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Hi Adman,
You ask some very technical Qs. They tell me you've mainly used refractors? I'll try to give some enlightenment.
1:Mass production OTA's that tend to be consistently good include GSO & Skywatcher. These are readily available in Oz. Look for primay mirrors advertized as parabolic.
2: A two inch low profile focuser. These tend to be less intrusive than focusers found on refractors meaning they will be less likely to intrude into the tube obstructing light. Easily accessible collimation screws for the secondary & primary mirror, including vertical adjustment for the secondary.
3: Fast (ie: f/5 or less) newtonians can & do 'suffer' from coma. The shallow light cone they form means that with most EPs & with astrophotography the outer edge of the field of view does not focus at the same time as the central area. This is not a defect but an artefact of the optical arrangement. 'Coma correctors' are lens arrangements which assist in eliminating this effect. Should you get one, be aware that there is not a one size fits all. There are a various out there & each better suits different focal ratios.
4: Newtonian optics are not held fixed in their cells like the lenses of refractors. Hence they do move, ever so slightly with transport, so they need to be recollimated frequently. Not hard to do, & there are many aids to help. It's a touchy feelie thing.
Just some points to which others can elaborate more on.
Mental
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