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