The sky is mapped out in a spherical coordinate system similar to the system of Latitude and Longitude on the surface of the Earth. On the imaginary celestial sphere, the coordinates are Declination, which is equivalent to Latitude and measured in degrees, and Right Ascension, which is equivalent to Longitude, but measured in hours. The celestial equator is a projection of the Earth's equator onto the celestial sphere. Because the positions of stars and other distant celestial objects, as plotted on this celestial sphere, change very slowly with time, their listed coordinates and star charts are only updated every fifty years. On the other hand, planets change position so rapidly that their coordinates must be obtained from current astronomy periodicals. The setting circles on your equatorial mount can be aligned with the celestial sphere to aid in finding astronomical objects.

Any optical defect and/or design error which causes any of the processed light to deviate from reaching the focal point, therefore reducing the quality of the image.

The apparent brightness a star would have if placed at a distance of 10 parsecs from the earth. Distance of 1 parsec: 1parsec = 3.23 light year

A refractor lens, made of two or sometimes three separate lenses, which has the effect of bringing most of the viewed colours to a sharp focus, thus reducing chromatic aberration.

A thin layer of film applied to an optical surface that reduces the loss due to reflection of light.

The diameter of the primary mirror or lens.

A group of stars that appear to make an easily recognized shape, such as the "Big Dipper" or the "Coathanger". Example: "Orion contains an asterism or well-known star pattern called the ‘saucepan’ or ‘iron pot’." Cygnus - the Swan - is a cluster of stars, or asterism , easily visible in the summer skies.

An imaginary ball with the earth at its centre. All astronomical bodies, disregarding their true distance, are assigned a two-dimensional location on the surface of this ball.

The tendency of a lens to bend light of different colours by unequal amounts. It can produce nasty haloes around bright objects. A well-made achromatic lens reduces this problem.

In lenses this is an antireflection coating. In mirrors a coating is applied that preserves the aluminum mirror surface.

The process of aligning all the elements of an optical system. Collimation is routinely needed in reflectors, often in Catadioptric systems but seldom in refractors.

Similar to Latitude on the Earth's surface, it is the distance in degrees North or South of the Celestial Equator (the projection of the Earth's Equator onto the Celestial Sphere). The degrees can be sub-divided into minutes and seconds.

A name given by amateur astronomers to objects beyond our Sun and its planets.

Two or more stars that appear very close in position. True double stars are in orbit about one another, while optical doubles simply seem due to line of sight.

The blocking of one astronomical body by another as seen from the earth. The most common of these events are Solar and Lunar eclipses.

The distance between the eyepiece lens and the position in which the eye must be placed to see through the telescope. Telescope users who wear eyeglasses while observing, appreciate the benefits of longer eye relief.

This is the diameter of the beam of light from the eyepiece which reaches the pupil of the eye. It is usually expressed in mm, and determined by dividing the diameter of the primary (in mm) by the Magnification. Knowing this value and the diameter of your dilated pupil allows you to choose the eyepieces which will work best for you with a specific telescope.

The maximum view angle of an optical instrument. The number, in degrees, supplied by the manufacturer is the Apparent Field of View. To find the True Field of View (also known as the Actual Field of View), divide the Apparent Field of View by the Magnification.

The distance of the light path from the objective (primary lens or mirror) to the convergence of the beam. The convergent spot is called the Focus or Focal Point.

This is found by dividing an optical system's Focal Length by its Aperture. The resulting value is sometimes called the system's "speed".

A system of latitude and longitude defined by the plane of our galaxy rather than the equatorial system (RA and DEC) based on the celestial equator. Coordinates can also be specified locally, for example by Altitude and Azimuth.

A very old, large, dense cluster of stars, bound by gravity. Many form spherical clouds around galaxies. Our galaxy is surrounded by at least 130 globular clusters.

When light enters or leaves a lens, there is a loss of some transmitted light due to reflection. By applying a surface coating of an antireflective material such as magnesium fluoride, the transmission can be greatly increased and internal flare can be reduced. When all lens surfaces have been coated they are said to be fully-coated and when the surfaces are coated with multiple layers to maximize transmission, the optics are said to be multi-coated. Coatings also play a big part in the performance of reflectors because not all of the light is reflected; there is a small loss at each mirror surface. Today's reflectors usually have a thin coat of aluminum as the mirror and then an overcoat of silicon monoxide or silicon dioxide to protect it. Silicon dioxide produces a more durable coat than silicon monoxide but requires specialized equipment to apply it and is therefore more expensive. Protection is needed because, in most reflectors, the mirror is open to the elements and deterioration of the reflective layer reduces the resolution of the telescope. All Sky-Watcher reflectors are multi-coated with silicon dioxide for more durability.

A transparent optical element consisting of one or more pieces of glass. A lens has curved surfaces that bring distant light to a focus.

The amount by which a system increases the apparent size of objects. Magnification is determined by dividing the Focal Length of the telescope by the Focal Length of the eyepiece.

The bright flash of light seen when a piece of material from space (a meteoroid) burns up in the earth's atmosphere. A piece of this material which reaches the ground, is called a meteorite.

In a telescope, it is a highly polished surface made to reflect light. Primary mirrors are usually made spherical or paraboloidal (parabolic) to focus the light rays.

A cloudy object composed of gas and dust which glows with its own light is called an emission nebula while one illuminated by the starlight of nearby bright stars is a reflection nebula. A cloud of dust which blocks light from star fields or bright nebulae beyond it is a dark nebula.

The primary or largest element in an optical system. Sometimes it is called the "fixed optics."

A group of stars, normally resolvable, which are bound together gravitationally. They are usually about the same age, having being born together from a collapsing nebula.

The housing and optical train of a telescope; not including the mount, diagonal, eyepiece or accessories.

A parabolic or more accurately a "paraboloidal" mirror, is ground to a shape which brings all incoming light rays to a perfect focus, on axis.

A circular or oblong region of gas that has been thrown off by a central star. Its name comes from its apparent similarity to the disk of a planet seen in a very small telescope.

A telescope mount's axis that is parallel with the earth's axis. With a drive motor, the motion of stars due to the earth's movement can be counteracted so that they remain in the field.

See Magnifying Power.

The focal point of the objective mirror or lens.

The ability of an optical system to reveal details.

The ability of a telescope to separate closely positioned points.

Similar to but not the same as Latitude on the Earth's surface. It is the position eastwards from the Vernal Equinox, in 24 one-hour units. The hours can be sub-divided into minutes and seconds.

Circular scales attached to the telescope. They are marked off in degrees of Declination and hours of Right Ascension. Together, the circles allow the position of a known object to be found by setting the dials to the equatorial coordinates.

A blurring of the image caused by the inability of a spherical mirror to focus all light from infinity to one focal point. Light rays from the edge of the spherical mirror focus to different points than those from the center.

A group of stars that travel together through space. Two types of Star Cluster Globular Cluster and Open Cluster.

How much sky, in angular measure, is available at the eyepiece. It is contrasted with Apparent Field, which measures the field of the eyepiece alone.

The point in the sky directly overhead.

Beginner's Guide to Amateur Astronomy: An Owner's Manual for the Night Sky by David J. Eicher and, Michael Emmerich (Kalmbach Publishing Co., Books Division, Waukesha, WI, 1993). NightWatch: A Practical Guide to Viewing the Universe by Terence Dickinson, (Firefly Books, Willowdale, ON, Canada, 3rd edition, 1999). Star Testing Astronomical Telescopes by Harold Richard Suiter, (Willmann-Bell, Inc., Richmond, VA, 1994). Star Ware: The Amateur Astronomer's Ultimate Guide to Choosing, Buying, and Using Telescopes and Accessories by Philip S. Harrington (John Wiley & Sons, New York, 1998 ). The Backyard Astronomer's Guide by Terence Dickinson and Alan Dyer (Firefly Books Ltd., Willowdale, ON, Canada, revised edition, 1994). The Beginner's Observing Guide: An Introduction to the Night Sky for the Novice Stargazer by Leo Enright, (The Royal Astronomical Society of Canada, Toronto, ON, Canada, 1999). The Deep Sky: An Introduction by Philip S. Harrington (Sky Publishing Corporation, Cambridge, MA, Sky & Telescope Observer's Guides Series, ed. Leif J. Robinson, 1997). The Universe from Your Backyard: A Guide to Deep Sky Objects by David J. Eicher (Kalmbach Publishing Co., Books Division, Waukesha, WI, 1988). Turn Left at Orion: A Hundred Night Sky Objects to See in a Small Telescope--and how to Find Them by Guy J. Consolmagno and Dan M. Davis, (Cambridge University Press, New York, 3rd edition, 2000)

New! The Great Atlas of the Stars by Serge Brunier, Constellation photography by Akira Fujii (Firefly Books; Willowdale, ON, Canada 2001). A Manual Of Advanced Celestial Photography by Brad D. Wallis and Robert W. Provin (Cambridge University Press; New York; 1984). Astrophotography An Introduction by H.J.P. Arnold (Sky Publishing Corp., Cambridge, MA,Sky & Telescope Observer's Guides Series, ed. Leif J. Robinson, 1995). Astrophotography for the Amateur by Michael Covington (Cambridge University Press, Cambridge, UK, 2nd edition,1999). Splendors of the Universe: A Practical Guide to Photographing the Night Sky by Terence Dickinson and Jack Newton (Firefly Books, Willowdale, ON, Canada, 1997). Wide-Field Astrophotography by Robert Reeves (Willmann-Bell, Inc., Richmond, VA, 2000).

Apparent Field (A.F.) is the angle viewed by the eye when looking into the eyepiece. An eye by itself has an A.F. of about 100 degrees, so any well corrected design up to this value would be a benefit.

A concave mirror can produce a real images in the same direction or in the inverted direction depending on the position of the object. When a light source is placed on the focus of the mirror, the reflected rays will be parallel. This feature is used in torches to produce beam of light.

In case of convex mirrors, since the focus is on the other side of the object, the mirror reflects the rays from the object outwards. Therefore, convex mirrors can only produce virtual images. Convex mirrors are extensively used as viewing mirrors in automobiles.

Since any telescope, even a small one, can produce any given magnification with the right telescope eyepiece, magnification is not a useful way to rate a telescope. Therefore, the size of the lens or mirror (the heart of the telescope) is used as a general way to measure telescope potential. This is usually part of the telescope's model description.

The larger the telescope lens or telescope mirror, the fainter the objects a telescope will reveal (the more objects that will be visible), and the greater the telescope magnification it can use and still produce a good image. For example, at 120x, a ten inch telescope will reveal hundreds of objects not visible in a three inch telescope. In addition, at 120x, the 10" inch telescope will produce images that are brighter and sharper than the 3" inch telescope.

For amateur astronomers, a small telescope generally refers to a telescope of 4 inches or less. A medium telescope generally means a telescope of five to eight inches and a large telescope usually means anything over ten inches in size. However, this is a very general way to look at a telescope. Optical quality and special features may allow a small telescope to outperform a larger telescope for some types of observing.