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You cannot remove the internal barlow from a short tube Newt. and achive focus. The barlow effectively double the focal lenght to reduce the inherent aberrations found in a spherical mirror. I have attempted it several times but get no where near focus even with extension tubes etc. I would avoid any sperical mirror of that F/L and get a decent 114 Newt. with a parabaloid mirror. They give surprising good images at high power with minimal coma whereas the barlowed versions image breaks down at even moderate magnifications.
Chris
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