REFRACTORS
These are telescopes that use one or more lenses as the objective. The objective is usually at the end which points towards the target and the other end has eyepiece where user can view. These are simplest and earliest form of telescope, though at apertures higher than basic ones (70+) they are expensive as costs involved to manufacture are high. They may have a single lens (singlet), 2 lenses (doublet) or 3 lenses (triplet) to correct for chromatic aberration.
REFLECTORS
These are telescopes that use a large mirror as the primary objective, and another smaller mirror that reflects the light rays coming from the primary, and focuses into the eyepiece. The primary is placed at the end farthest from the end that points at the target, and eyepiece is placed on the side of the OTA. Compared to refractors, reflectors are more economical to make and you can usually get a bigger aperture reflector than refractor in the same price range. Large reflectors are commonly called as light buckets because of the massive light gathering capacity they have due to big apertures
CATADIOPTRIC/ CASSEGRAINS
These are telescopes that use combination of lenses and mirrors for gathering and focusing light. Light usually bounces around a couple of times within the scope before being focused at the eyepiece. Cassegrains are most expensive to manufacture but are the most compact scopes enabling them to achieve far greater focal lengths at a fraction of the OTA length of refractors and reflectors.
