What differentiates, in terms of practical quality, not technical implementation, a refractor from a reflector telescope?
Why would one prefer a refractor over a reflector, when reflectors come with such large diameters at a smaller price?
[Physics] Lenses (refractor) or mirrors (reflector) telescope
opticstelescopes
Best Answer
There are actually three main telescope types, refractor, reflector, and catadioptric (which uses both lenses and mirrors to form its image). Each has its advantages and disadvantages, which is why all three are manufactured currently.
From a practical standpoint, refractors offer the sharpest and most contrasty images in a given size, because of their unobstructed apertures. Reflectors offer less sharpness and contrast because of the presence of a diagonal mirror and its supports in the middle of the main optical path. This can be minimized by reducing the size of the obstruction to under 20% of the aperture (by diameter). Catadioptric telescopes offer the poorest sharpness and contrast because of their more complex optical systems, which also almost always include a large central obstruction. In return, they provide extremely compact designs which are easier to transport and mount. Catadioptrics using a Maksutov corrector tend to perform better than those using a Schmidt corrector because their central obstructions are smaller and their correctors use simple spherical curves.
Part of the advantage of refractors is countered by their chromatic aberration. All refractors suffer from this to some extent, leading to colour fringing around bright objects. It can be reduced in two ways: going to a long focal ratio (f/15 or longer) or using special low dispersion glass (ED or Extra-low Dispersion or fluorite). These solutions each have their own disadvantage: telescopes with long focal ratios are difficult to mount solidly, so require large expensive mounts; telescopes with ED or fluorite lenses are often very expensive.
For most amateur astronomers, a high quality reflector offers the best compromise in terms of cost and aperture.