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Hi

I'm wondering what might cause two sets of diffraction spikes to appear on a bright star? One is dimmer than the other and rotated 45 degrees relative to the brighter set. Is this normal or are there something wrong with my optics?

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I take you have a reflector telescope in which case the spikes are caused by the four vanes that hold the secondary mirror in place. I would guess that you are referring the the smaller spikes between the larger vertical and norizontal spikes, and I would guess are artifacts caused by the vanes as well.

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What camera? I've seen some funky spikes caused by the camera iris - actually ignore me as this part is in the lens and will not be in the optical path when you connect to the scope...

What about mirror clips?

If you look down the scope, can you see anything?

Ant

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I am sure I have read that if you vanes are not quite aligned properly or at less than 90 degrees to each other this can create additional sets of diffraction spikes?

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The mirror clips seems to cause the dark shades under the dimmer spikes. This is fine, but I still don't understand the extra spikes. I understand that vanes not properly aligned to each other could cause extra spikes, but at such great angles?

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if for example, you made a spider with three vanes, you would get six diffraction spikes, equally spaced so I think this may be the issue - easily ruled out though if they are at 90 degrees - just check them. ;)

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not quite. for some reason (that I don't know) three vanes creates six spikes but four (correctly set up vanes) creates four spikes. as I understand it any more or less than four and it creates double the spikes.

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A single vane would cause spikes in opposing directions. So with two vanes directly opposite each other, the spikes of both overlap. With an odd number of vanes, the you don't get that overlap and thus appear to get more spikes.

But none of that explains the original question... the mention of mirror clips, while I don't know what one is I can imagine, and if they do intrude on the light path in any way, that could cause diffraction. If it is possible to align them with the vanes then resulting spikes should go away too.

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not quite. for some reason (that I don't know) three vanes creates six spikes but four (correctly set up vanes) creates four spikes. as I understand it any more or less than four and it creates double the spikes.

The easiest way to understand it is that each vane always creates two spikes at right angles to the vane. So three vanes => six spikes. With four correctly set up vanes however, the spikes overlap, so you only see four spikes.

Given these extra spikes are at 45 degrees to the main spikes, it implies whatever is causing them is at 45 degrees to the spider vanes. Take the camera out and have a look through the focuser and out of the telescope. See if you can see anything cutting into/crossing the image of the mirror. Anything which cuts across the mirror will cause a diffraction spike. I'd guess it might be something like the mirror clips.

(edit: cross post with GlassWalker...)

Edited by FraserClarke
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The theory about the mirror clips could be correct, but I have no way to rotate the mirror cell. It only fits in one way. I could do a test by removing the clips, but I'm too afraid of the mirror dropping out accidentally. :eek:

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