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Dark Matter A term used to describe matter in the universe that cannot be seen, but can be detected by its gravitational effects on other bodies.

Dark Nebulae - a cloud of gas and dust that does not re-emit light absorbed by nearby stars. It appears dark because it blocks the light of stars behind it.

Dark Site - an observing site with a dark sky. All telescope observers should have one. For the lucky, this will be the backyard. But, for most of us living in cities, this will be a distant site that requires driving to get there.

Darkness - a term referring to the actual brightness of the night sky.

Dawes Limit The limit of a telescope's angular resolution, expressed in arc seconds and equal to 4.56 divided by the telescope's aperture in inches: better called "Dawes' Empirical Criterion," a description of the smallest separation of equally-bright stars that can be achieved with a given telescope. A close modeling of the Dawes limit is given by 116/aperture_in_millimeters, or by 4.56/aperture_in_inches; the answer is in arc seconds. Not the final word in resolving closely-spaced point sources, but a very good guide none the less. It is not true that the criterion is valid only for sixth-magnitude stars (it is valid for any equal-brightness pair).

Dec - Declination

Declination The equivalent to latitude in the Celestial Sphere locating an object in the north-south directions. Celestial coordinates are listed in terms of Right Ascension and Declination. This is the angular distance of an object in the sky from the celestial equator.

Deep-Sky - A term used for dim, non-planetary, non-lunar astronomical targets, like nebulae and galaxies. Also: The region of space beyond the solar system.

Definition - The ability of a telescope to reveal, in an extended object image, the contrast between two areas having nearly the same brightness. Sometimes expressed as a combination of contrast and sharpness.

Deformed Optics - Optics that have become stressed from being pinched in their cells. This produces aberrations in the star images formed.

Density The amount of matter contained within a given volume. Density is measured in grams per cubic centimeter. The density of water is 1.0, iron is 7.9, and lead is 11.3.

Design Flaws - when an optical system is not designed for its intended use, and it does not perform as it should because of these, it is said to have design flaws. One good example of that is the presence of reflective surfaces in a telescope or eyepiece that allows reflected light to intrude into the visible field of view.

Dewing - Nighttime condensation of water on exposed optical surfaces after the dew point has been reached. It may be delayed through the use of a dew cap. Dewing happens whenever the temperature falls below that point where the air can hold the amount of water vapor it contains. Since cold air cannot hold as much water vapor as warm air, water condensation will form on any surface that is below this point. It is the reason your cold glass of soda drips in the summertime, and, unfortunately, why our optical surfaces sometimes can get covered with dew.

Dielectric: A reference to an electrically inert layering of transparent materials on glass that result in high reflectivity from that surface. Essentially the opposite of multi-coating, these coatings are designed to enhance reflection. The materials do not have the same reflectivity at all angles, so must be chosen to reflect at a particular angle. For the application to telescope optics, this is at a 45 degree angle in star diagonals and Newtonian reflector secondary mirrors. Reflectivities, without the application of any metallic film that can corrode, can approach 100% at specific frequencies, or average 98+% across the visible spectrum.

Diffraction - A modulation of light waves caused by interference in response to an obstacle in the path of propagation.

Diffraction Disk - The central, brightest part of a normal diffraction pattern of a star image. Its linear diameter is equal to 2.44X*f/D where X is the wavelength of light in microns, f is the telescope's focal length and D is the telescope objective's aperture. Also called Airy Disk.

Diffraction Limited: Performance in optical systems that is limited only by the wave nature of light. Often taken to mean satisfaction of Marechal's Criterion. a claim made by almost all telescope manufacturers, but probably lived up to by only a few makers. In its purest definition, "diffraction limited" refers to a telescope that is so perfect that only the wave nature of light, and not aberrations in the optical system, provide the limit to resolution of fine details and close separations in that instrument. In other words, the telescope itself is perfect in every way, and only the limitations of light itself impose these limits; therefore a diffraction limited telescope is a theoretical construction. In practice, this pure definition is rarely if ever used. Other definitions of "diffraction limited" have been proposed. These include the 1/14 wave RMS Marechal's Criterion, which states that a 1/14 wave wavefront RMS telescope satisfying certain other conditions passes as diffraction limited. In practice, a telescope built to this tolerance will have easily observable aberrations. Another definition proposed is the 1/4 wave p-v wavefront criterion, sometimes erroneously attributed to Lord Rayleigh. In practice, a telescope built to this tolerance is quite poor; Lord Rayleigh's own statement of this criterion was that such a poor telescope was "decidedly prejudicial" to the image quality. In another definition, diffraction limited refers to a telescope that meets the 1/50th wave RMS wavefront criterion, which corresponds to a Strehl ratio of 98.4%. Its current usage is largely as an advertising term to express ¡°high quality optics¡±.

Diffraction Pattern - The image of a point light source formed by a telescope. Also called Airy Pattern.

Digi-scoping - using a digital camera to shoot an image through the eyepiece of a telescope.

Digital Camera - a camera whose optical sensor is a silicon chip, and not photographic film.

Digital Photography Photographic imaging in which images are digitized and stored in a computer instead of recorded on film.

Digital Setting Circles - A computerized setting circle system on a telescope mount that simplifies locating celestial objects.

Diopter - a measurement of the refractive power of the eye in relation to its focal length. This typically describes the difference between perfect vision and what is measured in our eyes - negative for near-sighted vision, and plus for far-sighted vision.

Direct Objective Photography - A photographic technique that substitutes the telescope objective for the lens of the camera, focusing the image formed by the objective directly onto the film or chip. Also called Prime Focus Photography.

Disk The surface of the Sun or other celestial body projected against the sky.

Distortion - An optical aberration in which magnification varies across the field of view. In this aberration, the shape of the image is not a true copy of the object, even though it may be in sharp focus: see rectilinear distortion and angular magnification distortion.

Dob - Dobsonian Telescope Dobsonian mounted reflector. A simple alt/az mounted scope that is easy to use and offers very good value for the money.

Dobsonian - a simple altitude-azimuth telescope design made popular by John Dobson, a monk who extols the virtue of using simple materials to make a telescope. Commercial examples have become fancy and elaborate, but follow the idea of the simplicity of the alt-az movement and low centers of gravity.

Double Star A grouping of two stars. This grouping can be apparent, where the stars seem close together, or physical, such as a binary system, where the stars orbit each other.

Doublet: a two-element group of lenses.

Dovetail plate - a wedge-edged plate that attaches to the bottom of a telescope tube to allow its mounting on an equatorial mount. This can also refer to a plate that allows accessories to be mounted to the telescope.

DSC - Digital Setting Circles. Electronically display direction telescope is pointed in RA/Dec coordinates.

DSO - Deep Sky Object. Refers to most objects outside our solar system, including but not limited to galaxies, planetary nebula, globular clusters, etc.

Dwarf Planet - Any object in our solar system large enough for gravity to overcome hydrostatic equilibrium and pull the body into a spherical shape, that is in orbit around the Sun, and which does not constitute the major mass in its orbital zone. Objects included in this category would be the asteroids Ceres, Pallas, Vesta and Hygea, and the Kuiper Belt objects Sedna, Quaoar, Pluto, Eris, etc.


Eclipse The total or partial blocking of one celestial body by another.

Ecliptic An imaginary line in the sky traced by the Sun as it moves in its yearly path through the sky.

ED: short for "extra low dispersion," a reference to glass types that do not disperse light into its component colors so easily as regular glasses. ED glasses therefore exhibit less chromatic aberration by themselves than a typical crown or flint. Use of ED glass improves color correction in an achromatic refractor.

Edge-of-Field - the outer limits of the visible field in an eyepiece, next to the field stop, or edge, of the field of view.

Electromagnetic Radiation Another term for light. Light waves created by fluctuations of electric and magnetic fields in space. Technically, this refers to all frequencies of energy, even those we cannot see.

Electromagnetic Spectrum The full range of frequencies, from radio waves to gamma waves, that characterizes energy, whether thought of as waves or photons..

Element: in the context of optical systems such as eyepieces, "element" refers to an individual lens. For example, an Abbe orthoscopic is a four-element design; this means there are four lenses in the eyepiece.

Ellipse An ellipse is an oval shape. Johannes Kepler discovered that the orbits of the planets were elliptical in shape rather than circular. A circle is an ellipse, but one without any eccentricity.

Elliptical Galaxy A galaxy whose structure shaped like an ellipse and is smooth and lacks complex structures such as spiral arms. Currently thought to result from the collisions of smaller galaxies into one larger one.

Elongation The angular distance of a planetary body from the Sun as seen from Earth. A planet at greatest eastern elongation is seen in the evening sky and a planet at greatest western elongation will be seen in the morning sky.

Encircled Energy Ratio - The ratio of energy in an Airy Disk to the energy possible in a perfect one. For example, since a perfect Airy Disk contains 83.8% of the diffraction pattern's total energy, then if an Airy Disk contains only 79.6% of the total energy, the Encircled Energy Ratio is 0.95. Also called Strehl Intensity or Strehl Ratio.

Entrance Pupil - the opening through which light is admitted to the telescope - also called the aperture.

EP - Eyepiece

Ephemeris A table of data arranged by date. Ephemeris tables are typically to list the positions of the Sun, Moon, planets and other solar system objects.

EQ - Equatorial Mount

Equatorial Mount - A type of telescope mount designed to be oriented to the movement of the sky around the North Star (Polaris); a telescope mount with one axis parallel to the earth's polar axis. This provides easy tracking of celestial objects, and is preferred by many for astrophotography.

Equinox The two points at which the Sun crosses the celestial equator in its yearly apparent path in the sky. The equinoxes occur on or near March 21 and September 21. The equinoxes signal the start of the Spring and Autumn seasons.

Erecting Prism - A telescope accessory to give a correctly oriented image in a refractor or a catadioptric telescope. It can be a 45 degree, 60 degree, or 90 degree reflection, depending on the internal prism design.

Erfle - A type of wide-angle eyepiece: a class of eyepiece designs originated by Heinrich Valentin Erfle. It utilizes two doublets and a singlet in 2-1-2 configuration (see also Five-Element Pl?ssl). A variation that he also originated utilizes three doublets. Originally intended to provide apparent fields of up to 55 degrees, some modern designs offer fields up to 70 degrees or more. These eyepieces are commonly used in binoculars, spotting scopes, military gunsights, etc. Erfles exhibit flat fields, low distortion (except sometimes at the edges of Erfles with larger than average fields), but substantial off-axis astigmatism and lateral chromatic aberration.

Erfle, Heinrich Valentin: an optical designer, born 11 April 1884 in Duerkheim, Germany. He worked for the firm of Steinheil & Sohne until 1909, at which time he moved to Jena and worked for the telescope department at Carl Zeiss Inc. He became head of the department in 1918. He is credited with several advances in optical design and fabrication, affecting mostly military optics, and is the inventor of the well known Erfle eyepiece. He died on 8 April 1923.

Event Horizon The invisible boundary around a black hole past which nothing can escape the gravitational pull - not even light. This is not the actual mass of the Black Hole, but its effect on space-time. The mass resides in a point in the center.

Evolved Star A star that is near the end of its life cycle where most of its fuel has been used up. At this point the star begins to loose mass in the form of stellar wind due to the heating up of its core.

Eye Lens - in an eyepiece, the element or group which is closest to the user's eye.

Eye Relief - The distance needed between the eyepiece and the eye or the camera to achieve the optimal field-of-view (that point far enough to see all the rays from the eyepiece, and close enough to see the entire field of view): the distance from the vertex of the eye lens to the location of the exit pupil. This quantity can be made to vary for a given eyepiece by, e.g., the introduction of a Barlow lens, but this will be more pronounced with long focal length eyepieces, and in general is difficult to detect casually.

Eyepiece The lens at the viewing end of a telescope. The eyepiece is responsible for enlarging the image captured by the instrument. Eyepieces are available in different types and powers, yielding differing amounts of magnification and field of view.

Eyepiece Projection Photography Astrophotography where the telescope and eyepiece is used in place of the camera lens.

Exit Pupil - the minimum size of the width of the image formed behind an eyepiece when used in the telescope.

Exit Pupil Lower Limit: commonly quoted as 1mm, or 0.5mm, or some other arbitrary value. The lower limit to useful exit pupil is determined by seeing and by the presence or absence of floaters in the eye. On the planets, exit pupils of as low as .3mm or lower have sometimes been used to advantage; on close doubles, even smaller values have been used to make large, aesthetically pleasing Airy disks.

Exit Pupil Upper Limit: commonly quoted as 7mm, any larger exit pupil "wasting light." This is often quoted because the use of an exit pupil larger than this cannot be fielded by the maximum width of the dark adapted pupil, so rays outside this dimension do not enter the eye. But because the telescope is being used at low powers, where ultimate magnitude limits of a telescope cannot be seen, exceeding 7mm sometimes makes sense for low power, widefield views. In a reflector telescope, the shadow of the secondary mirror progressively becomes a larger and larger percentage of the exit pupil as it gets larger, and at some point the shadow becomes obtrusive and spoils the view. The 7mm figure isn't a bad average, though the maximum size of the dark adapted pupil shrinks with age - those older than 60 typically have dark adapted pupils of 4.5 to 5mm.

Extinction The apparent dimming of star or planet when low on the horizon due to absorption by the Earth's atmosphere.

Extragalactic A term that means outside of or beyond our own galaxy.

Extraterrestrial A term used to describe anything that does not originate on Earth.

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Agena Astro Products - The Great Atlas of the Sky, Jubilee Edition
Agena Astro Products - The Great Atlas of the Sky, Jubilee Edition
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Agena Astro Products BOOKS The Great Atlas of the Sky by P.Brych
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Sun Observer's Guide, Firefly Books
Sun Observer's Guide, Firefly Books
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A practical reference for how and why to view our nearest star. This book can start you making worthwhile observations.
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The History of Astronomy, Firefly Books
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Stargazing with a Telescope, Firefly Books
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Stargazing with a Telescope is a practical guide that demystifies the process of buying and using a telescope.
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Stargazing with Binoculars, Firefly Books
Stargazing with Binoculars, Firefly Books
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A practical, concise beginner's guide to viewing the night sky through binoculars
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Moon Observer's Guide, Firefly Books
Moon Observer's Guide, Firefly Books
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Manuf. No. Firefly 978-1-55297-888-7

Moon Observer's Guide offers practical guidance to amateur astronomers viewing Earth's only natural satellite.
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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
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Manuf. No. Firefly 978-1-55407-026-8

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Software Bisque TheSkyX Professional Edition for Mac
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Software Bisque TheSkyX Professional Edition for Mac
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TheSkyX Professional Edition for Mac is the next major release in TheSky's 25+ year history
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TheSkyX Professional for Windows will get you charged up about your passion. Explore, engage, enjoy while getting the most from your observing sessions.
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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.
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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
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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
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Field Version: Stars and deep sky objects are white on a black background. Unbound and printed on heavy paper
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