Why do stars look like crosses in photographs?

16 December 2007

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Question

Why is it that when you see pictures or photographs of stars they always appear as crosses?

Answer

This is an effect called a "diffraction spike" and it's to do with how big telescopes are made.

Reflecting telescopes, like the Hubble Space Telescope, contain two mirrors: one big, "primary" mirror, and a smaller secondary mirror which is held above the primary mirror.

The job of the secondary mirror is to reflect light back down onto the camera that takes the pictures.

So, in essence, the light enters the barrel of the telescope, hits the primary mirror, which reflects and focuses it onto the secondary mirror, which reflects and focuses it onto the camera.

But, the secondary mirror is sitting in the middle of the tube containing the big primary mirror, and it's got to be supported somehow.

To do this they tend to have four struts, or supports, projecting out of the side of the telescope tube to hold it up. The "star" or cross shapes you see on images of the stars are a really out-of-focus picture of the support struts. This effect is also called a diffraction pattern.

Diagram below: Comparison of diffraction spikes for various strut arrangements of a reflecting telescope. The inner circle represents the secondary mirror.


Comparison_strut_diffraction_spikes.png

Comparison of diffraction spikes for various strut arrangements of a reflecting telescope. The inner circle represents the secondary mirror.

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