Hi Steve
Diffraction spikes are inevitable with a conventional spider no matter what focal ratio you choose. If you want to get rid of them, think about a curved spider or a wire spider. The most effective solution is an optical widow but that is a lot of work and probably expensive.
For testing, I suggest you think more about building a Foucault / ronchi tester. You can star test with your finished scope prior to Aluminising.
Good luck!
Rod
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