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Lee,
That newt has a 50mm secondary which is somewhat oversized anyway, particularly for visual use. Before you go and buy a replacement secondary I would try what you suggest and just use the extension tube for visual work. I think the secondary will be fine as it is. The 10" version of the GSO scopes are similarly fitted with a 63mm secondary in a 10" scope (25% CO) which again is somewhat oversized for visual work in a 10"/F5 newt.
If I get a chance I will run all the numbers and calculate the illuminated field size with the 50mm secondary for you, if I can find all the old tube and focuser specs I had from my old scope. My gut feeling however is that it should be ok.
CS-John B
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