First flat telescope lens that captures color, detecting light from stars
phys.org
phys.org
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
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.
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.
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.
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...
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.
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.
Therefore indeed a star behind gas rich in oxygen may appear green, but that green light comes from the gas, not from the star.
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.