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I was wondering why the star has 6 crisp points and found an explanation here (a 3 minute video): https://www.youtube.com/watch?v=VVAKFJ8VVp4 https://www.youtu

I was wondering why the star has 6 crisp points and found an explanation here (a 3 minute video): https://www.youtube.com/watch?v=VVAKFJ8VVp4 https://www.youtube.com/watch?v=VVAKFJ8VVp4 The most interesting part of the video explains why even your naked eye viewing the sky at night will cause this effect -- it's due to imperfections in the lens of your eye.

yakubin 5y ago
To me it looks like this effect: <https://www.theclickcommunity.com/blog/creating-starbursts-in-your-photographs-tutorial/ https://www.theclickcommunity.com/blog/creating-starbursts-i...>
_xerces_ 5y ago
From another article I read, it appears that most of the pattern is due to diffraction from the edges of the hexagonal mirrors.
deleted 5y ago
[deleted]
superjan 5y ago
I see 8. With the smaller horizontal ones likely coming from one of arms of the secondary mirror (check the “selfie” image). The linked video is nice, by the way.
- deleted 5y ago
  [deleted]
arendtio 5y ago
Awesome video! Made me laugh and learn at the same time :-)
k__ 5y ago
How do stars look like in animal eyes?
staindk 5y ago
Good summary but I found it a bit weird how only 2 seconds were spent on the 'camera aperture' explanation - as AFAIK that's the main cause for photos of stars and lights to have 'points' - it depends on how many 'leaves' (? term) your camera/lens aperture has -> the more leaves the "more-sided" a shape the aperture makes (hexagon, octagon...) thus the more points you get when photographing lights.
- Green_man 5y ago
  The number of rays is determined by the number of aperture blades, as well as whether there's an even or odd number of aperture blades https://phillipreeve.net/blog/best-lenses-for-sunstars/#The_basics https://phillipreeve.net/blog/best-lenses-for-sunstars/#The_... Interestingly, some modern photography lenses have achieved aperture mechanisms with much rounder geometry, sometimes with near perfect circles at multiple apertures. This can result in a more desirable bokeh, at the cost of well defined sun stars.
  - blauditore 5y ago
    The article fails to explain that, but round aperture is equivalent to infinitely many straight blades (and thus rays), so there is a light halo instead of distinct rays.
    - Sharlin 5y ago
      Which is essentially also why you get more diffraction limited as you stop down the aperture -- a larger and larger fraction of the light that gets through passes near the edges (and gets diffracted) rather than the central area.
  - hatsunearu 5y ago
    yup, but when you step down the aperture a lot (which is when the sun star effect becomes more pronounced), the aperture transitions from more circular to more polygonal, so with a lot of lenses, the behavior is that when your aperture is a few steps of fully open, it is basically circular but when you need the sun starts it's definitely there.
hatsunearu 5y ago
I think the wikipedia explanation is much better. https://en.wikipedia.org/wiki/Diffraction_spike https://en.wikipedia.org/wiki/Diffraction_spike One deficiency is why apertures (effectively, the support structure in a telescope is an aperture) observe this behavior. > No matter how fine these support rods are they diffract the incoming light from a subject star and this appears as diffraction spikes which are the Fourier transform of the support struts. Like why is that the case? https://en.wikipedia.org/wiki/Airy_disk https://en.wikipedia.org/wiki/Airy_disk Read this if you don't immediately understand why. This is the shape of the image you see when the aperture is circular. This is the Fourier transform of a circular aperture, which, in 1D, is a sinc function. I can't give a straightfoward answer to elucidate further, but if you've done signal processing stuff before you can probably handwavey explain that if there is a Fourier-transform relationship with one function, due to linearity and spacial invariance, you can say that it holds for all functions.
- pmoriarty 5y ago
  So we're not actually seeing the image of a star but only its Fourier transform? I presume that only happens if the star is far enough away, as there are plenty of images of our Sun (which is much closer) that don't look like this.
  - Tagbert 5y ago
    For larger objects with a larger subtended angle, we are probably seeing the union of the Fourier transforms of each “pixel” (I’m not sure what the smallest element would be, in this case).
Maursault 5y ago
I wonder what they would look like if stars were pointed objects.
- rocqua 5y ago
  The same. Because any shape to a star is too small to be detectable with these kinds of resolution.

yakubin 5y ago

To me it looks like this effect: <https://www.theclickcommunity.com/blog/creating-starbursts-in-your-photographs-tutorial/ https://www.theclickcommunity.com/blog/creating-starbursts-i...>

_xerces_ 5y ago

From another article I read, it appears that most of the pattern is due to diffraction from the edges of the hexagonal mirrors.

deleted 5y ago

[deleted]

superjan 5y ago

I see 8. With the smaller horizontal ones likely coming from one of arms of the secondary mirror (check the “selfie” image). The linked video is nice, by the way.

deleted 5y ago
[deleted]

arendtio 5y ago

Awesome video! Made me laugh and learn at the same time :-)

k__ 5y ago

How do stars look like in animal eyes?

staindk 5y ago

Good summary but I found it a bit weird how only 2 seconds were spent on the 'camera aperture' explanation - as AFAIK that's the main cause for photos of stars and lights to have 'points' - it depends on how many 'leaves' (? term) your camera/lens aperture has -> the more leaves the "more-sided" a shape the aperture makes (hexagon, octagon...) thus the more points you get when photographing lights.

Green_man 5y ago
The number of rays is determined by the number of aperture blades, as well as whether there's an even or odd number of aperture blades https://phillipreeve.net/blog/best-lenses-for-sunstars/#The_basics https://phillipreeve.net/blog/best-lenses-for-sunstars/#The_... Interestingly, some modern photography lenses have achieved aperture mechanisms with much rounder geometry, sometimes with near perfect circles at multiple apertures. This can result in a more desirable bokeh, at the cost of well defined sun stars.
- blauditore 5y ago
  The article fails to explain that, but round aperture is equivalent to infinitely many straight blades (and thus rays), so there is a light halo instead of distinct rays.
  - Sharlin 5y ago
    Which is essentially also why you get more diffraction limited as you stop down the aperture -- a larger and larger fraction of the light that gets through passes near the edges (and gets diffracted) rather than the central area.
- hatsunearu 5y ago
  yup, but when you step down the aperture a lot (which is when the sun star effect becomes more pronounced), the aperture transitions from more circular to more polygonal, so with a lot of lenses, the behavior is that when your aperture is a few steps of fully open, it is basically circular but when you need the sun starts it's definitely there.

hatsunearu 5y ago

I think the wikipedia explanation is much better. https://en.wikipedia.org/wiki/Diffraction_spike https://en.wikipedia.org/wiki/Diffraction_spike One deficiency is why apertures (effectively, the support structure in a telescope is an aperture) observe this behavior. > No matter how fine these support rods are they diffract the incoming light from a subject star and this appears as diffraction spikes which are the Fourier transform of the support struts. Like why is that the case? https://en.wikipedia.org/wiki/Airy_disk https://en.wikipedia.org/wiki/Airy_disk Read this if you don't immediately understand why. This is the shape of the image you see when the aperture is circular. This is the Fourier transform of a circular aperture, which, in 1D, is a sinc function. I can't give a straightfoward answer to elucidate further, but if you've done signal processing stuff before you can probably handwavey explain that if there is a Fourier-transform relationship with one function, due to linearity and spacial invariance, you can say that it holds for all functions.

pmoriarty 5y ago
So we're not actually seeing the image of a star but only its Fourier transform? I presume that only happens if the star is far enough away, as there are plenty of images of our Sun (which is much closer) that don't look like this.
- Tagbert 5y ago
  For larger objects with a larger subtended angle, we are probably seeing the union of the Fourier transforms of each “pixel” (I’m not sure what the smallest element would be, in this case).

Maursault 5y ago

I wonder what they would look like if stars were pointed objects.

rocqua 5y ago
The same. Because any shape to a star is too small to be detectable with these kinds of resolution.