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

16 December 2007

Question

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

Answer

This is to do with how big telescopes are made. They're actually two mirrors and we have one big primary mirror and there's a secondary mirror held above that primary mirror. That reflects the light back down where you'd put a camera. The secondary mirror is in the middle of the tube with the big one on. They've got to support it somehow. They tend to have four supports, vertical supports that hold it up or else it's gonna fall down. That star shape you see round the stars is basically a really, really out of focus picture of this star shape. It's also called diffraction pattern and that's what's producing all these star shapes.

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