Optics Question: Why Do Stars (Hubble Photos) Appear 4-Pointed?

If you look at high definition photos from the Hubble space telescope, the starts appear four pointed-why is this?

There is this thing called diffraction. Simple explanation is it light diffracting (scattering if you will) off of edges.

Imagine a telescope that is just a big lens. The only diffraction is off the edges of the lens as it focuses a star to a “point”. This diffraction actually is what defines how small a point star can be focused to (and the big the lens the smaller the point but thats a different issue).

Now imagine a lens that has lets say a wire runing across it.

It will now have an image of a star that is a finite point. But it will also have some diffracted light perpendicular to the wire.

Two wires perpendicular to the center equals two multidirection diffractions which equals 4 spikes.

Three wires equals 6 spikes.

The Hubble is a newtonion in which part of the optics are “obstructing” the view of the “lens” (in this case a mirror but it doesn’t make any difference).

Well that thing in the way (in this case what is called a secondary) HAVE to be supported by something.

And in this case it two perpendicular thin piece of metal oriented sideways. But they are still there.

Therefore the spikes.

And to make things more complicated, even if that thing in front of the lens/mirror was just floating out there without any support it would still impact the actual star image (minus spikes, it would just make the image more bloated).

That’s one cause, and I think it’s the dominant one for the Hubble. Another potential cause for that sort of thing, though, is CCD bleed. The pixels in a CCD (light detector, like in a digital camera) are arranged in a grid, and if you get too much light on one pixel, it can bleed out to other pixels on that row and column of the grid.

I think you’re referring to blooming. Which can happen, but usually produces an assymetric vertical streak as shown in this link, not a symmetric 4-pointed star.

The 4-pointed star is definitely due to diffraction from the secondary mirror support. Many astronomers hate these diffraction spikes, and some use curved spiders which eliminates it. Well, it doesn’t actually eliminate diffraction - it’s just spread out as faint circular halo rather than being concentrated into 4 sharp spikes.

p.s. This image from the Solar Dynamics Observatory shows both. The 8 diagonal streaks are diffraction spikes. In this case, the diffraction mostly comes from a wire mesh that supports a very thin filter. There are two sets of filters, and they aren’t aligned to each other, so we end up with 2 sets of 4 spikes. There is also a secondary mirror support, but that, I believe, is aligned to one of the filters.

You can also see a shorter vertical streak. That’s the blooming that Chronos mentioned.