Changing to curved spider vanes

[niallk]

Interesting explanations!  For observing Jupiter, I often think it would be nice to reduce the diffraction spikes - I've a regular straight 4 vane spider in my Obsession (17.3% obstruction).

Here's a pic of a Lukehurst:

& an Obsession 12.5" curved spider approach:

And Teeter:

Interesting to see such variety in the approaches - I wonder if Orion Optics went for two full circles to spell their initials 'OO' !  I suppose the Obsession one also gives a big 'O'

 

[Ruud]

On 23/03/2016 at 19:44, Ruud said:

The combined angle of the arced vanes still needs to be a multiple of 180°, but also: when you translate the arcs (you may rotate them during the translation, but only over 180°) and arrange them head to tail, they must form a half circle, a whole circle, one-and-a-half circle, two circles, etcetera.

I've tried to add an illustration to clarify this, but although I still had an edit button, I could not save the edit.

The idea is this: if you translate the vanes and arrange them head to tail, their combined arcs should form a multiple of 180°. You can flip a vane (turn it 180°) if needed. Flipping a vane will not affect its diffraction pattern.

This is how you can check if a three 60° vane solution will send the diffracted light uniformly in all directions. The vanes are at positions 120° apart around the secondary:

The three vanes to the left have the same diffraction pattern as the half circle to the right, which is an evenly spaced pattern, equally bright in all directions, without brighter or darker sectors.

Tal 250k

[Duke2k]

I have finally managed to make the vanes for my 6 inch newt out of stainless steel food serving rings. They are thin and very rigid and come in various diameters so do the job well.

 

[Stu]

This is a very interesting thread to me. I've recently acquired a Portaball with curved vane support. It looks like 3 x 60 degree arcs.

Whilst it gives very nice planetary views, the stellar images and those of the Gallilean moons for instances are not particularly pleasing. There are no spikes as such, but a kind of fragmented diffraction pattern around the star or moon.

I'm wondering if, although the 60 degree arcs give approaching the minimum theoretical diffraction, any error or misalignment can lead to a failure to completely cancel out the diffraction spikes? I'm thinking that perhaps 3 x 90 degrees might just give a little better performance in this regard? John's spider has two complete 360 arcs, giving slightly more overall diffraction but ensuring complete cancellation of spikes, so possibly performs better?

Regarding the Lukehurst image, I don't see how this can work as it does not appear to go through 180 degrees so cannot completely cancel out the spikes?