Interested in a diagonal for my f/5 richest field refractor and am not sure of the issues with using a mirror vs prism diagonal. Some say that for fast scopes a mirror is best, while others say that faster than about f/7 though, the prism will induce spherochromatism. Does this apply more to a doublet (non apo) refractor, or is the difference negligible at relatively low power? I intend using the scope for comet patrol and ideally require orientation to be as per binocular view.