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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,
or30,800,000,000,000km.(Apartfromthe
Sun,nostarlieswithinoneparsecofus.)
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 The angular distance of a celestial
Ascension:(RA) objectmeasuredinhours,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 The totality of astronomical things, events,
relations and energies capable of being
described objectively.
V –
Variable Star 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.
W –
Waning Moon The period of the moon’s cycle between
full and new, when its illuminated portion
is decreasing.
Waxing Moon The period of the moon’s cycle between
new and full, when its illuminated portion
is increasing.
Z –
Zenith The point on the Celestial Sphere directly
above the observer.
Zodiac 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
byabout30degreessincethattime;the
signs have moved with it and thus no longer
coincide with the constellations.
