Quote:
Originally Posted by AstralTraveller
I think some confusion has crept in here because the discussion has shifted from reflectors to refractors. Only refractors suffer from chromatic aberration and so only refractors benefit from minus violet filters. You don't need one for a reflector (although they have their own issues).
Actually you only need a minus violet for a poorly corrected refractor. So nice (expensive) triplets don't need a filter (or so I believe) and probably an ED doublet can also do without. For simple achromatic doublets the amount of chromatic aberration is a function of aperture and f ratio. So as a scope gets bigger it needs a longer ratio. According to one discussion I read a 125mm doublet is OK at f/8 but a 150mm needs f/10 to have acceptable colour. That would make the scope 1.5m which is possibly too big, certainly for a portable scope. So the compromise is f/8 and a filter.
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So, just to be sure, I dont need one of these filters for my Saxon Newtonian Reflector, but the quality standard of the mirrors will still restrict me to 200X?