The rays of light that stick out of bright stars in a telescope with a spider. Four straight vanes give four spikes; three vanes give six. They are normal and do not point to any fault; a curved spider replaces them with a faint halo.
Related terms
- Spider
The holder of the secondary mirror, made of thin vanes (often three or four) stretched across the tube. The vanes diffract light and produce the spikes around bright stars. It carries the secondary’s adjustment screws.
- Airy disc
Even perfect optics do not turn a star into a point but into a tiny disc surrounded by rings, because of diffraction. Its angular radius is 1.22 λ/D: the larger the aperture, the smaller the disc. About 84% of the light falls in the central disc.
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