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Author Topic: Why do point light sources appear as crosses?  (Read 1997 times)

Robert Smith

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Why do point light sources appear as crosses?
« on: 11/12/2009 10:30:02 »
Robert Smith asked the Naked Scientists:
   
Hi Dr Chris,
  
I work from home and listen to your show's podcasts again and again and still find them fascinating, perhaps I'm dull??
  
Anyway, my question is:  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.
  
Also, whilst I'm on, have any of your shows 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?
  
Cheers and keep up the good work.
  
Bob

What do you think?


 

Offline graham.d

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Why do point light sources appear as crosses?
« Reply #1 on: 11/12/2009 13:22:49 »
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.
 

Offline Fozzie

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Why do point light sources appear as crosses?
« Reply #2 on: 11/12/2009 16:08:08 »
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.
 

Offline syhprum

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Why do point light sources appear as crosses?
« Reply #3 on: 11/12/2009 21:55:42 »
Many telescopes use an off axis secondary ,does this cure this problem ?
 

Offline Fozzie

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Why do point light sources appear as crosses?
« Reply #4 on: 18/12/2009 09:53:08 »
Many telescopes use an off axis secondary ,does this cure this problem ?

Yes.

 

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Why do point light sources appear as crosses?
« Reply #4 on: 18/12/2009 09:53:08 »

 

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