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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 layer.

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, SN.

Catadioptric - Any optical system that uses a combination of refractor and reflector lenses to produce a large aperture, compact optical system.

Catadioptric Telescope - A telescope employing a combination of mirrors and lenses to form an image, as in a Maksutov-Cassegrain or Schmidt-Cassegrain.

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 sphere.

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 Focus #2.

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 parts.

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 system.

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 a volcano.

Cross Hair Reticle - Cross hairs of a guiding eyepiece used in long-exposure astrophotography or for visual alignment of a computerized telescope.

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.

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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
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.
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The History of Astronomy, Firefly Books
The History of Astronomy, Firefly Books
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Manuf. No. Firefly 978-1-55407-537-9

This extraordinary book traces humans' interaction with the endless wonders of the night sky.
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Stargazing with a Telescope, Firefly Books
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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300 Astronomical Objects: A Visual Reference to the Universe, Firefly Books
300 Astronomical Objects: A Visual Reference to the Universe, Firefly Books
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A handy and comprehensive reference to the 300 most interesting celestial objects. This book provides a tour through the galaxy.
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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
Item No. 896-10719
Manuf. No. Firefly 978-1-55407-026-8

This Firefly book is a very nice introduction to astronomy for beginners.
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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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Software Bisque TheSkyX Professional Edition for Windows
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Software Bisque TheSkyX Professional Edition for Windows
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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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Sky & Telescope - SKY ATLAS 2000.0 FIELD LAMINATED
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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
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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
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Sky & Telescope - Astronomy Library Sky Atlas 2000.0 Deluxe Edition
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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
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