Naked Science Forum
General Science => General Science => Topic started by: Seany on 03/09/2007 13:48:32
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Like the one where we see the sun extra big?
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We call them 'powerful' telescopes but they are, really, SENSITIVE telescopes. Magnifying an image of the Sun is dead easy - there is loads of energy available. The real problem comes when you want to see a really distant / faint object. To make what we call a powerful telescope, it needs to be wide enough to gather lots of energy from the wanted direction. There is another issue, too, in that light is a wave and gets diffracted as it goes through a telescope lens. It blurrs the image and makes it hard to 'resolve' distant details. A wide telescope (a big 'aperture') solves this problem too.
Basically, powerfull means big and very expensive!
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The problems are not only diffraction in the lens, but diffraction in the upper atmosphere (for which they try and produce adaptive lenses).
You have another trade off, as you can obtain more light through a larger, and heavier (and more difficult to move) lens; or you can do the same by just increasing the exposure time (but that means you have to be able to keep the lens locked onto the object you want to see for a long time, and you get even more problems with atmospherics, and you have greater problems with thermal noise).
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I believe the most powerful telescopes are radio telescopes. They can be linked in an array to mimic a telescope with the diameter of the array. Radio telescopes on opposite sides of the Earth have been linked so that they act as if it were 1 really huge telescope the diameter of the planet.
Here's a link to info on the VLA (Very Large Array) & EVLA (Extended Very Large Array) in New Mexico...
http://www.vla.nrao.edu/ (http://www.vla.nrao.edu/)
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I believe the most powerful telescopes are radio telescopes. They can be linked in an array to mimic a telescope with the diameter of the array. Radio telescopes on opposite sides of the Earth have been linked so that they act as if it were 1 really huge telescope the diameter of the planet.
Here's a link to info on the VLA (Very Large Array) & EVLA (Extended Very Large Array) in New Mexico...
http://www.vla.nrao.edu/ (http://www.vla.nrao.edu/)
Certainly very large synthetic aparture radio telescopes do exist, but the also operate on very large wavelengths (radio telescope in the 2cm wavelength would need to be 40,000 larger than an optical telescope with the same resolving power).
A radio telescope with a synthetic aperture of 12,000Km (the diameter of the Earth at the equator) would have the same resolving power as an optical telescope with a 300 metre aperture. While this is certainly larger than any present optical telescope, I doubt it will be larger than future optical telescopes (at present, the limit for optical telescopes seems to be about 10m, but 50m and larger are in planning and construction, with 100m optical telescopes currently merely being hypothetical proposals. I would imagine that within a few decades we would have optical telescopes in the region of 300m (possibly even synthetic aperture optical telescopes).
It should be added that while synthetic apertures will increase resolution (i.e. magnification, and increased resolving power) they will not themselves increase sensitivities.
It should be noted that synthetic apertures are not limited to the size of the Earth, and it is possible to get some useful information from synthetic apertures based on the Earth's rotation around the Sun.
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I was talking in the present tense. But you're right, undoubtedly in the future there will be opticals more powerful.
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There is a new multi telescope optical grouping in Chile that uses synthetic aperture between multiple optical telecopes