part ;)

Not sure, but wouldn't the usefullness of parallax decrease faster with distance than measuring disk size?

For main sequence stars I would expect the opposite. The disk size would drop below the telescope's resolution limit before the parallax. For a bloated supergiant such as Betelgeuse the parallax would go first.

Hornblower
2016-Jun-04, 05:48 PM
When we infer that a main sequence G2 star is virtually identical to the Sun by examining its spectrum, we get its size and luminosity. We can already determine its distance from its apparent brightness. What would measuring its angular diameter tell us about its distance that we do not already have?

AFJ
2016-Jun-04, 06:56 PM
From individual stars in other / distant galaxies? ( hypothetical case in relation to very large telescopes). Different sequence stars might have some variation in size?

Tom Mazanec
2016-Jun-04, 10:01 PM
Would a mission to the Sun's gravitational focus allow a performance equivalent to the telescope being considered?

01101001
2016-Jun-05, 05:18 AM
Wikipedia: FOCAL (spacecraft) (https://en.wikipedia.org/wiki/FOCAL_(spacecraft))


In order to use the Sun as a gravity lens, it would be necessary to send the telescope to a minimum distance of 550 astronomical units away from the Sun, enabling very high signal amplifications: for example, at the 203 GHz wavelength, amplification of 1.31015. Maccone suggests that this should be enough to obtain detailed images of the surfaces of extrasolar planets.

01101001
2016-Jun-05, 05:43 AM
FOCAL would have to get out there maybe roughly in the Planet Nine distant neighborhood.

New Horizons is whipping through space at about 3 AU/year, so you can probably gauge the effort.

Hornblower
2016-Jun-05, 05:41 PM
It just occurred to me that resolving the star's disk could be a sanity check against the possibility of extinction by some sort of stuff that does not cause the reddening (color excess) that is characteristic of interstellar dust. If the disk is larger than expected, with a correspondingly lower apparent surface brightness that is at odds with the effective temperature that is indicated by the spectrum, then we could infer that the star is closer and is being dimmed by anomalous particulate stuff.

AFJ
2016-Jun-05, 07:23 PM
It just occurred to me that resolving the star's disk could be a sanity check against the possibility of extinction by some sort of stuff that does not cause the reddening (color excess) that is characteristic of interstellar dust. If the disk is larger than expected, with a correspondingly lower apparent surface brightness that is at odds with the effective temperature that is indicated by the spectrum, then we could infer that the star is closer and is being dimmed by anomalous particulate stuff.

Well i guess check check double-check always counts as a good reason to perform such observations, so there you go :) Would be a lot of fun if such a thing was discovered, maybe we then have to incorporate some form of grey matter to explain the anomaly?

One other thing i was wondering about; would the disk size of a star in a (high redshift) distant galaxy that is either moving away at relativistic speed or have a lot of expanding spacetime ( (almost) the same in observational sense) in between it and us be altered in any way by this? Would the disk size for instance be smaller or bigger as seen by an earth observer?

Hornblower
2016-Jun-05, 10:05 PM
Well i guess check check double-check always counts as a good reason to perform such observations, so there you go :) Would be a lot of fun if such a thing was discovered, maybe we then have to incorporate some form of grey matter to explain the anomaly?

One other thing i was wondering about; would the disk size of a star in a (high redshift) distant galaxy that is either moving away at relativistic speed or have a lot of expanding spacetime ( (almost) the same in observational sense) in between it and us be altered in any way by this? Would the disk size for instance be smaller or bigger as seen by an earth observer?I am not sure of the answer here, but once again the disk resolution of our dreams could be a nice sanity check for our body of theory. If the angular diameter of a strongly redshifted G2V star of a certain apparent brightness was consistent with the theory, it would reassure us that we have a good theory. If not, then back to the drawing board.

AFJ
2016-Jun-06, 10:46 AM
I am not sure of the answer here, but once again the disk resolution of our dreams could be a nice sanity check for our body of theory. If the angular diameter of a strongly redshifted G2V star of a certain apparent brightness was consistent with the theory, it would reassure us that we have a good theory. If not, then back to the drawing board.

Now that you mention it, aside from some unknown non-reddening stuff in between; what about stars that aren't highly (cosmologically) redshifted, the ones in the Milky Way for instance.

Classifying it by its spectrum as equal to the Sun and assuming equal disk size / diameter, wouldn't this be a normal logical check for that assumption? Or is this already done with the Hubble telescope or others?

publiusr
2016-Jun-10, 10:40 PM
A 'scope in a bubble
http://www.space.com/33121-giant-balloon-technology-near-space-telescopes.html