Why are refractor telescopes used for astrophotography?

Each design of telescope comes with its pros and cons. As you say, reflectors do offer greater aperture than comparably priced reflector telescopes, but they suffer from things that refractors don't. Generally reflectors suffer from more collaborating issues, where the primary mirror tends to need aligning more than a refractor. read more

But in terms of astrophotography refractors tend to have a a larger f number. This is the ratio of the focal length to the aperture of the scope. The larger the f number the sharper the image. read more

In terms of astrophotography performance, an apochromatic refractor has many advantages such as a wide field of view, and sharp, pin-point stars. I have used Newtonian Reflectors and Ritchey-Chrétien telescopes in the past, and none can match the clarity of an apochromatic refractor. read more

This freedom of choice is not possible in astrophotography as, due to telescopes special optical design and the specific characteristics of the objects they are used for, they need a fixed focal length that can be varied with the use of additional accessories but only to some extent. read more