If you look at the supplementary information document, there's more pictures.
There's definitely quite a haze around light sources. It seems they have tried applying some deconvolution to try to increase the sharpness, but the resulting images are still quite muddled.
I really think they could have done better with `Visualization3.mp4`. The black and white ICCapture screen recording of an LED light (?) doesn't really do a whole lot to show off the achromatic capabilities of their lens.
"The key innovation lies in the microscopically small concentric rings that the researchers can pattern on the substrate. Unlike the ridges of FZPs (Fresnel Zone Plate), which are optimized for a single wavelength, the size and spacing of the flat lens' indentations keep the diffracted wavelengths of light close enough together to produce a full-color, in-focus image."
Yeah I read that, and upon re-reading it I have some vague idea on how that might work, but I would still need to read the paper and see some illustrations to really get it.
Sounds like this may enable full-colour astrophotography (always wondered why it's all in B&W!) and/or make achromatic telescopes a lot lighter.
I published a paper in this area a few years ago (but with a different lens design), so I think I can answer.
In conventional diffractive optics, the focal plane (where the sensor should be placed for a focused image) is wavelength-dependent. This means that you receive an in-focus image of the scene for a specific wavelength λ_i when your sensor is located a distance d_i from the lens [0]. By varying d_i you can scan the scene for all wavelengths even if your sensor is monochromatic. You get blurry versions of all other wavelengths as well, but the idea is to use computational techniques to separate these out.
Here, they've designed an optic which tries to eliminate the wavelength dependence of the lens focal plane. Instead of you place an RGB sensor at this one focal plane to get all the wavelengths.
You can use a regular bayer-matrix colour camera (or colour film) for astrophotography, it’s very common in amateur circles.
However, mono cameras and filters are also often used because the interesting colours are often not R,G,B of the visible spectrum but certain emission frequencies of hydrogen, silicon, oxygen and others. These are often combined to make false-colour images.
Filters also help penetrate light pollution by keeping only the frequencies you’re interested in and rejecting much of the light reflected by the atmosphere.
In addition, getting lenses to focus all frequencies of light equally well at the same time is quite tricky. For a mono camera with narrowband filters, this is less of a problem: you can focus precisely on one colour at a time and you can get away with simpler optics or cheaper glass.
The key point is that the sub-wavelength nature allows broader wavelength behavior and is distinct from conventional fresnel lenses. I suspect these plastic lenses will never be as sharp as a conventional lens, but presumably loads better than fresnel lenses and presumably worth it for weight-/size-restricted applications.
I hope they point it at the night sky and see a green star. That would be a very exciting event, as naturally green stars are not possible. I know that if I was an advanced "angler fish" type of civilization, that's definitely the lure I would use in a dark forest universe. on that note i hope the scientists are not stupid enough to send a signal to that star.
It's not really possible to make a green star because the color is mostly just determined by the temperature, with colder stars being redder and hotter being blue. A green star would just be a white star, or a star with the same temperature as the sun, whose peak output is in the green range, but having almost equal amounts of light in the blue and red range, appearing white. There is no way to "squish" the black body spectrum into a narrower range, it is determined by the Maxwell–Boltzmann distribution of velocities of particles in a gas.
That's my point. A green star would signal to anyone in the universe that something is really off here and most likely achieved by unnatural means (aka and advanced civilization). This would attract attention. which the civilization that sent the beacon can take advantage of for various reasons. There are many such beacons that could be placed all over the universe to attract attention.
I see what you're getting at. My thoughts on that topic are... it would be really hard to make a recognizable signal that crosses a significant fraction of the universe, or even galaxy. There could be a laser-pointer-green sun on the other side of the Milky Way, but a single sunlike star is already impossible to detect on the other side of the galaxy, much less in another galaxy.
95% of the brightest stars in the sky are less than 1000ly away, while the galaxy is 100,000ly across. The ones that are over 1000ly away and still visible are extremely large, unusual stars.
R136a1 is one of the brightest stars known, at almost 5 million times the luminosity of the sun, it's just outside the Milky Way in the LMC, and it required 3.6 meter telescopes to tell that it was multiple extreme stars around a single, very extreme star. In any telescope picture where R136a1 is visible, a regular sun-like star in the vicinity would be invisible. There are no images of "normal" stars more than 1000ly away because they just combine into an ambient glow of space.
The amount of energy needed to broadcast a signal to the entire galaxy (much less universe) requires something like a supernova, which... just looks like a supernova.
That is true they are feint to civilizations that don't have the capacity to view them because of technological constraints. Those are exactly the civilizations you don't care about anyways, as they wont have the capacity to visit the site at that technological level nor respond with a signal in that general direction. So the beacon you place is trying to attract attention from a specific tech civilization you are interested in (from their perspective). Also when i say beacon i mean the green star. But not an active beacon, you place a dump particle swarm around the star that refracts light in to the green spectrum that's not possible by natural star type of deal. So energy wise not too intensive once you get the particles around the star. But there could be other methods as well that have nothing to do with green stars. For example another "beacon" could be augmentation of some sort near the most interesting objects in the universe... the black holes at the heart of almost every galaxy. You know as a conscious entity that others will look in that general direction as that is the most interesting place in the "neighborhood" so it warrants to do something round those parts to attract attention.
Most likely they appear blue-green (a.k.a. turquoise a.k.a. teal), not green.
As another poster has mentioned, with increasing temperatures the color sequence for a black body is black => red => yellow => white => blue-green => blue.
Neither the greenish colors intermediate between yellow and blue-green nor the purplish colors intermediate between red and blue can appear as colors of a hot body, but only the colors that can be seen when low-pass filtering white light (i.e. between red and yellow, inclusive) or when high-pass filtering white light (i.e. between blue-green and blue, inclusive).
While above I have simplified by saying that purplish colors should not appear, that is only partially true, because very hot stars could appear as violet, i.e. reddish blue, due to the defect of human vision that the red photoreceptors are sensitive not only to red light, but also to light with a higher frequency than where the maximum sensitivity for blue is, so the high-frequency violet light excites both the blue and the red photoreceptors, being indistinguishable from a mixture of blue light with low-frequency red light.
With a video camera where the output signals would not be processed in such a way as to mimic the bugs of human vision (like in all normal color video cameras), purplish colors would not be obtained for hot bodies.
When you have double (or more) stars and gases involved, you can end up with a green-ish tint. Obviously, different people will describe the color in a different way due to everyone's own seeing conditions, instruments, and color sensitivity.
Oxygen has a strong green emission, so gases rich in oxygen may appear green, which is why many nebulae are green, like also most auroras in the Earth's atmosphere.
Therefore indeed a star behind gas rich in oxygen may appear green, but that green light comes from the gas, not from the star.
If you look at the supplementary information document, there's more pictures.
There's definitely quite a haze around light sources. It seems they have tried applying some deconvolution to try to increase the sharpness, but the resulting images are still quite muddled.
Here's one of the libraries they used: https://github.com/apsk14/rdmpy
Applied Physics Letters: https://pubs.aip.org/aip/apl/article/126/5/051701/3333379/Co...
I really think they could have done better with `Visualization3.mp4`. The black and white ICCapture screen recording of an LED light (?) doesn't really do a whole lot to show off the achromatic capabilities of their lens.
There is a related article from 2020 linked at bottom about flat lens cameras
https://phys.org/news/2020-03-focus-free-camera-flat-lens.ht...
It says it can focus everything at once. I don't understand how this works either but these two are probably similar science.
f/2. I wonder why they went with such a short focal ratio. Something inherent with Fresnel lenses?
How does it work?
"The key innovation lies in the microscopically small concentric rings that the researchers can pattern on the substrate. Unlike the ridges of FZPs (Fresnel Zone Plate), which are optimized for a single wavelength, the size and spacing of the flat lens' indentations keep the diffracted wavelengths of light close enough together to produce a full-color, in-focus image."
Yeah I read that, and upon re-reading it I have some vague idea on how that might work, but I would still need to read the paper and see some illustrations to really get it.
Sounds like this may enable full-colour astrophotography (always wondered why it's all in B&W!) and/or make achromatic telescopes a lot lighter.
I published a paper in this area a few years ago (but with a different lens design), so I think I can answer.
In conventional diffractive optics, the focal plane (where the sensor should be placed for a focused image) is wavelength-dependent. This means that you receive an in-focus image of the scene for a specific wavelength λ_i when your sensor is located a distance d_i from the lens [0]. By varying d_i you can scan the scene for all wavelengths even if your sensor is monochromatic. You get blurry versions of all other wavelengths as well, but the idea is to use computational techniques to separate these out.
Here, they've designed an optic which tries to eliminate the wavelength dependence of the lens focal plane. Instead of you place an RGB sensor at this one focal plane to get all the wavelengths.
0: https://ieeexplore.ieee.org/abstract/document/9191355?figure...
Could these help with field curvature as well? To make the focal plane more, uh, planar?
You can use a regular bayer-matrix colour camera (or colour film) for astrophotography, it’s very common in amateur circles.
However, mono cameras and filters are also often used because the interesting colours are often not R,G,B of the visible spectrum but certain emission frequencies of hydrogen, silicon, oxygen and others. These are often combined to make false-colour images.
Filters also help penetrate light pollution by keeping only the frequencies you’re interested in and rejecting much of the light reflected by the atmosphere.
In addition, getting lenses to focus all frequencies of light equally well at the same time is quite tricky. For a mono camera with narrowband filters, this is less of a problem: you can focus precisely on one colour at a time and you can get away with simpler optics or cheaper glass.
seems to me it's a Fresnel Lens, unfortunately the article is behind a paywall to say more
Here's a decent open-access review: https://doi.org/10.3390/sym16101377
The key point is that the sub-wavelength nature allows broader wavelength behavior and is distinct from conventional fresnel lenses. I suspect these plastic lenses will never be as sharp as a conventional lens, but presumably loads better than fresnel lenses and presumably worth it for weight-/size-restricted applications.
So in a sense they might become a new "Pareto frontier" in that particular trade-off?
I hope they point it at the night sky and see a green star. That would be a very exciting event, as naturally green stars are not possible. I know that if I was an advanced "angler fish" type of civilization, that's definitely the lure I would use in a dark forest universe. on that note i hope the scientists are not stupid enough to send a signal to that star.
It's not really possible to make a green star because the color is mostly just determined by the temperature, with colder stars being redder and hotter being blue. A green star would just be a white star, or a star with the same temperature as the sun, whose peak output is in the green range, but having almost equal amounts of light in the blue and red range, appearing white. There is no way to "squish" the black body spectrum into a narrower range, it is determined by the Maxwell–Boltzmann distribution of velocities of particles in a gas.
That's my point. A green star would signal to anyone in the universe that something is really off here and most likely achieved by unnatural means (aka and advanced civilization). This would attract attention. which the civilization that sent the beacon can take advantage of for various reasons. There are many such beacons that could be placed all over the universe to attract attention.
I see what you're getting at. My thoughts on that topic are... it would be really hard to make a recognizable signal that crosses a significant fraction of the universe, or even galaxy. There could be a laser-pointer-green sun on the other side of the Milky Way, but a single sunlike star is already impossible to detect on the other side of the galaxy, much less in another galaxy.
95% of the brightest stars in the sky are less than 1000ly away, while the galaxy is 100,000ly across. The ones that are over 1000ly away and still visible are extremely large, unusual stars.
R136a1 is one of the brightest stars known, at almost 5 million times the luminosity of the sun, it's just outside the Milky Way in the LMC, and it required 3.6 meter telescopes to tell that it was multiple extreme stars around a single, very extreme star. In any telescope picture where R136a1 is visible, a regular sun-like star in the vicinity would be invisible. There are no images of "normal" stars more than 1000ly away because they just combine into an ambient glow of space.
The amount of energy needed to broadcast a signal to the entire galaxy (much less universe) requires something like a supernova, which... just looks like a supernova.
https://en.wikipedia.org/wiki/R136a1
That is true they are feint to civilizations that don't have the capacity to view them because of technological constraints. Those are exactly the civilizations you don't care about anyways, as they wont have the capacity to visit the site at that technological level nor respond with a signal in that general direction. So the beacon you place is trying to attract attention from a specific tech civilization you are interested in (from their perspective). Also when i say beacon i mean the green star. But not an active beacon, you place a dump particle swarm around the star that refracts light in to the green spectrum that's not possible by natural star type of deal. So energy wise not too intensive once you get the particles around the star. But there could be other methods as well that have nothing to do with green stars. For example another "beacon" could be augmentation of some sort near the most interesting objects in the universe... the black holes at the heart of almost every galaxy. You know as a conscious entity that others will look in that general direction as that is the most interesting place in the "neighborhood" so it warrants to do something round those parts to attract attention.
On the other hand, if you could reliably trigger a supernova once every exactly once per year for 10 years, that's a strange and unique signal.
Why would they detect a green star ?
Some stars, eg Lambda B, do appear green when looking thru amateur telescopes.
Most likely they appear blue-green (a.k.a. turquoise a.k.a. teal), not green.
As another poster has mentioned, with increasing temperatures the color sequence for a black body is black => red => yellow => white => blue-green => blue.
Neither the greenish colors intermediate between yellow and blue-green nor the purplish colors intermediate between red and blue can appear as colors of a hot body, but only the colors that can be seen when low-pass filtering white light (i.e. between red and yellow, inclusive) or when high-pass filtering white light (i.e. between blue-green and blue, inclusive).
While above I have simplified by saying that purplish colors should not appear, that is only partially true, because very hot stars could appear as violet, i.e. reddish blue, due to the defect of human vision that the red photoreceptors are sensitive not only to red light, but also to light with a higher frequency than where the maximum sensitivity for blue is, so the high-frequency violet light excites both the blue and the red photoreceptors, being indistinguishable from a mixture of blue light with low-frequency red light.
With a video camera where the output signals would not be processed in such a way as to mimic the bugs of human vision (like in all normal color video cameras), purplish colors would not be obtained for hot bodies.
When you have double (or more) stars and gases involved, you can end up with a green-ish tint. Obviously, different people will describe the color in a different way due to everyone's own seeing conditions, instruments, and color sensitivity.
Oxygen has a strong green emission, so gases rich in oxygen may appear green, which is why many nebulae are green, like also most auroras in the Earth's atmosphere.
Therefore indeed a star behind gas rich in oxygen may appear green, but that green light comes from the gas, not from the star.