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double diffraction spikes


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Swinging the scope to Venus this morning I noticed a double set of diffraction spikes. Here's a back-of-the-envelope (literally) sketch.


The second set are dimmer and so I guess its only looking at a really bright object that they become noticeable - but I'm scratching my head to work out why you'd ever see more than four.

Any ideas?


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Each secondary vane creates a diffraction spike on either side of the star. If the vanes are perfectly aligned with their opposite then the spikes from the vanes will overlay each other and you'll only see four. If they aren't then you'd get the effect you're seeing.


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Thanks everyone

I've had a look this morning and on the face of it the spider vanes looked reasonably square...


However, I did notice on closer inspection that the two vanes that run perpendicular to the focuser are very slightly bent (near the secondary holder) in a direction away from the focuser. I think a bit of fiddling with the tension on the vane screws may help.

Im surprised that such a small bend would produce a second set of diffraction spikes that were offset at such a large angle (around 30 degrees or so), but I keep getting surprised by things with this astro-lark, so I'll tweak and see what happens

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