- APPARENT FIELD OF VIEW: The
size in degrees of the field as seen
through the eyepiece of your telescope.
- APERTURE: The diameter of the
main light gathering element in a
- ASTERISM: A grouping of stars
resembling a familiar shape.
- ASTIGMATISM: A defect in optics
in which a lens or mirror has two
different amounts of curvature at 90
degrees to each other.
- CATADIOPTRIC TELESCOPE: A
telescope using both mirrors and lenses
to focus light. The most popular of
these is the Schmidt-Cassegrain
- CELESTIAL SPHERE: The imaginary
sphere on which the the night sky is
seen to be projected on.
- CHROMATIC ABERRATION: The
discoloration of a bright object caused
by dispersion in a refracting
telescope's objective lens(es). This
dispersion causes different colors of
light to come to a focus at different
- CIRCUMPOLAR: Star or objects
that are close enough to the pole so
that they never set from your particular
- COLLIMATION: The systematic
aligning of optical elements in a
telescope to emsure the brightest and
sharpest views possible.
- COMA: A defect causing stars to
be deformed into a fanlike or comet
shape. This is typically caused by
- DECLINATION: The celestial
equivalent of latitude. Declination is
measured from the celestial equator to
the celestial pole, spanning 90 degrees.
- DIFFRACTION-LIMITED: A term
used by telescope manufacturers to
represent the 1/4-wavelength Rayleigh
limit of optics. This represents an
error where the amount of detail seen is
limited only by the wave nature of
light. Errors greater than this result
in optics that degrade sharply.
- DOUBLE-STAR: Two stars close
together so that they appear as one at
lower magnifications. Many of these
stars are actually associated with each
other gravitationally forming a binary
- ECLIPTIC: The path in the sky
the sun follows.
- EXIT PUPIL: The diameter of the
cone of light exiting the eyepiece. This
can be calculated by dividing the
diameter of the objective by the
- FOCAL LENGTH: The distance from
the objective lens or mirror to the
point where the light converges, the
focal point. This distance is usually
measures in millimeters.
- FOCAL RATIO: The ratio of the
focal length divided by the diameter of
the primary mirror or lens.
- GLOBULAR CLUSTER: An enourmous
grouping of 25,000 to upwards of over
100,000 old stars. Globular clusters are
gravitationally held together and orbit
the outer reaches of most, if not all,
- LIMITING MAGNITUDE: The
magnitude of the dimmest object which
can be seen using a telescope of a given
aperture or with your naked eye.
- MAGNIFICATION: The amount of
increase in apparent size of an object.
This quantity can be derived by dividing
the telescope focal length by the
eyepiece focal length.
- MAGNITUDE: A scale used to
compare the brightness of objects in the
sky. One magnitude is a difference of
2.512 times in brightness.
- MESSIER OBJECT: Any of the 110
objects catalogued by French astronomer
Charles Messier in the late 18th
century. Messier was a comet hunter and
catalogued these as objects to
"avoid," or objects that could
be easily mistaken as a comet. They
include many of the best nebulae, star
clusters, and galaxies that are visible
from mid-northern latitudes.
- NEBULA: A large cloud of gas
and dust in space. Most are dark and
obscure the light from background stars.
Others glow brilliantly from the energy
of hot star within them.
- OPEN CLUSTER: A loose grouping
of relativly young star grvitationally
bound together. Most stars in open
clusters formed as a group inside large
- OPPOSITION: The point in time
when a planetary body is at the same
heliocentric longitude as the Earth.
Bodies at opposition rise at sunset.
- PLANETARY NEBULA: The last gasp
of a dying star, a planetary nebula
consists of the outer layers of a star.
These layers glow brilliantly from the
hot ultraviolet radiation from the
stellar core in the middle.
- REFLECTING TELESCOPE: A
telescope which uses mirrors to gather
and focus light.
- REFRACTING TELESCOPE: A
telescope which uses lenses to gather
and focus light.
- RIGHT ASCENSION: The
astronomical equivalent of longitude. It
starts where the ecliptic intersects the
celestial equator in Pisces and moves
through 24 hours.
- SEEING: A rating of how steady
the atmosphere is at your observing
site. Most astronomers use a scale of
1-10 or 1-5. Cold clear nights typically
have the worst seeing. The better the
seeing, the better high power views of
the planets and double stars.
- SPHERICAL ABERRATION: The
departure of a wavefront of foucused
light from a spherical shape.
- TRANSPARENCY: A rating of how
clear the night sky is. This is usually
given by the magnitude of the faintest
star easily visible to the naked eye.
Cold clear nights have some the best
- TRUE FIELD OF VIEW: The size of
the actual area of the sky you are
viewing measured in degrees. This
quantity can be approzimated by dividing
the apparant field of the eyepiece by
- ZENITH: The point in the
celestial hemisphere that is directly
above you or your observing location.