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sv165 guide scopes

sv165 guide scopes

The sv165-40mm guide scope is specified as 160mm focal length, but it measures as 170mm when focused on a distant object (also noted in cloudynights forum by unimatrix0 on 25th June 2023.) When focused, the objective tube and camera (sv905c) are at the extremity of travel so I inserted an extension tube to make it more secure.

The sv165-30mm guide scope is specified as 120mm focal length but it measures as 130mm.

Is there an explanation for the discrepancy between specified and measured focal length for this guide scope.