Celestron 114LCM Computerized Telescope LCM Series Manual (English) - Page 26

Parfocal Refers to a group of eyepieces that all require the same distance from the focal plane of the telescope to be in focus. This means when you focus one parfocal eyepiece all the other parfocal eyepieces, in a particular line of eyepieces, will be in focus. Parsec The distance at which a star would show parallax of one second of arc. It is equal to 3.26 light-years, 206,265 astronomical units, or 30,800,000,000,000 km. (Apart from the Sun, no star lies within one parsec of us.) Point Source An object which cannot be resolved into an image because it to too far away or too small is considered a point source. A planet is far away but it can be resolved as a disk. Most stars cannot be resolved as disks, they are too far away. R - Reflector A telescope in which the light is collected by means of a mirror. Resolution The minimum detectable angle an optical system can detect. Because of diffraction, there is a limit to the minimum angle, resolution. The larger the aperture, the better the resolution. Right Ascension: (RA) The angular distance of a celestial object measured in hours, minutes, and seconds along the Celestial Equator eastward from the Vernal Equinox. S - Sidereal Rate This is the angular speed at which the Earth is rotating. Telescope tracking motors drive the telescope at this rate. The rate is 15 arc seconds per second or 15 degrees per hour. T - Terminator The boundary line between the light and dark portion of the moon or a planet. U - Universe V - Variable Star W - Waning Moon Waxing Moon Z - Zenith Zodiac The totality of astronomical things, events, relations and energies capable of being described objectively. A star whose brightness varies over time due to either inherent properties of the star or something eclipsing or obscuring the brightness of the star. The period of the moon's cycle between full and new, when its illuminated portion is decreasing. The period of the moon's cycle between new and full, when its illuminated portion is increasing. The point on the Celestial Sphere directly above the observer. The zodiac is the portion of the Celestial Sphere that lies within 8 degrees on either side of the Ecliptic. The apparent paths of the Sun, the Moon, and the planets, with the exception of some portions of the path of Pluto, lie within this band. Twelve divisions, or signs, each 30 degrees in width, comprise the zodiac. These signs coincided with the zodiacal constellations about 2,000 years ago. Because of the Precession of the Earth's axis, the Vernal Equinox has moved westward by about 30 degrees since that time; the signs have moved with it and thus no longer coincide with the constellations. 24