The Photon Sieve Could Revolutionize Optics

Ever since astronomers first began using telescopes to get a better look at the heavens, they have struggled with a basic conundrum. In addition to magnification, telescopes also need to be able to resolve the small details of an object in order to help us get a better understanding of them. Doing this requires building larger and larger light-collecting mirrors, which requires instruments of greater size, cost and complexity.

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CrazyNeutrino said:

The Photon Sieve Could Revolutionize Optics

Ever since astronomers first began using telescopes to get a better look at the heavens, they have struggled with a basic conundrum. In addition to magnification, telescopes also need to be able to resolve the small details of an object in order to help us get a better understanding of them. Doing this requires building larger and larger light-collecting mirrors, which requires instruments of greater size, cost and complexity.

more…

One always ponders upon the Frazier lens.

> need to be able to resolve the small details of an object in order to help us get a better understanding of them. Doing this requires building larger and larger light-collecting mirrors

Or an interferometer.

> Basically, the photon sieve is a variation on the Fresnel zone plate, a form of optics that consist of tightly spaced sets of rings that alternate between the transparent and the opaque. Unlike telescopes which focus light through refraction or reflection, these plates cause light to diffract through transparent openings. On the other side, the light overlaps and is then focused onto a specific point – creating an image that can be recorded.

I hadn’t heard of this. It seems that the result would have lower light gathering power than a Fresnel Lens, but potentially higher resolution at a much lower cost. Last time I went to an astronomy lecture, in question time I suggested that the Fresnel Lens would be a good alternative that combines the light weight of the telescope mirror with the wide angle possible with lenses. The Fresnel zone plate would be cheaper and may be as good or better.

> Although such a device would be potentially useful at all wavelengths

But if I understand the optics correctly, different wavelengths would focus at different focal points, so multiple exposures would be needed to build up a coloured image. (Let me check. Looking up https://en.wikipedia.org/wiki/Zone_plate). Not so good. The necessary spacing of rings depends nonlinearly on the wavelength, you can’t get away with using the same zone plate again with a different focus length. So a single zone plate will only work with a single wavelength, you’d need a different zone plate for each wavelength – not an insurmountable obstacle, just slightly annoying to change the lens every time you change the filter.

> However, in the long-run, they hope to create a sieve that will measure 1 meter (3 feet) in diameter. With an instrument of this size, they believe they will be able to achieve up to 100 times better angular resolution in the ultraviolet than NASA’s high-resolution space telescope – the Solar Dynamics Observatory. This would be just enough to start getting some answers from the Sun’s corona.

Nice.

I’ve forgotten the theoretical physics in 35 years of clinical practice, but in the real world, Fresnel lenses do not give anything like the image quality of an ordinary ground lens. There have been Fresnel prisms for very specialised uses in clinical optics. They fail more often than not. Some of the new intra-ocular lenses for cataract surgery are Fresnel in order to get multifocal qualities. Yes, you get that. But the brain has to completely relearn how to see (yes, I get that’s not a problem with telescopes), the patients tend to be older, and there are diffraction side effects with night driving.

buffy said:

I’ve forgotten the theoretical physics in 35 years of clinical practice, but in the real world, Fresnel lenses do not give anything like the image quality of an ordinary ground lens. There have been Fresnel prisms for very specialised uses in clinical optics. They fail more often than not. Some of the new intra-ocular lenses for cataract surgery are Fresnel in order to get multifocal qualities. Yes, you get that. But the brain has to completely relearn how to see (yes, I get that’s not a problem with telescopes), the patients tend to be older, and there are diffraction side effects with night driving.

In glasses and contact lenses, I can see why that might be, though I don’t fully understand why. Fresnel lens transfers for the back windows of vans work beautifully.

mollwollfumble said:

buffy said:

I’ve forgotten the theoretical physics in 35 years of clinical practice, but in the real world, Fresnel lenses do not give anything like the image quality of an ordinary ground lens. There have been Fresnel prisms for very specialised uses in clinical optics. They fail more often than not. Some of the new intra-ocular lenses for cataract surgery are Fresnel in order to get multifocal qualities. Yes, you get that. But the brain has to completely relearn how to see (yes, I get that’s not a problem with telescopes), the patients tend to be older, and there are diffraction side effects with night driving.

In glasses and contact lenses, I can see why that might be, though I don’t fully understand why. Fresnel lens transfers for the back windows of vans work beautifully.

Perhaps the quality of image required is not so critical?

(Are you still around…I’ve sent you an email)

(Are you still around…I’ve sent you an email)

:-) There are times when I can’t get to the computer.

mollwollfumble said:

> However, in the long-run, they hope to create a sieve that will measure 1 meter (3 feet) in diameter. With an instrument of this size, they believe they will be able to achieve up to 100 times better angular resolution in the ultraviolet than NASA’s high-resolution space telescope – the Solar Dynamics Observatory. This would be just enough to start getting some answers from the Sun’s corona.

Nice.

I only just noticed the word “ultraviolet”. Interesting choice for several reasons.
1) There are a hundred or so IR telescopes around but very few in UV.
2) Because UV has a shorter wavelength you can get very much better resolution in UV than you can in IR.
3) Making a Fresnel in UV would be more difficult than in IR and V, but possible through the use of nanotech.

That’s three very good reasons for choosing UV.

On the negative side, how do they handle atmospheric turbulence? In IR adaptive optics is easy to use but that becomes more difficult as the wavelength decreases. So either they’re planning world beating adaptive optics or, more likely, going with multiplayer stacking together with “lucky imaging”.

> multiplayer stacking

oops.
multilayer stacking.