HN Theater @HNTheaterMonth

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

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.

⬐ staindk

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

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_...

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.

⬐ hatsunearu

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.

⬐ blauditore

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

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.

⬐ k__

How do stars look like in animal eyes?

⬐ yakubin

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

⬐ Maursault

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

⬐ rocqua

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

⬐ superjan

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.

⬐ macromaniac

The optical sensor on NIRcam has a cross on it https://en.m.wikipedia.org/wiki/NIRCam#Electronics my guess is this adds the 2 extra bars since I'd expect the arm to add only 1.

⬐ _xerces_

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

⬐ arendtio

Awesome video!

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

⬐ aaaaaaaaaaab

Not the lens. The aperture (i.e. pupil).

⬐ hatsunearu

I think the wikipedia explanation is much better.

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

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

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

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).

There is a nice Minute Earth video about this, for those who want a quick and entertaining way to get the concept: https://www.youtube.com/watch?v=VVAKFJ8VVp4