Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: Robert Smith on 11/12/2009 10:30:02
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Robert Smith asked the Naked Scientists:
Hi Dr Chris,
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I work from home and listen to your show's podcasts (http://www.thenakedscientists.com/HTML/podcasts/) again and again and still find them fascinating, perhaps I'm dull??
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Anyway, my question is: Â Why do stars and other light sources appear as crosses on photographs or sometimes through windows? I assume it is something to do with the structure of glass, but though I'd ask anyway as one of your boffins will probably know.
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Also, whilst I'm on, have any of your shows (http://www.thenakedscientists.com/HTML/podcasts/) ever discussed "string theory". You tend to have people on that can explain things in layman terms and I'd be interested to have a listen. If it's been done can you tell me which show?
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Cheers and keep up the good work.
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Bob
What do you think?
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The light gets diffracted around edges or lines in the lens or on glass window. A "+" shape would indicate a rectangular aperture or an 8 pointed star shape an 8-sided aperture for example. If you are in a car at night with a dirty windscreen, give it a rub in (say) just a vertical direction and look at a street light through it. You will see its image spread sideways by diffraction by all the fine vertical marks you put on the window.
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The main reason why stars appears as crosses when their photos are taken through a reflecting telescope is because of the cross-shaped bracket at the top end of the telescope which holds the secondary mirror. This interrupts the field of view and turns bright stars into crosses of the same shape.
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Many telescopes use an off axis secondary ,does this cure this problem ?
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Many telescopes use an off axis secondary ,does this cure this problem ?
Yes.
(https://www.thenakedscientists.com/forum/proxy.php?request=http%3A%2F%2Fstarizona.com%2Facb%2Fbasics%2Foptics%2Foffaxis_newt_scope.jpg&hash=e397b3a696f63e6e6b25fb80eaaf5f0e)