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.'"

Comments

Will they build it.

[FiestaFan]

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

Re:Will they build it.

[TapeCutter]

"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 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 suffers from the halo effect but radically new scope designs are rare and I like their thinking!

Of course with such a long focal length a large scope of this design would have to be space based but I don't see any insurmountable problems lanching and deploying such a beast in two parts, except of course the usual cost/benifit arguments. As for the objections elsewhere in this thread that a two part scope would drift out of sync, precisely syncronised space flight been already been done with a pair of gravity probes. Besides we also have something called adaptive optics.

Re:Will they build it.

[ArAgost]

Chromatic aberration is usually (in everyday optics) caused by refraction. Of course, since IIRC different wavelenghts diffract differently, there will be some problem of this kind, but still it's a neat idea.

Re:Will they build it.

[Malekin]

Any aperture will cause diffraction. Reflector or refractor. The halos aren't visible in binoculars because they have magnification ratios too small.

Reflectors are preferred over refractors because it's cheaper and easier to make a large mirror than it is to make a large set of refracting optics. A larger diameter aperture will result in less diffraction but the primary motivation for large diameter scopes (and thus the popularity of reflector designs) is that a large diameter is a large "light bucket". The more light you capture, the more (dimmer) objects you can observe.

Re:Will they build it.

[hubie]

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 it is very challenging (at least to do on a reasonable budget) which is why many of the NASA and ESA separated telescope projects have been drastically scaled back or delayed (SIM, TPF, Darwin, etc.).

In general, long focal lengths aren't that much of a problem because of the many telescope designs that fold up the optical path.

Looks like a sail...

[sapphire wyvern]

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.

Re:Looks like a sail...

[Agripa]

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...

[fyngyrz]

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. :-)

Problems

[FearForWings]

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

I discovered that as a kid ..

[Saffaya]

.. 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.

Not for amateurs...

[syousef]

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

Re:Not for amateurs...

[Overzeetop]

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, the earths gravitational field could be mapped very accurately. In other words, two satellites in the exact same orbit will vary in distance with one another constantly throughout an orbit based on the gravitational field. As the orbit precesses, the variation will change from orbit to orbit.

I don't know how this would be dealt with, but it's a bit of a potential stumbling block. (well, that and getting a thin, light, high precision piece of anything into orbit without damaging it)

Re:Not for amateurs...

[Luyseyal]

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 middle, will the shearing effects of gravity be too much for attitude correction for such a sensitive scope?

Not an astronomer... yet.
-l

This is crazy

[Plazmid]

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 to crinkle and are susceptible to micro-meteoroids. But, if they could make it cheap enough, they could launch a bunch of them and do "brute force astronomy."

Re:This is crazy

[evanbd]

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

[Genda]

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!

Much more fragile than a sail

[Mathinker]

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 even started to think about how it can survive the stresses of being launched at multiple G's.

more fun with diffraction

[heeeraldo]

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

Only advantage is the light weight

[helioquake]

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.

I'll say

[thegnu]

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. 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

The chromatic aberration would be horrible

[exp(pi*sqrt(163))]

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.