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Hi Martyn
Some of the Vixen refractors are delivered with a screw-on extension tube (approx 2 ½ inches long) which screws directly into the end of the focuser. This is used when viewing "straight through" without a diagonal.
When you use a diagonal, you need to remove the screw in extension tube due to the longer light path of the diagonal. See attached image using a 1.25" diagonal.
On my Vixen, the base focuser is a chrome finish and the extension tube is black anodised so it was easy to see that they are separate units that can be unscrewed.
Hope it is as simple as that.
Cheers
Dennis
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