![]() But more often than not, the blades are straight and the resulting opening is polygon-shaped. If the blades are curvy, they create circular opening that gives no diffraction spikes. (This is similar to how the iris in our eye controls the size of the pupil.) The most commonly used aperture diaphragm is made of movable blades. The diffraction spikes are caused by the aperture diaphragm in the lens, a mechanism that allows a photographer to change the aperture in order to control the amount of light entering the camera. For these starbursts camera lenses are to blame. You can see them around many bright light sources, such as the Sun, stars, street lights, traffic lights and so on. In photography and astrophotography, the ‘star effects’ are very common. This is because different telescopes use different support structures for the secondary mirrors, and those, in turn, create unique ‘signature’ diffraction patterns. Interestingly, you can often tell what telescope the image was taken with by looking at the stars’ diffraction spikes. Even if the rods are hair-thin, the incoming light bends, or diffracts, as it hits the vanes, resulting in a star’s hedgehog-like appearance. They are also lighter than the lenses, which is important for the really big ground-based telescopes and the telescopes that we launch into space.ĭespite these obvious advantages, it is the reflecting telescopes that create pointy stars. Inside a reflecting telescope there is a smaller secondary mirror that is held in place by the support rods (aka spider vanes). ![]() They quickly gained popularity as they were easier and cheaper to manufacture. ![]() The early telescopes were refractors, the ones with the lenses. Reflectors use mirrors (the light bounces o ff the mirror) and refractors use lenses (the light passes through a lens). Telescopes come in two flavours- reflectors and refractors. If you’ve never seen a telescope up-close, let alone looked through one, there is one thing you need to know: Let’s see how the stars get their pointy bits in different optical systems. To get a ‘pointy star’, you need to bend light even more! DIFFRACTION SPIKES But this bending alone does not give us diffraction spikes. As a result, instead of a bright point-like star image, we get a pattern consisting of a bright spot (Airy disk) surrounded by several concentric circles (Airy pattern). A good way to visualize it would be to imagine a water wave spreading out through a small opening in a barrier. * Strictly speaking, every time the light from a star enters a circular aperture, it diffracts. They are the artifacts of our imaging devices! The diffraction spikes occur because light exhibits wave-like behavior through the phenomenon called diffraction. ![]() These radial lines have to do NOT with the way a star shines, but with how we detect this starlight with our eyes or a camera or a telescope. Why do we draw pointy stars at Christmas? Why do stars look spiky on Hubble photographs and appear spiky to our eyes? Do they really have “pointy bits” around them?Īstronomers call those bright spikes that we so often see around bright stars in astro-images DIFFRACTION SPIKES. Why are stars pointy? | (Diffraction spikes explained)
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