Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: Geezer on 09/07/2012 02:20:45
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Phew! You know it's hot when the water coming out of the garden hose burns your hand, but I digress. This is an old hobby-horse of mine. It's been around here before, but I thought it might be worth a re-run during the dogdays.
We know how to create really enormous radio-telescopes by combining the signals from arrays of smaller receivers. Why can't we do the same thing at optical frequencies to synthesize gigantic optical telescopes?
I realize there are a few "details" that need to be worked out because of the rather high frequencies involved, but, other than some technological barriers, is there any fundamental reason why we can't build an optical array of any arbitrary size? (It would not necessarily be terrestrial.)
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We know how to create really enormous radio-telescopes by combining the signals from arrays of smaller receivers. Why can't we do the same thing at optical frequencies to synthesize gigantic optical telescopes?
Been done ... https://en.wikipedia.org/wiki/List_of_astronomical_interferometers_at_visible_and_infrared_wavelengths (https://en.wikipedia.org/wiki/List_of_astronomical_interferometers_at_visible_and_infrared_wavelengths)
e.g. https://en.wikipedia.org/wiki/Cambridge_Optical_Aperture_Synthesis_Telescope
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Well yes, but how large can they be, and what limits the resolution?
EDIT: Come to think of it, infrared ain't exactly "optical". The BBC long-wave transmission could be described as infrared.
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I think it depends on what you are looking at. [:o]
One of the basic requirements of a telescope is that the image striking any part of the mirror has to be essentially the same. There may be a limit on the size of the telescope based on the focal size and distance. So, for example you were looking at the moon, or at nearby asteroids, then a telescope beyond a few meters may have some of a binocular effect, and be out of focus.
Looking at very distant objects may be so far red-shifted that the incoming light would lie outside of the visible spectrum. Still, IR might be considered optical, but what about microwaves? There is also significant atmospheric interference in the IR spectrum, although apparently there are telescopes that use IR.
Is there a limit on the size of casting a single piece of glass/pyrex/quartz? 10 meters? Of course, that doesn't prevent segmented mirrors.
Anyway, my dream would be to see extremely large telescopes on moon. Perhaps a km or more in diameter. I'd probably place them in craters, for protection and temperature/ambient control. There would be many advantages of very large telescopes on the moon. Fewer storms, less gravity to interfere with the secondary mirror, no atmospheric interference, broader spectrum of incoming light, less light pollution, slower planetary rotation, etc.
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Ah! This is more along the lines of what I had in mind. I found it in one of the links RD posted.
https://en.wikipedia.org/wiki/Fast_Fourier_Transform_Telescope (https://en.wikipedia.org/wiki/Fast_Fourier_Transform_Telescope)
I'm not sure what the detectors are. I was thinking it might be possible to have no optical elements at all and recieve the light with nanoantennae so that the whole thing was actually a radio telescope, but that may well be a very bad idea.
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I think one of the limits on the EM spectrum is that the wavelength determines the pixel size.
So, microwaves, with a wavelength in the cm range might be about as big as one could practically make the pixels for a single receiver. I suppose that if one made a 1km dish, then one might attempt to pick up longer wavelength EM. Meter range, or even a few meters?
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I well remember the anticpation of the construction of the giant 200" (6m) telescope that was completed in 1947 now 40 meters is on the way.
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ClffordK.
I believe the Arcibo 300m radio telescope can be operated from 3cm to 30cm this being the highest resolution single dish radio telescope ever kikely to be constructed.
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now 40 meters is on the way.
Sadly, I suspect it will be delayed by economics.