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DEFINITION OF TERMS (GLOSSARY): U - Z

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U

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.

V

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 point.

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 eyepieces, etc.

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.

W

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 down)

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 and resolutions.

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

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.

Y

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.

Z

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 13 constellations.

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 centre.

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ASTRONOMICAL BOOKS

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Agena Astro Products - The Great Atlas of the Sky, Jubilee Edition
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Agena Astro Products - The Great Atlas of the Sky, Jubilee Edition
Item No. 786-20000

Agena Astro Products BOOKS The Great Atlas of the Sky by P.Brych
 
Our Price: $239.00
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Sun Observer's Guide, Firefly Books
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Sun Observer's Guide, Firefly Books
Item No. 896-23211
Manuf. No. Firefly 978-1-55297-941-9

A practical reference for how and why to view our nearest star. This book can start you making worthwhile observations.
 
Our Price: $14.95
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The History of Astronomy, Firefly Books
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The History of Astronomy, Firefly Books
Item No. 896-23210
Manuf. No. Firefly 978-1-55407-537-9

This extraordinary book traces humans' interaction with the endless wonders of the night sky.
 
Our Price: $29.95
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Stargazing with a Telescope, Firefly Books
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Stargazing with a Telescope, Firefly Books
Item No. 896-23208
Manuf. No. Firefly 978-1-55407-577-5

Stargazing with a Telescope is a practical guide that demystifies the process of buying and using a telescope.
 
Our Price: $19.95
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Stargazing with Binoculars, Firefly Books
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Stargazing with Binoculars, Firefly Books
Item No. 896-23207
Manuf. No. Firefly 978-1-55407-368-9

A practical, concise beginner's guide to viewing the night sky through binoculars
 
Our Price: $19.95
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300 Astronomical Objects: A Visual Reference to the Universe, Firefly Books
300 Astronomical Objects: A Visual Reference to the Universe, Firefly Books
Item No. 896-23206
Manuf. No. Firefly 978-1-55407-175-3

A handy and comprehensive reference to the 300 most interesting celestial objects. This book provides a tour through the galaxy.
 
Our Price: $29.95
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Moon Observer's Guide, Firefly Books
Moon Observer's Guide, Firefly Books
Item No. 896-23203
Manuf. No. Firefly 978-1-55297-888-7

Moon Observer's Guide offers practical guidance to amateur astronomers viewing Earth's only natural satellite.
 
Our Price: $14.95
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Night Sky Atlas: The Moon, Planets, Stars and Deep Sky Objects, Firefly Book
Night Sky Atlas: The Moon, Planets, Stars and Deep Sky Objects, Firefly Book
Item No. 896-10719
Manuf. No. Firefly 978-1-55407-026-8

This Firefly book is a very nice introduction to astronomy for beginners.
 
Our Price: $29.95
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Software Bisque TheSkyX Professional Edition for Mac
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Software Bisque TheSkyX Professional Edition for Mac
Item No. 773-55161
Manuf. No. Software Bisque SKYX_PRO_MAC

TheSkyX Professional Edition for Mac is the next major release in TheSky's 25+ year history
 
Our Price: $349.00
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Software Bisque TheSkyX Professional Edition for Windows
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Software Bisque TheSkyX Professional Edition for Windows
Item No. 773-55160
Manuf. No. Software Bisque SKYX_PRO_WIN

TheSkyX Professional for Windows will get you charged up about your passion. Explore, engage, enjoy while getting the most from your observing sessions.
 
Our Price: $349.00
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Sky & Telescope - Sky Atlas 2000.0 Companion, 2nd Edition
Sky & Telescope - Sky Atlas 2000.0 Companion, 2nd Edition
Item No. 846-10899
Manuf. No. Sky Publishing 46956

This essential reference features an alphabetical listing of every deep sky object plotted in Sky Atlas 2000.00.
 
Our Price: $29.95
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Sky & Telescope - SKY ATLAS 2000.0 DESK UNLAMINATED
Sky & Telescope - SKY ATLAS 2000.0 DESK UNLAMINATED
Item No. 846-10898
Manuf. No. Sky Publishing 46883

Includes close-up charts of such areas as the celestial poles and the Virgo-Coma galaxy region, as well as an acetate coordinate-grid overlay for determining accurate positions
 
Our Price: $34.95
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Sky & Telescope - SKY ATLAS 2000.0 FIELD LAMINATED
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Sky & Telescope - SKY ATLAS 2000.0 FIELD LAMINATED
Item No. 846-10895
Manuf. No. Sky Publishing 46921

The standard against which all other star atlases are measured, each version of Sky Atlas 2000.0 contains 26 charts covering the whole sky and showing 81,312 single, multiple, and variable stars of magnitude of 8.5 and brighter and 2,700 deep-sky objects
 
Our Price: $69.95
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Sky & Telescope: SKY ATLAS 2000.0 FIELD UNLAMINATED
Sky & Telescope: SKY ATLAS 2000.0 FIELD UNLAMINATED
Item No. 846-10890
Manuf. No. Sky Publishing 46891

Field Version: Stars and deep sky objects are white on a black background. Unbound and printed on heavy paper
 
Our Price: $34.95
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Sky & Telescope - Astronomy Library Sky Atlas 2000.0 Deluxe Edition
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Sky & Telescope - Astronomy Library Sky Atlas 2000.0 Deluxe Edition
Item No. 846-10880
Manuf. No. Sky Publishing 46875

Sky Atlas 2000.0 Deluxe Edition contains 26 charts showing close to 85,000 objects (stars and deep sky objects) down to magnitude 8.5
 
Our Price: $69.95
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