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As to the design constraints, an 1 1/4" focuser drawtube is going to vignette the light cone entering the drawtube. The governing factor is the height of your focuser, if a standard low profile focuser you still have to allow approx 80mm to the eyepiece focal plane. If you have some other design such as a low profile helical focuser that will help but from what NEWT shows, even allowing for a focuser height of 50mm and a tube ID of 300mm you will need a 63mm diagonal mirror, otherwise you are just throwing light away ( not reaching the eyepiece focal plane ).
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