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All spiders will cause some diffraction. In a normal, straight, four-vane spider, the diffraction will form four distinct spikes emanating from any bright object. This is really eight diffraction spikes not four. Each vane causes two spikes, perpendicular to the vane: A curved-vane spider will spread the diffracted light throughout the field of view, eliminating the spikes. For example, my 8 inch telescope with a standard 10 inch four vane spider, the spider absorbs just under 1 square inch of viewable light path. That equals about 2 % of the total light path. My secondary mirror absorbs about 2 square inches or 4 % of the light path. So the combination of spider and secondary absorbs about 6% of my light path. When I converted to a large curved vane spider, the spider absorbed 2.2 % of my light path, making the new total 6.2 %. But, the little light that is diffracted by the spider is spread over the whole field of view there by becoming invisible