R

Rack-and-Pinion Focuser - This type of focuser is found on most refractors and Newtonians. It has a rack of teeth running along the underside of the focus tube and a gear connected to the focus knobs is used to move the drawtube in and out. It is a simple design but is prone to backlash from slop in the gear teeth, so for high precision purposes such as planetary observing or CCD imaging, a Crayford focuser is often used instead.

Reflector - Any telescope that uses mirrors exclusively in its design, such as a Newtonian or Ritchey-Chrétien. Newtonians are the most common type of reflecting telescope, and sometimes the terms Newtonian and reflector are used interchangeably, but technically reflectors are a larger group including all the mirror telescopes.

Reflex Finder - Also called zero-power finders, reflex sights project a red dot or bull's-eye onto a flat piece of glass which makes the dot or bull's-eye appear to float on the sky. These types of finders tend to be much easier to use than standard finderscopes since they do not invert the image or give a magnified view. One of the most popular is called a Telrad.

Refractor - A telescope that uses only lenses in its design. Refractors are the oldest telescope design but suffer from chromatic aberration unless special glasses are used in the design. This makes good quality refractors among the most expensive types of telescopes.

Right Ascension - The celestial sphere has two coordinate axes, right ascension and declination. Right ascension is analogous to longitude on Earth. Just like longitude, right ascension is divided into 24 one-hour segments. Objects with higher right ascensions rise after those with lower right ascensions. It is named such because if you stand facing north, objects on your right will be ascending as Earth rotates. Right ascension is often abbreviated RA.

Rods - Rods are the cells in the retina of the eye which are sensitive to black and white. They are more sensitive than the color-detecting cones when dark adapted, which is why color is difficult to see in deep-sky objects.

Roof Prism - These are the type of prisms used in a straight-through binocular design. They provide a more compact design and closer focusing range than porro prisms but are typically more expensive and require an extra reflection in the optical path, so they are usually not quite as bright (although modern coating technology makes the difference very small).