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.'"

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.

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 [patentstorm.us]

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."

more fun with diffraction

[heeeraldo]

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.

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.

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.

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.