Proposed Telescope Focuses Light Without Mirror Or Lens

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.'"

Will they build it.

Great, but will it get build before I'm dead?

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"The only major obstacle being the materials used in building one."

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 re

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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

Looks like a sail...

Hmm, a large flat surface with holes in it.

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.

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Hmm, bad form to reply to my own post.

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 k

Much more fragile than a sail

I think you are missing a big point here. We're not talking about a solid sheet like a sail, but rather, a sheet which is X% holes, and for which the exact geometric arrangement of the holes is critical for the physics to work. Looks to me like one has eve

Re:Looks like a sail...

You can build ground based radio telescopes or satellite antennas using this technique. I have an old Radio Electronics with an article and plans for a greater than 4 foot refraction based satellite antenna using concentric strips of plywood with the focus behind the flat surface. The advantage lies in not having to form a curved three dimensional surface. The math is relatively straightforward.

The difference with the space based proposal is using optical wavelengths instead of radio wavelengths so the edge spacing is much smaller.

ok...

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

"anoptikon"

Problems

I think it would be clear to anyone who examines it, the idea clearly has some holes in it.

I'll say

FTFS:

The scientists suggest that an orbital 30-meter imager could resolve planets the size of Earth within 30 light-years.

O RLY?! I suppose they haven't considered how unbearably LONG 30 light years is. I'm certainly not prepared to wait that long. Besi

I discovered that as a kid ..

.. when I didn't have my glasses handy and still wanted to look at something in particular.

I would form a small hole by curling my index then look through it for visual correction to my myopea.

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You discovered the pinhole camera, aka tiny aperture = increased depth of field. This is different -- they actually have a large imaging aperture and still keep good focus.

Not for amateurs...

I was thinking hey neat till I read this in the article.

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]

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This is also somewhat complicated by the actual performance of objects in orbit. A project I worked on had two satellites in LEO - one main sat with a laser ranger, and one passive "following" sat with a corner cube. By ranging the distance between the two

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These large earth-finder telescopes are all being proposed for Lagrange points, not LEO. However, I do wonder how big the fudge factor is for being sufficiently close to the Lagrange. E.g., if these satellites are both +/- 15km with the actual point in the

This is crazy

So basically they're building A HUGE FRAKKIN' PINHOLE CAMERA. Frankly I find it strange that they would build a telescope that only collects 10% of the light, as this might present problems for planet finding. Not to mention that huge sheets of foil tend

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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

Re:This is crazy

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?

1. A) Not a Pinhole camera, It uses difraction caused by wave interaction through the holes of the lense.
2. B) The lens has an aperture of 30 meters, with a surface area of over 700 Square meters. Even at 10% transmission, it would have more than 15 time the light gathering power of the Hubble, and more than 150 times the resolution.
3. C) The best way to transport the lense would be to wrap the foil on a cylindrical spindle keeping it free of wrinkles, then having it unwound onto some kind of frame for mounting and stretching.
4. D) It would have to be placed in some kind of protection housing to prevent damage from space debris.
5. E) It would have to use some kind of laser/optical alignment system to get the lense and camera operating in conjunction. However this is not a big problem, long baseline interferometry in space would require much stricter positioning for constellations of satellites and such devices are already on the drawing boards.

In short, this is a perfectly viable technology, and it poses a fascinating solution to a really challenging problem.

Bravo!

more fun with diffraction

Canon has been using the same principle in a couple of lenses [canon.com] for some time now. The lenses themselves are pretty damn expensive but well regarded; I hope the telescope meets similar success.

Exoplanets

In the first 8 years of the 21st century I have witnessed an almost feverish acceleration of astronomer attention on the discovery of "exoplanets" [exoplanets.org] - planets around stars other than our own Sun. Already some solar systems very similar to our own have been d

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Some serious questions would need to be directed towards the SETI program.. as it seems highly unlikely that a modern society could exist without emanating some signals that SETI should have picked up.

Right now, SETI isn't really looking for "random" sign

Seems that there may be a little problem

This isn't a pinhole camera - it's a giant diffraction grating that acts as a lens. What I can't figure out is how they're going to keep the "lens" accurate; very small bumps / wrinkles in the foil would disrupt the operation of the lens, so it'd have to b

Only advantage is the light weight

The article makes it sound like only a 30-meter "Fresnel" optics can allow to resolve an earth-size object within 30 light-years.

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

The focusing of diffraction gratings is heavily wavelength dependent. The article makes it sound easy to shove in an extra Fresnel lens, but it's not that easy. Maybe it'd be better to use this only as a narrow band imager using suitable filters.

Overall, I like this idea a lot.