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Diffraction spikes only on stars (?)


EA2007
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Hello,

Forgive this rather daft question (as some may find it), but my dad just asked why diffraction spikes only occur on images of stars and not other DSO's such as galaxies, nebulae and such-like.

TBH I couldn't answer him. I was showing him an image of the Hubble Ultra Deep Field and showed him how the Hubble telescope is a Newtonian design because of the spider struts that produce the 4 lines around the foreground stars.

Even the smaller spherical galaxies in the image didn't have any spikes, yet the larger foreground stars did. Any answers?

On a side-note: We were looking at the image in the 'Wonders of the Universe' book by Coxy and Cohen and I noticed that on the inside dust cover then the opening paragraph states "you find something of unimaginable age and size: 13.7 billion years old, 93 billion light years across and filled with over 100 billion galaxies etc etc", however on page 8 then the opening paragraph states: "At 13.7 billion years old, 45 billion light years across and filled with 100 billion galaxies etc etc"

Surely the 93 billion LY's and 45 billion LY's are contradictory?

I also find the statement somewhat irregular as I have always found the size of the universe (not that its measurable btw!) but I have always found it to be stated as the observable universe to be 13.7 LY across. However there is no centre point, even if there was a centre point some 13.7 billion LY's away then the other side would be 13.7 G LY's away from that, so in essence its 27.4 G LY's in size (G being Billion's of years).........but all of this is incorrect.

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You get diffraction spikes only on stars because the are 'point sources'. Ie. so small and far away that the light appears to come from a single point.

I think this means the light waves are split by the spider vanes and this causes the diffraction. Galaxies and Nebs being larger the light is more spread out so the light waves don't get split by the spider vanes.

(I know that's not exactly right, but the truth is more complicated and I can't remember my A level physics after so many years)

(Edit: must type faster)

Edited by RikM
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And the Hubble is a Ritchey Chretien, not a Newtonian. It does have a secondary mirror with vanes generating spikes.

As for the 'size' of the universe, the whole thing is a minefield! If the light travel time to an object is 13.7 billion years the object is, due to expansion during the light flight, vastly more than 13.7 billion LY. And since two observers in vastly separated parts of the universe don't share a common clock they can't agree on simultanaity and... etc etc. Groan!

Olly

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You get diffraction spikes only on stars because the are 'point sources'. Ie. so small and far away that the light appears to come from a single point.

I think this means the light waves are split by the spider vanes and this causes the diffraction. Galaxies and Nebs being larger the light is more spread out so the light waves don't get split by the spider vanes.

(I know that's not exactly right, but the truth is more complicated and I can't remember my A level physics after so many years)

(Edit: must type faster)

The diffraction spikes do appear, but smeared out and at a very low surface brightness.

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You'll get spikes around anything sufficiently bright, and the effect can be exacerbated if there's stray light in the system. I used to get bright spikes round planets with my 12" f4.9. I introduced baffling and the spikes are now fainter but still there.

You can eliminate the spikes using a curved-vane spider, though what you get instead is a slight smearing of star images. Diffraction is unavoidable as long as you have some kind of strut holding the secondary.

Diffraction spike - Wikipedia, the free encyclopedia

How Does a Secondary Affect Telescope Diffraction?

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