| Radial Velocity |
Portion of an object's velocity which is parallel to the line of sight. |
| Radiation |
Light. |
| Reflecting Telescope |
An optical system where light is bent with a curve mirror. |
| Reflex Finderscope |
Any one of several pointing devices for a telescope that uses no magnification and uses an illuminated "marker" reflected off a glass window. The observer looks through the glass window to the sky and moves the telescope until the marker is on the desired celestial target. The original and most popular of these pointing devices is the "Telrad". Other popular reflex finders are the "red dot" finders produced by several companies, sometimes referred to as a "BB gun sight." |
| Refracting Telescope |
An optical system where light is bent through clear lenses. |
| Reseau |
A system of lines forming small squares of standard size photographed by a separate exposure on the same plate with star images to facilitate measurements. |
| Resolution |
The ability of an optical instrument to show fine detail. |
| Reticle |
A wire or cross hair in the focus of an eyepiece. |
| Reticle Eyepiece |
An eyepiece with cross hairs used for guiding on stars in long-exposure astrophotography. |
| Reticulum |
The constellation "The Net" |
| Retrograde Motion |
This is a phenomena where a planet appears to move West-to-East against the field of stars instead of the normal East-to-West direction. This occurs when the orbit of the Earth overtakes the orbit of the observed planet. |
| Rhea |
A moon of Saturn was discovered by Giovanni Cassini in 1672. Rhea also is designated as "Saturn V". In Greek mythology, Rhea was a Titan and mother to Zeus. |
| Rich Field Telescope |
A low-magnification, wide-field telescope designed to reveal as many Milky Way stars in the field of view as possible. |
| Right Ascension |
The coordinate on the sky analogous to longitude on Earth, measured around the celestial equator from a specific place in the sky known as the vernal equinox. |
| Ritchey-Chretien |
A Cassegrain telescope variation incorporating a hyperbolic primary mirror and a strongly hyperbolic secondary mirror. |
| Roof Prism |
A compact prism used to produce a correctly oriented image. |
| Rosalind |
A moon of Uranus was discovered by Stephen P. Synnott (Voyager 2) on January 13, 1986. Rosalind is also designated as "Uranus XIII" and "S/1986 U4". Rosalind is the daughter of the banished Duke in William Shakespeare's play As You Like It. |
| Rose Moon |
The name of the Full Moon usually in June. Specifically the last Full Moon before the Summer Solstice (June 22 by "The Maine Rule"). Sometimes called the "Strawberry Moon." |
