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This thread is showing the unfortunate inescapable geometry problem with a Newtonian. Basically the focsuser (and OTA upper end I suppose) needs to be a lot stiffer than in a Cassegrain.
Ignoring at this point off axis loads (like ONAG mounted cameras that's another story), in a Cassegrain/refractor the side ways torque applied by the mass on the focuser/OTA, is proportional to the sin(za) where za is the zenith angle for the observation (i.e. za = 0 when looking straight up, za=90 looking at the horizon). For a Newtonian it is proportional to cos(za) because of the 90deg bend in the light path by the secondary mirror.
So the ratio cotan(za) is not at all favourable for the prime observing area far from the horizon. It means maximum deflection for the Newt at the zenith (the ratio being infinity there), >11x more than a Cass ~5 deg from zenith, and still ~1.7x 30deg from zenith. Once you get down to 45 deg elevation/za then things are even, except the focuser is attached to some big'ole back plate in a Cass, where the Newt can still have truss flex.
And the fast Newt doesn't make things any easier!
EB
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