Is there a limit to how good a space based telescope can see? If we could build a mirror the size of a football field would it be possible to see planets orbiting other stars? And I don’t mean just as some dot, but something where you could see features. Or is there some limit, where no matter how big the telescope is, it won"t make any difference.
There is a kind of absurd limit; if you had a telescope with a sensor a light-year across, then light would be hitting different parts of it at different times, making it hard to get a focused picture.
Within that kind of limit, bigger is better. Also, you don’t have to have a solid sensor, but can have a composite of many smaller sensors, so that’s one good way to save engineering costs. But in general, the more light you capture, the better your image will be (perhaps after some data correction.)
Really big baselines would also give more accurate measurements of parallax, which would be nice. Also large-baseline interferometry would produce some fun results.
But that doesn’t require a humongous telescope. Nor does it require two, just one that moves.
To get a good picture of a revolving planet you need to gather a lot of photons over a short period of time. You can’t rely on a long exposure to gather a lot of photons, otherwise the image will be blurred.
It could be possible, once the period of rotation has been established, to take a snapshot once every revolution and ‘stack’ them; but this will mean that any rapidly moving cloud layer (if present) would become indistinct. From space the rapidly moving cloud layer is often the most conspicuous feature, so this kind of ‘stacking’ wont give an accurate picture.
Actually, not an issue. Light might hit different parts of the mirror at different times, but it’ll all take the same amount of time to reach the focus. This is in fact an inherent property of all focused optical systems.
Well, strictly speaking, it’s not true for segmented optics (e.g. Fresnel lenses) - it’s true within each facet, but different facets have different path lengths.
There’s also diffractive optics (gratings, zone plates, etc.)
But to answer the OP, no, there is no theoretical limit to how large a telescope can be. Resolution will increase linearly with aperture, if the telescope is diffraction-limited. That means the quality of the optics is good enough that the telescope’s performance is only limited by diffraction of light. For a telescope mirror, it means the mirror surface deviates from an ideal shape by no more than 1/8 of the wavelength of light. This becomes really difficult once the mirror becomes too large to be constructed as a single solid block of material. But that’s just an engineering problem, not theoretical. (In case you’re wondering, segmented telescopes like the Keck Telescope are not diffraction limited; resolution is only as good as each of the segments by itself.)
True…with complications, because, with some objects, while you’re moving, the object is moving too. But, yeah, most stellar proper motion isn’t enough to make that an issue. It might make your measurement of the position of Planet X a bit more difficult, but I suppose it would be something you could correct for.
There’s no fundamental difference between ground based and space based telescopes, save the location. Inside the atmosphere is a pretty terrible place to put a telescope, it turns out.
The Hubble Space Telescope is not a remarkable instrument in terms of its optics- a 2.4 meter Ritchey-Chretien reflector isn’t particularly big. There are numerous considerably bigger telescopes on the ground (like say… the 5.1m Hale Telescope), and have been for decades, but they’re handicapped to a great extent by the atmosphere.
Space telescopes are likely more able to perform a lot closer to their theoretical maximums than Earth-bound telescopes are, and that’s why a middling-sized telescope is so amazing in space, versus telescopes many times larger on Earth.
But the same theoretical limits apply in space as they do on Earth.
NASA studied ways to directly observe exoplanets, under the Terrestrial Planet Finder (TPF) project. The goal here is just to see the planet as a dot, separate from the star it’s orbiting. Which is actually very difficult because of the extreme difference in brightness. (Often compared to seeing a firefly perched next to a searchlight aimed at you.) One method is to use interferometry to cancel out the light from the star while seeing the planet. One concept was an array of four 4-meter telescopes.
They also did a concept study for a Terrestrial Planet Imager, which means resolving detail on the planet. This would need to be an interferometer array of interferometer arrays, placed thousands of miles apart.
There are practical engineering difficulties in making a single large telescope. Also the resolution required to resolve extra-solar planets is extreme.
The current best approach is using an optical interferometer, which is the optical equivalent of a radio telescope array: http://images.nrao.edu/images/VLASouth_med.jpg
Due to the extremely short wavelength relative to radio waves, optical interferometers are very difficult. Some have been used on terrestrial telescopes such as the Gemini
Planet Imager: Gemini Planet Imager - Wikipedia
The Very Large Telescope array in Chile can function as an interferometer, but cannot resolve earth-like extra-solar planets.
Terrestrial applications also require advanced imaging adaptive optics to compensate for atmospheric distortion: Adaptive optics - Wikipedia
The ideal approach might be a free-flying space interferometer array, which was discussed for NASA’s Terrestrial Planet Finder, which was cancelled in 2011: Terrestrial Planet Finder - Wikipedia
It is technically very challenging but probably feasible to eventually build a long-baseline, free-flying space optical interferometer of sufficient size to directly resolve extra-solar planets.
The 2006 paper “The Future of Space-Based Interferometry” by Kenneth Carpenter reviewed work and projections as of that date:
More recent studies indicate possible solutions to engineering hurdles in optical interferometry: A Dramatic Upgrade for Interferometry | Centauri Dreams
The referenced paper is “A Dispersed Heterodyne Design for the Planet
Formation Imager” (Ireland, Monnier, 2014):
http://planetformationimager.org/Planet_Formation_Imager_Project/Files/PFI_SPIE2014_Ireland.pdf
If you had the infrastructure for it, it might also be easier to manufacture an extremely large mirror in zero-g. I’m picturing a balloon hundreds of meters across with just a trace of pressure inside, and then blowing a bubble of molten glass inside of that. Let it cool, and then slice it up.
But that would be a huge pane to build.
That would produce a spherical rather than parabolic mirror. You’d have to machine it to a paraboloid somehow (probably infeasible), or add a lens to correct the spherical aberration.
–Mark
Seven years bad luck for that pun!
If you spun it while it was molten, you’d get a second order of approximation to a parabolic cross-section. Still not exact…but pretty close. A whole bunch of these, ten meters across, all ganged in an array, would be better than what we have now!
Sure, but the corrective optics could be much smaller than the primary.
People have already calculated the approximate telescope diameter to resolve detail on earth-like extrasolar planets. It is huge, far beyond any achievable monolithic or segmented mirror. Basically the only solution is using optical interferometry to achieve a large “virtual diameter”.
“A 100-pixel image of a planet twice the width of Earth some 16.3 light years away would require the elements making up a space telescope array to be more than 43 miles apart. Such pictures of exoplanets could make out details such as rings, clouds, oceans, continents, and perhaps even hints of forests or savannahs. Long-term monitoring could reveal seasonal shifts, volcanic events, and changes in cloud cover.”
There is another solution, using a gravity lens. If you could set up a telescope at a particular distance, out past the orbits of the planets the gravity of the Sun can be used as a vast lens.
However, that has the downside of being really hard (you need to not only get out there, but have enough fuel left to stop), and of there being only a really limited ability to change what you are looking at.
I don’t care what you metric system lovers may think, but giving the diameter of the 200" Hale telescope in meters is just wrong!
5.1 meters?! Blasphemy
I actually knew it was 200 inches, but since the Hubble’s given in meters, I looked up the Hale telescope’s diameter in meters to keep the same units.
Next thing we know, you’ll be talking about light-fortnights or some such other antiquated stuff.
Oh, I know that my monster bubble-scope would still be inadequate (at least by itself) for resolving details on planets. But there’s still a Heaven of a lot of astronomy you could do with a hundred-meter lightbucket.
And gravitational lensing is great for collecting photons, but gives a horribly distorted image.