Length doublets (F8 or more) are often a better choice. Chromatic aberration in significant quantities will killĭon’t have to be long focal length, but it can giveġ) Long focal lengths are easier to figure well andĢ) Aberrations are easier to control in longer focalģ) Longer focal ratios make precise focus lessĪre easier to manufacture, cool more quickly and have fewer optical surfaces to Optically-perfect long-focal-length Newtonian can get quite close to aĪ refractor need to be an apochromat? No, but if it’sĪn achromat it needs to be a long focal length one (F12-F15 plus) to How much depends on the design and quality of the reflector – an Of these factors make refractors definitively better than reflectors, but sizeįor size a good refractor will always outperform a good reflector, though by
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