Why do very large astronomical telescopes use a mirror rather than a lens for the objective optic?
Light must pass through the lens, this causes loss and distortion. A mirror does not have this problem. Large lens are very hard to make and the above problems are intensified with size. You will only find small refractors for this reason.
In addition, there is the "strength" issue. A large mirror can be supported all along its back while a lens can only be supported arround its perimeter.
Also, a refracting telescope has its lens at the "top", pointed toward the heavens, while a reflector has its mirror at the bottom. Since those are the heaviest parts of large telescopes, large refractors are "top heavy".
Finally, the reason Newton built the Newtonian reflector to begin with: chromatic aberation. One type of glass refracts different colors of light differently. Using a lens made of a single type of glass instead of a point of light, you see a series of circles of different colors. You can fix that by making the lens of different kinds of glass so that the chromatic aberation of one "corrects" that of the other but for large lenses that is difficult and expensive.
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