Why Do Most Astronomers Use Reflecting Telescopes Instead Of Refracting Telescopes?

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Astronomers use both refracting and reflecting telescopes, but you are correct they use more reflecting telescopes than refracting.  In general, the larger the objective lens (the main mirror in a reflecting telescope and the large lens in a refracting telescope) on a telescope the more useful it is to astronomers because it can see dimmer objects than a smaller objective, which in turn means it can see more things further away.  The main reason that more reflectors are used is that it is easier and much less expensive to make an objective lens of a given size, mount it, and keep it aligned in a reflecting telescope than a refracting telescope.
 
A reflecting telescope can have supports in various places behind the mirror, where as a refracting telescope objective lens can only be support around the edges.  For very large lenses, this means that a refractor lens will distort under its own weight before a reflector mirror does.  The objective on the refractor has to be at the top of the tube so the tube must be much stronger to support the weight without bending too much.  The objective on a reflector is at the bottom of the tube and its weight does not cause as much of an alignment problem.
 
However, refractors do some things better than reflectors, so they have a place in astronomy as well, especially for objects that are bright, like the planets for example.  So, there are many refractors being used by astronomers as well.

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