Ultra High Contrast (UHC) Filter - A type of nebular
filter that only allows light frequencies common to nebulae through - H-Beta
and O-III lines at 486, 496 and 501nm.
Ultrawidefield - a term used to describe an eyepiece that
has an apparent field of view of 75 degrees or more.
Umbra The area of total darkness in the shadow of one
body on another. The darkest area in a lunar eclipse is caused by the
umbra of the Earth¡¯s shadow.
Universe - The totality of everything that is. As of this
writing, it is figured to be 156 billion light-years wide and about 14
billion years old.
Universal Time (UT) Also known as Greenwich Mean Time,
this is local time on the Greenwich meridian. Universal time is used by
astronomers as a standard measure of time. Mean solar time on the Prime
Meridian. Also called Greenwich Mean Time.
UTA - Upper Tube Assembly; frequently-used acronym for
the secondary cage of a Dobsonian telescope. See also Dob.
Van Allen Belts Radiation zones of charged particles
that surround the Earth. The shape of the Van Allen belts is determined
by the Earth's magnetic field.
Variable Star A star that fluctuates in brightness.
These include eclipsing binaries.
Vernal Equinox - that point in the sky where the sun crosses
the celestial equator on its way north in the sky. It occurs in late March.
The 0 Hour of Right Ascension extends to both celestial poles from this
Vignetting An optical problem where an optical image¡¯s
edges are fainter due to partial or complete blockage of light in the
optical system. This is caused by the eye or camera not being properly
aligned with the focal plane. Its effects sometimes appear as tunnel vision.
The most common cause is that one element of the optical train is insufficiently
large to field all the rays from the previous one. In a typical telescope,
vignetting can come from a tube that is not large enough in diameter,
baffles with openings too small, secondary mirrors with inadequate size,
small diameter focusers, improper use of focal reducers and large diameter
Violet Fringe Filter - see Minus Violet Filter
Visible Light Wavelengths of electromagnetic radiation
that are visible to the human eye.
Visual Acuity - the sharpness of ones vision. This can
be natural, or enhanced by glasses.
Visual Pointing - a procedure of telescope use wherein
the telescope is trained on a site in the sky by looking through a finder
scope and centering on a particular location. This is the fastest way
of finding objects, but requires familiarity with the location of an object.
Virgo Cluster A gigantic cluster of galaxies that is
located mainly within the constellation of Virgo. This cluster is located
about 45 to 55 million light years from Earth.
Visual Magnitude A scale used by astronomers to measure
the brightness of a star or other celestial object. Visual magnitude measures
only the visible light from the object. On this scale, bright objects
have a lower number than dim objects.
Wavefront - light¡¯s passage through a scope can be though
of as a wave. Deviations from the perfect wavefront are described as aberrations.
Wavelength The distance between consecutive crests of
a wave. This serves as a unit of measure of electromagnetic radiation.
It is inversely proportional to frequency (as one goes up, the other goes
Webcam - a small camera usually used for taking pictures
and broadcasting them on the Internet. These are often used in a telescope
for simple lunar and planetary photography. That is, they were until inexpensive
cameras became available for the same purpose that had greater sensitivities
White Dwarf A very small, white star formed when an
average sized star uses up its fuel supply and collapses. This process
often produces a planetary nebula, with the white dwarf star at its center.
No fusion takes place any longer at the core of a white dwarf. The star
has essentially become a solid of incredible density.
White Light - Visible light that includes all colors
and, therefore, all visible wavelengths. A mixture of all wavelengths
of the visible spectrum.
White Light Filters The most common type of solar filter,
composed of Mylar or glass, which rejects almost all light across the
entire spectrum. These filters work well for viewing the sunspots, photosphere
granulation, solar limb darkening, calcium clouds in the solar chromosphere,
the twenty-five day solar rotation period and events like solar eclipses
and Mercury and Venus transits.
Wide-Field - a term used to describe an eyepiece that
has a field of view between 60 and 75 degrees of apparent field.
Wide-Field Astrophotography - Astrophotography with a
camera alone which is either mounted stationary for short exposures or
is mounted piggyback on an equatorially-mounted telescope to take advantage
of its clock drive for long exposures.
Wratten System A numbering system for specifying color filters.
X-ray Electromagnetic radiation of a very short wavelength
and very high-energy. X-rays have shorter wavelengths than ultraviolet
light but longer wavelengths than gamma rays.
X-ray Astronomy The field of astronomy that studies
celestial objects by the x-rays they emit.
X-ray Star A bright celestial object that gives off
x-rays as a major portion of its radiation.
Yellow Dwarf An ordinary star such as the Sun at a stable
point in its evolution.
Yoke mounting A form of equatorial mounting in which
the polar axis consists of a yoke in which the telescope tube is mounted.
Zenith A point directly overhead from an observer.
Zodiac An imaginary belt across the sky in which the
Sun, Moon, and all of the planets can always be found. It passes through
Zodiacal Light A faint cone of light that can sometimes
be seen above the horizon after sunset or before sunrise. Zodiacal light
is caused by sunlight reflecting off small particles of material in the
plane of the solar system.
Zone A definite area on the mirror, measured from the