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Prostar have this to say about 3 vs 4 vaned spiders:
Should I use a 3-vane or 4-vane spider?
The choice between a 3-vane or 4-vane spider is largely an aesthetic preference, and the consequence of your decision will be the pattern of the diffraction spikes that emanate from bright objects. A 4-vane spider produces four brighter spikes. A 3-vane spider produces six, dimmer, spikes. A 3-vane spider is usually a good choice for telescope apertures up to about 15.0 inches.
3-vane spiders are a particularly good choice for planetary and lunar observers. The brightness of the spikes is inversely proportional to the viewing magnification. Thus, at the high powers typical of planetary observation, the spikes almost disappear, and the view is more "refractor like." (Conversely, if you mostly observe at low magnifications, you might prefer a 4-vane spider.)
I think all you need to do is get some thinner vanes for that mount, they look excessively thick to me. My 16" has 4 vanes less than 1mm thick to support a 2.6" secondary.
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