|
Morton,
Depends how much CA you'll tolerate. For a standard doublet 4", f/12 is about the minimum, and I could see CA in my f/15 4" doublet.
ED glass helps, for sure, but I still wouldn't go below f/9, personally.
Obviously, if usage is primarily lower powers, then your f/7 to f/9 range is OK. And there are a plethora of minus violet filters available for specific scopes and targets.
I just rarely meet someone using a scope like this who is not into viewing the Moon and planets.
Given the prices on decent Asian-made triplet refractors these days, there isn't really much reason to stick to doublets unless weight is an issue (doublets are considerably lighter). I'm seeing the triplet edging out the ED doublet in terms of sales since so many triplets are out there now from several companies. Also because ES is selling the 102ED triplet for $1099.99 in the States right now. Is it more than $1800AUD for you guys?
|