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Proposed Telescope Focuses Light Without Mirror Or Lens
Posted by
Soulskill
on Fri May 02, 2008 02:08 AM
from the i-can-see-clearly-now dept.
from the i-can-see-clearly-now dept.
A team of scientists from Observatoire Midi Pyrénées in Toulouse, France have been working with an unusual technique for focusing light. It takes advantage of diffraction - the bending of waves when they encounter an obstacle in their path - to focus light as it passes through a foil sheet with precise holes in it. The scientists suggest that an orbital 30-meter imager could resolve planets the size of Earth within 30 light-years. In addition, the foil is much lighter than traditional materials, and thus easier to transport.
"A Fresnel imager with a sheet of a given size has vision just as sharp as a traditional telescope with a mirror of the same size, though it collects just 10% or so of the light. It can also observe in the ultraviolet and infrared, in addition to visible light. The imager can take very detailed images with high contrast, which is great for 'being able to see a very faint object in the close vicinity of a bright one.'"
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Will they build it. (Score:2, Insightful)
Re: (Score:3, Interesting)
I was under the impression that the main impediment to large refractors is the "halo" effect (coloured rings around the edge of the image), this was the problem Newton solved with the reflector and it is why Newtonian telescopes are the norm. The halo is unoticable with a small high-quality refractor (eg: binoculars) but the effect rapidly deteriorates the usefullness of refractors as the size increases.
No mention of wether this design
Re: (Score:3, Informative)
The gravity probes, as far as I am aware, do not have precisely synchronized flight, but very good knowledge of where each of them are. The science is extracted by measuring the changes in the spacecraft separation (I think the relative distance is known at the tens or hundreds of microns). Flying a separated telescope requires measuring and controlling separations and rotations to a level much more demanding than the GRACE satellites. In principle it can be done now (such as in the lab), but in practice
Looks like a sail... (Score:5, Interesting)
It looks like launching one of these babies would require solutions to the same technical problems as solar sails, ie stowing & unfolding once in orbit.
Would it be possible to have the sheet do double duty, acting as both a Fresnel "lens" and a means of propulsion for the spacecraft? That might be a neat way of getting the instruments to a good location.
Re: (Score:2)
I note that one objection raised in the article is that since the focal length of this thing is measured in kilometres, the instruments would have to be borne on a separate spacecraft to the focussing sheet, and that keeping the two aligned when changing the orientation of the instrument would require a lot of fuel.
This seems like it would be a perfect use for the solar sail technique; hopefully it would allow you to keep the instrument craft on a pretty much ballistic
Much more fragile than a sail (Score:2, Insightful)
Re:Looks like a sail... (Score:4, Informative)
The difference with the space based proposal is using optical wavelengths instead of radio wavelengths so the edge spacing is much smaller.
Parent
ok... (Score:5, Insightful)
Make a sphere with a central axis. Place the fresnel lens on the surface of the sphere. Rotate the sphere about the center (where the focal point is.) No more formation flying, etc. Since you don't need any part of the sphere but the place where the fresnel lens is, just create a radius - lens at one end, focal point at the other end. Use a track to adjust the focal point distance from the foil. Rotate the entire assembly to re-point. No formation flying. Precision alignment all the time. Slow adjustment means good fuel economy.
It seems to me that this is a great excuse for a foil-making plant in space. Imagine a veewwwwy large foil sheet. Then think of the available resolution. This is better than a dispersed array.
Well, one can hope. :-)
two words (Score:2)
Problems (Score:4, Funny)
I'll say (Score:3, Funny)
O RLY?! I suppose they haven't considered how unbearably LONG 30 light years is. I'm certainly not prepared to wait that long. Besides, we'll all be dead in 30 light years, what with the Hopi prophecy foretelling the end of time, and all.
While I'm here, let me get this out of the way, save us some time:
(joke) ------------->
(you)----> O__O
I discovered that as a kid .. (Score:3, Informative)
I would form a small hole by curling my index then look through it for visual correction to my myopea.
Re: (Score:2)
Not for amateurs... (Score:5, Interesting)
For one thing, the light comes to a focus far away from the foil sheet - with distances measured in kilometres, which means the camera and other instruments have to be mounted on a separate spacecraft. The instrument spacecraft would have to stay precisely aligned with the foil sheet, to within a millimetre or so.
Certainly not impossible, and still exciting, but this isn't going to be a mainstream or amateur tool any time soon.
Looks like there also may be a related patent to get past...
http://www.patentstorm.us/patents/6375326-claims.html [patentstorm.us]
Re: (Score:3, Insightful)
Re: (Score:3, Insightful)
Not an astronomer... yet.
-l
This is crazy (Score:2, Interesting)
Re: (Score:3, Informative)
10% of the light from a 30 meter telescope is the same amount of light as a regular 10 meter telescope. Hubble is a 2.4m telescope. I think it will have plenty of light.
Foil doesn't have to crinkle. Look at the center of a mylar balloon -- not exactly crinkly. Obviously if you want telescope-grade not-crinkly you'll have to spend a bit more, but that's not really a problem. This is also a bit more sophisticated than a pinhole camera -- those have trouble collecting much light.
Re:This is crazy (Score:5, Informative)
This is actually a really clever solution to a number of thorny problems. The first being, how do you get a really big telescope into space without breaking the bank??? Another being how do you get great contrast to show up faint sources?
In short, this is a perfectly viable technology, and it poses a fascinating solution to a really challenging problem.
Bravo!
Parent
more fun with diffraction (Score:3, Interesting)
Exoplanets (Score:2)
Re: (Score:2)
Right now, SETI isn't really looking for "random" signals. It's looking for signals deliberately sent our way, with plenty of power. So it wouldn't really be surprising if they're not picking up TV signals from Alpha Centauri.
Seems that there may be a little problem (Score:2)
This looks good on the drawing board but making a real-world example is going to require some very fancy engineering. Building larger scale structures in spac
Only advantage is the light weight (Score:3, Interesting)
The fact is that any conventional 30-meter telescope can resolve an earth-size object within 30 light-years (circa 6000Angstrom in wavelength). Spatial resolution can be determined by the ratio of wavelength to diameter of the optics:
6000A / 30m ~ 2e-8 radian ~ 0.004 arcsec.
So a 30m telescope can resolve an object in angular size of 0.004arcsec at 6000Angstrom.
At the distance of 30 light-years, the earth-size object looks like
6400km / 30lyr ~ 2e-8 radian ~ 0.004 arcsec.
So that's that. This telescope doesn't give us any special resolving power per optics size. So the advantage is merely its light weight.
Since the precise alignment of holes is required for this optics to work, I can see why this project got kicked out by ESA. It's probably too premature to attempt in deploying this kind of precision engineering in space today.
The chromatic aberration would be horrible (Score:3, Informative)
Overall, I like this idea a lot.