Back Focal Length The distance from the final surface
of an optical system to the focal plane.
Bandpass The selectivity of a filter or spectrohelioscope - the
frequencies passed and not rejected.
Barlow, Peter: a mathematician born at Norwitch around
October, 1776. He began his optical work around 1827, and first occupied
himself with the color correction of refractors through the use of smaller
lenses positioned a considerable distance from the objective. The famous
Barlow lens was the result of collaboration between Peter Barlow and George
Dolland. Barlow designed a concave achromatic lens; Dolland made it in
1833 and attached it to a telescope. Its first user was Dawes, who employed
it while measuring close double stars. Peter Barlow died in 1 March 1862
at the age of 86 years.
Barlow lens: A small negative lens that amplifies and
relocates a telescope's beam when placed just inside focus. It is used
to eliminate the necessity of the use of short-focus eyepieces and in
negative lens projection photography. It is used to increase the magnification
of a telescope-ocular system. In pure terms, the "Barlow" lens
is this design, invented by Peter Barlow and first made by George Dolland.
A side effect of the Barlow is to vary the eye relief of eyepieces (it
increases it). This happens because the Barlow bends off-axis beams outward,
and when this light enters the eyepiece at an angle it was not designed
for, the exit pupil is moved back. Another, beneficial, side effect of
Barlow lenses is to frequently cause eyepieces to perform better off-axis
than they would without the Barlow; this is because in most telescopes
the focal surface is inward-curving, and the Barlow flattens (or nearly
flattens) it by reducing the size of field viewed, and because the greater
effective focal ratio of the system reduces the effects of astigmatism
present in the eyepiece. In general terms, in modern amateur astronomy
circles, "Barlow lens" refers to any telenegative lens designed
to increase magnification, and is applied, more or less loosely and improperly,
to amplifiers with 3 or more lenses.
Binary A system of two stars that revolve around a common
center of gravity.
Binoculars - Any optical instrument used for viewing that
utilizes both eyes.
Binoviewer - an optical device that splits the image coming
from a telescope so that both eyes can be used for viewing.
BK7: a borosilicate crown glass made by Schott Optical
Glass, with an Abbe number of 64.2 and a refractive index of 1.517. Found
in binoculars as the glass used in the internal prisms. BAK-4 glass is
preferred for short f/ratio lenses in expensive binoculars in order to
better control color aberrations, but in the longer f/ratios of telescopes,
BK-7 glass is preferred for diagonal prisms.
Boundary Layer, Thermal The layer of relatively warm
air on a telescope objective that is cooling down. The layer becomes harmful
to definition due to the refraction of light as it passes through the
Black Hole The collapsed core of a massive star. Stars
that are very massive will collapse under their own gravity when their
fuel is exhausted. The collapse continues until all matter is crushed
out of existence into what is known as a singularity (essentially, a point).
The gravitational pull is so strong that not even light can escape.
Blur-Circle - The "most-focused" star image in a telescope
possessing astigmatism. Focus is not exact, but the image is merely reduced
to its smallest apparent size. The lack of sharpness to this image is
why it is referred to as a "blur circle"
Bright nebulae - Nebulae that appear bright in the telescope.
They can either be "emission nebulae" (that absorb light from stars and
re-emit that light later) or "reflection nebulae" which reflect the light
of nearby stars off of dust and ice grains in the nebula.
Brunification - the yellowing of the lens of the eye that
comes with increased age. This is believed to be caused by continuous
exposure to ultra-violet light, which is why it is so important to wear
sunglasses that block this light every time you are outside.
Camera Adapter - A telescope accessory that receives
a T-Ring Adapter for attaching a Single Lens Reflex (SLR) camera to a
telescope. Also called T-Adapter
Camera Adapter Ring See T-Ring Adapter.
Cass - Cassegrain Reflector. See also Mak, RC, SCT.
Cassegrain: a telescope comprising a concave primary
mirror and a convex secondary mirror. Cassegrains can be made to satisfy
any three of four conditions: short tube length, small secondary mirror,
flat focal surface, and focal surface behind the primary. There are many
variants. The Classical Cassegrain produces images almost identical to
that of a Newtonian of the same focal ratio, except that the Classical
Cassegrain has a more strongly curved field. The Dall-Kirkham and Pressman-Carmichael
exhibit strong coma off-axis and are best for narrow-angle viewing, e.g.,
of the planets. The Ritchey-Chr¨¦tien exhibits round star images a good
distance off the axis, but has strong field curvature and exhibits off-axis
astigmatism. For a given focal ratio, a low amplification secondary mirror
will provide better off axis correction than a high amplification mirror.
Cassegrain Telescope - In its classical form, a compound
reflecting telescope employing a parabolic concave primary mirror and
a small hyperbolic convex secondary mirror to form images. Variations
of the Cassegrain design have been developed since its invention, two
of them being the Maksutov-Cassegrain and the Schmidt-Cassegrain.
Cat - Catadioptric Telescope. See also Mak, MN, SCT,
Catadioptric - Any optical system that uses a combination
of refractor and reflector lenses to produce a large aperture, compact
Catadioptric Telescope - A telescope employing a combination
of mirrors and lenses to form an image, as in a Maksutov-Cassegrain or
Cataracts - small opaque spots that occur in the lens
of the eye. The only cure is the removal/replacement of the lens in the
eye. This is an "end stage" to brunification, and points out how critical
the use of sunglasses are.
CCD - Stands for "Charge Coupled Device". CCD
chips are the detectors used in digital cameras.
Celestial Equator An imaginary line that divides the
celestial sphere into a northern and southern hemisphere.
Celestial Poles The North and South poles of the celestial
Celestial Sphere An imaginary sphere around the Earth
on which the stars and planets appear to be positioned: an observationally
practical model of the sky as a sphere with fixed stars that rotate around
the North Star (Polaris).
Central Obstruction In Newtonian and Catadioptric telescopes,
the obstruction caused by the secondary mirror.
Cepheid Variable This is a variable star whose light
pulsates in a regular cycle. The period of fluctuation is linked to the
brightness of the star. Brighter Cepheids will have a longer period. This
is one of the "standard candles" astronomers use to gauge the distance
to other nearby galaxies.
Chaos A distinctive area of broken terrain, as on the
Moon or planets.
Cheshire Eyepiece - a collimation tool for the alignment
of the Newtonian telescope¡¯s primary mirror. It consists of a bright ring
surrounding a peep hole (on the optical axis). The reflected image allows
the adjustment of a primary mirror by moving the centermark¡¯s reflected
image into the center of the dark area near the peep hole.
Chip - another term for the silicon wafer at the focal
plane of a digital camera.
Chromatic Aberration - An optical problem cause by light
going through a refractor lens with not all the light frequencies coming
to focus at the same point. Usually it is apparent at high magnifications
as rainbow edges on objects. An apochromatic lens system is designed to
solve chromatic aberration. In a simple lens, shorter wavelengths (blue)
focus closer to the lens than longer wavelengths (red). The result at
the eyepiece varies depending on the objective lens design. In a visually
corrected achromatic refractor, typically a haze of purple will be seen
around bright objects. In a photographically corrected achromatic refractor,
the haze will be more red or ruddy. In an apochromatic refractor, the
experience of chromatic aberration can be nil, or an extremely thin fringe
of color around the most highly contrasting edges of the brightest objects
(typically greenish, but varying depending on the lens). Mirrors are inherently
free from this aberration.
Chromosphere - The part of the Sun's atmosphere just
above the surface, between the Photosphere and the Corona.
Clarity - the absence of aerosols in the air.
Classical Cassegrain The original Cassegrain telescope
design consisting of a parabolic concave primary mirror and a small hyperbolic
convex secondary mirror. The design is geometrically perfect on-axis,
since any light ray that is reflected by a paraboloid toward Focus #1
of a convex hyperboloid must meet the optical axis at the hyperboloid¡¯s
Clock Drive - A motor that drives the polar axis of an
equatorial telescope mounting, enabling long-exposure photography and
continuous viewing at high magnifications. Essentially, it turns the telescope
mount at the same speed the Earth rotates.
Coating, antireflection - a thin dielectric or metallic
film applied to lenses that reduces reflections and increases the effective
transmission of the lens. For minimal reflection of a single wavelength
of light normally incident to the surface, the coating can consist of
a single layer, must have a refractive index equal to the square root
of the product of the materials on either side of it (typically, the product
of the refractive index of the glass to which it is applied and the refractive
index of air), and the thickness must be 1/4 the wavelength in question.
Multilayer coatings, which can correct for a wider range of wavelengths
or can be made to include or exclude specific bands, are deposited in
layers having alternating high and low refractive index. It was noticed
in the early 1900's that refractor telescopes transmitted more light 20
or 30 years after they were built than when they were new. The cause was
variously explained by a thin layer of pollution, or a thin layer of chemically
altered glass, which developed as the objective was exposed to the environment.
Experiments were done to simulate the aging of glass to gain the higher
transmission immediately and in a controlled manner, but in 1935 both
Carl Zeiss and Bausch & Lomb developed the antireflection coating
nearly simultaneously. By 1939, Carl Zeiss was "multi-coating"
their optics with a double layer of coatings, and in 1942 was using a
triple layer coating. It was not until the 1960's that multicoatings became
common or popular. The advantage of coatings is increased net transmission.
An uncoated air-glass surface loses about 4.0%; a multicoated surface
loses between 0.2 and 0.5 %, but some excellent coatings can lose as little
as 0.12%. The benefits of multicoating are therefore most dramatic on
complex multi-element systems, such as binoculars or zoom camera lenses,
or systems where high efficiency is required, such as telescope optics.
The most common single layer coating substance is magnesium fluoride (MgF2).
Other substances used include LiFl2, TiO2, SiO2, and others. Modern multicoatings
can have as many as 120 layers for specialty optics, but more typical
applications use between three and seven layer coatings. Some multicoatings
are soft and will not stand up well to cleaning; others are quite hard
and robust (ask the manufacturer for cleaning instructions).
COL - Computerized Object Locator. Essentially Digital
Setting Circles added to a non-computerized telescope mount to enable
the user to find objects in the sky.
Collimate The term used for adjusting a telescope to
gain maximum optical performance by means of alignment of the optical
Collimation - The proper alignment of the optical elements
of a telescope.
Color Aberration - See Chromatic Aberration.
Coma An asymmetrical off-axis aberration inherent in
certain telescope designs: an aberration which results in a point object
being turned into a pear-shape or comet shape at the focal plane, most
commonly off-axis. It is caused by unequal magnification in different
zones of a lens for obliquely incident rays from an off-axis object. An
easier way of putting this may be to say that coma is caused when light
enters a lens or mirror from the side, and rays from different parts of
the lens intersect the axis of those rays at different distances. In a
Newtonian with a paraboloidal primary mirror, coma is an inherent property.
A Newtonian with a spherical mirror can be made with no coma if an aperture
is placed at the radius of curvature of the mirror, creating a symmetrical
system. A common commercial Schmidt-Cassegrain telescope will exhibit
as much coma as a Newtonian of 60% the focal ratio (i.e., an f/10 SCT
has as much coma as an f/6 Newtonian) at the same distance from the axis.
However, this means that at a given angular distance from the center of
the field, coma is quite a bit more than in the Newtonian. Coma may be
corrected with a Ross coma corrector, but to eliminate spherical aberration
at the focal plane a hyperboloidal mirror in the Newtonian should be used.
See also Paracorr.
Coma An area of dust or gas surrounding the nucleus
of a comet.
Combination Collimation Tool - A combination of Sight
Tube and Cheshire collimation tools into one tool. It is a little less
sensitive than the other tools used separately, but is an economical way
to get the appropriate collimation tools for a Newtonian reflector.
Comet A gigantic ball of ice and rock that orbits the
Sun in a highly eccentric orbit. Some comets have an orbit that brings
them close to the Sun where they form a long tail of gas and dust as they
are heated by the Sun's rays and the pressure of the solar wind drives
those particles away from the body of the comet.
Compound Optics - any telescope containing lenses and
mirrors in the same scope. There are many, many configurations.
Computerized Mounts - a telescope mount that has a small
computer inboard that allows for computations of positions of objects
in the night sky, moves the telescope to the selected objects, and which
tracks those objects as the Earth turns.
Conjunction An event that occurs when two or more celestial
objects appear close together in the sky, with one directly north of the
object along a line of Right Ascension.
Constellation A grouping of stars that make an imaginary
picture in the sky.
Continuous Spectrum - a reference to an object which emits
light at all wavelengths, as opposed to an object that emits energy at
just a few discrete wavelengths.
Contrast - the difference between levels of brightness
in an image. It can be color, or black and white. When black and white,
the number of visible, discrete, steps between the brightest and faintest
parts of an image are referred to as the contrast level. Brightening the
brightest part, as with a larger aperture, or increasing the visibility
of the faintest parts, as with superior light transmission or darker skies,
will increase the contrast level of the image.
Contrast Factor - A consequence of the Strehl Ratio,
the ratio of the energy contained in a diffraction pattern's Airy Disk
to that contained in its bright diffraction rings.
Convection Currents Warm air rising from a reflecting
telescope's primary mirror, made turbulent by the open main tube.
Corona - The outer part of the Sun's atmosphere. The
corona is visible from Earth during a total solar eclipse. It is the bright
glow seen in most solar eclipse photos.
Corrector Plate - A refractive lens in a typical catadioptric
Cosmic Ray Atomic nuclei (mostly protons) that are observed
to strike the Earth's atmosphere with extremely high amounts of energy.
Cosmic String A tube like configuration of energy density
that is believed to have existed in the early universe. A cosmic string
would have a thickness smaller than a trillionth of an inch but its length
would extend for huge distances.
Cosmogony The study of the origin and evolution of celestial
systems, including the solar system, stars, galaxies, and galactic clusters.
Cosmology A branch of science that deals with studying
the origin, structure, and nature of the universe.
Crater A bowl-shaped depression formed by the impact
of an asteroid or meteoroid. Also the depression around the opening of
Cross Hair Reticle - Cross hairs of a guiding eyepiece
used in long-exposure astrophotography or for visual alignment of a computerized
CSC - Clear Sky Clock - an on-line weather prediction
tool for thousands of observing sites all over the US and Canada.
Culmination - the crossing of the meridian by any star
or object (caused by the turning of the Earth).
Curvature of Field: curvature of field is present when
the sharpest focus is formed along a curved surface rather than a flat
plane. Unless deliberate steps are taken to eliminate it, it will be present
in any optical system, including the Newtonian, refractor, Cassegrain,
Schmidt-Cassegrain, Maksutov, and others. Refractors, Newtonians, and
Cassegrain telescopes exhibit inwardly-curved (concave to the sky) field
curvature. The focal surface in the Schmidt-Cassegrain is quite strongly
curved. In the case of the achromatic refractor, the field curvature's
radius is around 0.32 times the focal length, and the curvature is concave
to the sky. Eyepieces have either positively or negatively curved fields
as well, and this can add to the telescope¡¯s.
CW - Counterweight - a weight added to the telescope to
balance its load to provide less strain for the gears and more accurate
movement as the telescope tracks the object.