Hi Pete, welcome to the group.
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I’m not sure what vintage your mount is but, it very likely accepts Losmandy style dovetail plates only. The one on your scope seems to be Vixen style and will not work on the mount as is. The best way to use your C8 on the GM8 is to get a C8 specific dovetail plate with its associated radius blocks. Losmandy sells those on their ?website.
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Like Chip mentions, you could also upgrade the base on your mount to the newer version which accepts both Losmandy and Vixen plates but, imo Losmandy’s bigger sized plates offer better stability and stiffness. However, if you have other, smaller scopes with the Vixen plates, this option may be more versatile. That base is also sold on Losmandy’s website ;)
