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The best geometry for curved spiders one that minimises spider vane length (to maximise contrast) subject to the constraint that the vanes cover orientations spanning 180 degrees (to completely eliminate diffraction spikes; span not 360 because a 180 degree rotated vane is still running along the same direction). There are also practical considerations, e.g. single vane spider does not provide enough rigidity. Efficient curved spiders can be found on some Russian Cassegrains. Some of the TAL scopes have them. 3 vanes, each covering a 60-degrees.
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