You've seen these spikes in pictures of bright stars:
Thursday's Astronomy Picture Of the Day: link
Once again, if you're not hitting NASA's APOD site every day, you're missing out.
From the Wikipedia article:
Diffraction spikes are lines radiating from bright light sources in reflecting telescope images. They are artifacts caused by light diffracting around the support vanes of the secondary mirror. Refracting telescopes and their photographic images do not have the same problem.Lots more there, including a discussion of why the spikes appear at 90deg relative to the mirror supports. Now you know.