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Information You Supply When Ordering Secondary Mounts ("spiders")

Secondary mounts are custom-made to the size of your secondary mirror and tube (or cage) inside dimension. Read below about the 'A', 'B', and 'C' dimensions below, and enter it into in the "Ordering Instructions" box of the shopping cart. If you are specifying a built-in offset into the spider, please also include the offset amount and the orientation as described near the bottom of this page. Optional features are added to the shopping cart as separate items.

Screenshot of the shopping cart checkout


Note: You don't need to supply any dimensional information if you're ordering a direct replacement spider for a commercial telescope.


Dimensional information needed to be supplied with spider orders

Dimensions to provide for a
solid tube telescope


Dimensions to provide for a
truss-style telescope
(upper cage shown)


Dimensions to provide for a
curved secondary mount in a
solid tube telescope



A = The actual inside diameter of your tube or upper cage. Provide the actual inside diameter of your tube or cage. We will make your spider slightly smaller so it can be properly tensioned and centered. A measurement accuracy of +/- 1/16" (+/- 1.5 mm) is sufficient.

B = The size of your secondary mirror (minor axis). If you recently purchased your secondary mirror, then you probably know it's size. If you're not sure, please make this measurement yourself. This dimension is important in order for the mirror shroud to fit properly.

C = The tube wall thickness. If it is a truss-style scope, please provide the strut diameter (the strut is what the spider will attach to). This dimension is not important for making your spider, but it lets us include the proper length mounting screws into your kit.


Optional features

Protostar straight-vane secondary mounts can be ordered with finger-adjustable collimation screws, built-in antidew heater, and custom offsets. Read the options page for more information.

When specifying an offset, please include the offset dimension along with the orientation (see image to the right).

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