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adi11235
2010-Oct-28, 03:02 PM
I was wondering if zone plates can be built to replace mirrors (or radio dishes) currently or sometime in the near future.

Telescope could focus light without a mirror or lens (http://www.newscientist.com/article/dn13820)

Given that a flat reflector or grating can be made much larger than a parabolic shape, wouldn't it work to just have a huge plastic sheet? I was also thinking of reconfigurable arrays made out of OLEDs, provided the "pixels" can be made small enough.

RAF_Blackace
2010-Oct-29, 11:42 PM
Why can a flat mirror be made much larger than a parabolic shape ? I always understood a flat plane is much harder to create, a parabola is natural and you can grind a fairly good mirror in your shed. It would be hard for even the best labs on Earth to make a flat mirror to the same tolerances regardless of the size.

cjameshuff
2010-Oct-30, 02:30 AM
Why can a flat mirror be made much larger than a parabolic shape ? I always understood a flat plane is much harder to create, a parabola is natural and you can grind a fairly good mirror in your shed. It would be hard for even the best labs on Earth to make a flat mirror to the same tolerances regardless of the size.

It's not a mirror, it's a zone plate. It's a foil sheet with a pattern of holes cut in it, or a transparent plastic film with an opaque pattern on it. Light diffracts through it and interferes with itself on the other side to result in a focused image.

Parabolas occur naturally in some situations but not in others...spherical lenses and mirrors are common because they're easier to make. This thing doesn't have to be ground, cast, or assembled from precision parts, it just has to be unfolded.

cjameshuff
2010-Oct-30, 02:44 AM
Given that a flat reflector or grating can be made much larger than a parabolic shape, wouldn't it work to just have a huge plastic sheet?

The article seems to answer your question...it mentions a possible 30 meter imager.



I was also thinking of reconfigurable arrays made out of OLEDs, provided the "pixels" can be made small enough.

OLEDs would not be much use...do you mean LCDs? LCD-based spatial light modulators have been used in holographic devices. They might be useful for adjusting to different wavelengths...a relatively simple LCD element arrangement would suffice, just a large number of concentric rings.

Could have uses beyond telescopes...you could use such a zone plate in a camera to adjust to different focal planes and build up an image with high depth of field, with no mechanical parts.