Choosing the right spider is a process of choices. The spider must be able to support your secondary mirror without movement. If you have a normal average telescope then the general specs on the spider will be perfect for making this choice. If you have a Planetary scope then your secondary mirror is very small, meaning it is light weight. In this case the strength of the spider is less important than the size of the center block of the spider; the center block must be small enough that the secondary mirror will hide it from the primary. If on the other hand you have a very fast scope with a large secondary mirror, you will want to get the next larger spider that will be more suited to support a larger and therefore heavier secondary mirror. Many customers want to use the four vane spider because their old spider has four vanes. This is not necessarily the best choice. A three vane spider has less obstruction, less surface to cause diffraction, and is more open from the front making it easier to work on your scope.
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