About the glow of stars in astronomical images

[longtry]

I've seen many pictures of galaxies and clusters, and in all of them there are diffraction spikes. It's explained to be caused by the vanes supporting the telescope's secondary mirror. Most of the images have 4 spikes, but some not taken by Hubble have 2. That begs the question of why 2 or 4 but not other numbers, like 3? 3 vanes is way more stable than 2 and almost as good as 4, while producing 25% less spikes than 4.

About the spikes themselves: are they taking light energy from the star in an image? In other words: suppose that there existed a technology that allows the secondary mirror to float precisely in place without any vane. Let's call the picture with that tech img2, while the Hubble one img1. Is the combined brightness of a star plus its spikes in img1 equal to that of the star in img2? Or more? Or less?

On a related note about the glow of stars in astro images: most of them look like this. I describe the glow as having some distinct concentric areas: one pure white in the center, the one surrounding it is mostly white but having some color, and one blue with thin black rings on the outside. Why do stars glow like that in images? Aren't they supposed to be points of light, which translate to just 1 pixel on screen? If it's true, then is what we're seeing the brightness 'overflowing' from that pixel into adjacent ones? What causes the black rings in the glow?

[Chris Peterson]

I've seen many pictures of galaxies and clusters, and in all of them there are diffraction spikes. It's explained to be caused by the vanes supporting the telescope's secondary mirror. Most of the images have 4 spikes, but some not taken by Hubble have 2. That begs the question of why 2 or 4 but not other numbers, like 3? 3 vanes is way more stable than 2 and almost as good as 4, while producing 25% less spikes than 4.

I don't think I've seen images with just two diffraction spikes. Telescopes with 3 vanes produce six spikes, and while not super common, we certainly see those. You also get six spikes from some of the big professional scopes that have hexagonal segment mirrors. And a four vane spider actually produces eight spikes, but they overlap and appear as four.

About the spikes themselves: are they taking light energy from the star in an image? In other words: suppose that there existed a technology that allows the secondary mirror to float precisely in place without any vane. Let's call the picture with that tech img2, while the Hubble one img1. Is the combined brightness of a star plus its spikes in img1 equal to that of the star in img2? Or more? Or less?

Yes, the light in the spike is coming from the star. Without diffraction the stars would appear just a little brighter. But the amount of energy in the spikes is just a tiny fraction of the total- it only appears otherwise in many images because of the non-linear intensity transformations that are applied. And there are certainly telescopes that don't produce diffraction spikes- refractors, of course, but also SCTs and Maksutovs, which have the secondary mounted on a glass surface at the aperture. These still produce diffraction, but not in the form of linear spikes.

On a related note about the glow of stars in astro images: most of them look like this. I describe the glow as having some distinct concentric areas: one pure white in the center, the one surrounding it is mostly white but having some color, and one blue with thin black rings on the outside. Why do stars glow like that in images? Aren't they supposed to be points of light, which translate to just 1 pixel on screen? If it's true, then is what we're seeing the brightness 'overflowing' from that pixel into adjacent ones? What causes the black rings in the glow?

That image is showing the diffraction rings produced by the circular aperture. The color is largely meaningless- the white is simply saturated pixels, so can't show any color. As the rings become dimmer moving outwards, you can see color. FWIW, that image may be unusual. It is showing Sirius and its companion, and in order to do that it may be using a special type of mask on the aperture, and that may result in a strong diffraction pattern.

[longtry]

Thank you Chris!

I don't think I've seen images with just two diffraction spikes. Telescopes with 3 vanes produce six spikes, and while not super common, we certainly see those. You also get six spikes from some of the big professional scopes that have hexagonal segment mirrors. And a four vane spider actually produces eight spikes, but they overlap and appear as four.

If possible, could you please lead me to a source that explain the phenomenon in details, with pictures illustrating the light paths? I have trouble connecting the diffracting waves into patterns seen in astro images, and a quick google only leads to articles that display the end results of diffraction on images.

Yes, the light in the spike is coming from the star. Without diffraction the stars would appear just a little brighter. But the amount of energy in the spikes is just a tiny fraction of the total- it only appears otherwise in many images because of the non-linear intensity transformations that are applied. And there are certainly telescopes that don't produce diffraction spikes- refractors, of course, but also SCTs and Maksutovs, which have the secondary mounted on a glass surface at the aperture. These still produce diffraction, but not in the form of linear spikes.

What do you think is the best type of telescope if an important goal is to have the diffraction not interfere with observing faint background objects?

That image is showing the diffraction rings produced by the circular aperture. The color is largely meaningless- the white is simply saturated pixels, so can't show any color. As the rings become dimmer moving outwards, you can see color. FWIW, that image may be unusual. It is showing Sirius and its companion, and in order to do that it may be using a special type of mask on the aperture, and that may result in a strong diffraction pattern.

Oh, I didn't realize that. Anyway, does that mean the sensor and software are using the 'overflowing' method where pixels otherwise not getting any real light are receiving redundant signal from overly saturated nearby pixels?