Spider sharpening?

[michaeldurban]

Was just thinking..

If the total diffraction depends upon the thickness of the spiders'

vanes, then wouldn't it be a good idea to sharpen the spider?

Bit like a double edged knife, all four or three vanes...

any ideas or experiences with this?

[Moonshane]

most manufacturers do use 0.5mm vanes these days?

[JamesF]

If the total diffraction depends upon the thickness of the spiders'

vanes, then wouldn't it be a good idea to sharpen the spider?

If it did, it might.  As far as I am aware however, it doesn't.

I think it would be correct to say that edge length is what contributes to the amount of diffraction, and edge "parallelism" (there must be a proper word for that) is what causes the diffraction to appear as spikes.

Increasing vane thickness reduces the amount of incoming light and may therefore reduce contrast.

I think

James

[Lonestar70]

Sharpening the edges will have no effect on the diffraction, unless you also reduced the thickness whilst doing the sharpening... which would weaken the spider vanes and possibly result in collimation issues.

You could actually introduce other problems, such as unwanted light reflections off of the resulting angled faces, even if you paint them matt black.

You would get a small reduction in diffraction (though probably not worth the effort) by using a thinner material for the spider legs, but of course, they would need to remain strong enough to support the secondary mirror and housing (or the same collimation issues would result).

Sharpening the edges would also be more of a danger when handling the scope.

Not Recommended.

Best regards.

Sandy.

[JamesF]

I'm struggling to get my head around some of this, but it might be an interesting read:

http://www.telescope-optics.net/spider.htm

James

[Astrobits]

Basically, diffraction occurs at the boundary of the spider vane as seen from infinity. Interference between the two sides of each vane modifies the absolute effect and the thickness also contributes to the final effect.  Sharpening the vanes will have no effect at all on diffraction as it will not change the profile as viewed from infinity. Whether the variations typical in commonly encountered, straight,  vanes will be visually detectable is open to experiment, I am not aware of any actual field testing with different thicknesses of vane.

Nigel