Spikes on stars

[Tenacious Del]

Does anyone know why the spider on newtonians leave spikes on stars? I would have thought the opposite optical effect would have happened.

[AstroTasmania]

Hi Dellio,

Here is a link that will take you to all you ever wanted to know about diffraction spikes in optical systems. Just follow some of the links within the article.

http://en.wikipedia.org/wiki/Newtonian_telescope

A Google search will bring up pages of links, check out the ones at your level of interest.

Clear skies...

[Radar]

Actually, thats a good question. I never really thought about it. Thanks for the links Shev.

[AstroTasmania]

Hi Radar,

The shape & number of vanes on a spider support do make a difference to the quality of the final image. The best spider/diagonal supports are curved, which avoids the spikes. More spikes add central obstruction and reduce image contrast, very important for planetary observers. The support for my 4" MA diagonal for the 16", is of a curved design to avoid the spikes. It adds a lot to the mechanical design problems too! I will be taking some more construction pics soon.

Clear skies...

[Tenacious Del]

Cool, thanks guys.

[Tenacious Del]

Okay, so we know that a spider on a newt causes spikes, but I saw a photo earlier of a spider with S shaped legs?

http://www.geckooptical.com/anttlers_optics/images/telescopes/ao_hpn/scope_home_antares_spider.gif

What would be the effect on stars with this?:hmm:

[Noel Carboni]

With a curved spider there will not be hard spikes, but rather a soft flare emanating in all directions from stars. This adds to the flare already there from around the outside of the aperture.

I for one like spikes. For imagery, spike brightness and length can help a viewer visualize the relative brightness of two stars that would otherwise just be bright burned-out circles. The longer the spikes, the brighter the star.

-Noel

[CanisMajorTom]

Spikes are beautiful. It took me ages before I realised what they were even caused by. I've seen numerous photos where the photographer put more than four spikes on the front of his scope. 8 spikes looked fantastic.

[Draig]

I agree, spikes are a great visual appeal. Even those unforunate ones who do not appreciate Astronomy seem to like spikes. They somehow seem to think that it is not a star unless it has got spikes. :duh:

CHeers Colin

[Radar]

They somehow seem to think that it is not a star unless it has got spikes.

lol, that is so typical.:duh:

[Noel Carboni]

They somehow seem to think that it is not a star unless it has got spikes. :duh:

Well... The idea has a little merit. Spike brightness and size can help differentiate stars that have been exposed too brightly for direct measurement.

Consider a Hubble image. The scope is so good that most stars are pinpoints. Without spikes, it becomes difficult to differentiate a light pixel that is a star from a light pixel that is a distant galaxy. That's a bit extreme, but consider images such as this one, and what I'm saying becomes a bit more clear:

http://www.nasa.gov/vision/universe/starsgalaxies/hubble_UDF.html

-Noel

[Radar]

Hmmmm, interesting. I never noticed that before. Good stuff Noel. :thumbsupmate:

[Draig]

Consider a Hubble image. The scope is so good that most stars are pinpoints. Without spikes, it becomes difficult to differentiate a light pixel that is a star from a light pixel that is a distant galaxy. That's a bit extreme, but consider images such as this one, and what I'm saying becomes a bit more clear:-Noel

Thanks Noel I have never thought of looking at it from that point of view.

Cheers Colin