Here are some of the terms used in our astronomy pages. We are working on a comprehensive glossary and will post it when it is complete and verified for accuracy. In the mean time, there are a number of outstanding glossaries available on line.

SEDS ( Students for the Exploration and Development of Space ) has a wonderful glossary at:

At the end of the SEDS glossary are a number of  additional links to other great sources of information.

(AU) astronomical unit:

The average distance from the Earth to the Sun. 1 AU =  149,597,870 km or 92,960,116 miles

Aperture represents the size (diameter) of the primary mirror or objective lens of a telescope.

Aperture Fever:
Many amateur astronomers suffer from a condition known as Aperture Fever. This is an ongoing desire for more light gathering power. The larger the aperture, the more faint objects one can see in their telescope.

Barlow Lens:
A lens designed to increase the magnification of an eyepiece. The eyepiece is slipped into the Barlow lens which is in turn placed in the focuser. Barlows come in many flavors. There are Barlows that will double (2X), triple (3X), and even quintuple (5X) the power of your eyepieces.

Binary Star:
Two stars which revolve around each other's center of gravity. Mizar, the 2nd star in the handle of the Big Dipper is a binary star. Have a look at it. The sky is brimming with binary star systems.

Catadioptric telescopes are essentially a combination of a refractor (lenses) and a reflector (mirrors). These telescopes fold the light path 3 times allowing for a much shorter tube. Further, because of their clever use of corrective lenses and lack of a spider to hold the secondary mirror, they are free of many of the optical defects that can plague refractors and reflectors.

Chromatic Aberration:
Chromatic Aberration is a defect in some optical systems that adds false color to images. These colors usually take the form of a light purple halo around the observed object.

Collimation is the term used for the alignment of the optics in a telescope. Improperly collimated optics will not deliver optimum performance and will add a number of undesirable distortions to the image.

A condition associated with some optical systems that has the effect of making objects at the outside edge of the field view have the appearance of being wedge shaped or look like little comets.

For astronomical purposes, contrast is simply how dark the sky appears behind the objects that are being viewed. The more contrast, the darker the sky and the more detail one can make out in the object being viewed.

The effect caused by light bending around obstructions in an optical system. (More coming on this including some graphic illustrations.)

Basically,(well almost) an oval. Webster's calls it a closed plane curve generated by a point moving in such a way that the sums of its distances from two fixed points is a constant: a plane section of a right circular cone that is a closed curve. Let's go with "kind of like an oval" till I can explain this with a few graphics.

A device used to move an eyepiece towards or away from the focal plane of an optical system like a telescope.

The distance light travels in a year.  9,460,920,000,000 km or 5,880,000,000,000 miles  or 63,239 AU

186,000 miles/second  or 299,274 km/second 

Newton (Issac):

Magnitude (or apparent magnitude):
Magnitude or apparent magnitude is the term used to indicate the degree of brightness of a celestial body. This system of measuring brightness was conceived by  the Greek astronomer Hipparchus.  Since his time, this system has been expanded to accommodate fainter objects. This is a numerical scale where the brightest star ( Sirius ) other than our sun (Sol) has a magnitude of -1 and the faintest star visible to the naked eye (of most human beings) have a magnitude of  6. An increase in one magnitude is equal to 2 1/2 times the brightness of the previous step in the magnitude scale. Our sun has a magnitude of -26. This is almost 6 trillion times brighter than a 6th magnitude star.

The chart below demonstrates how this scale functions from a magnitude 6 star to a magnitude 1 star in terms of increase in brightness.

Magnitude of star

Times brighter than magnitude 6













An objective is the main or largest lens or mirror in a telescope. If it is a mirror, it is often called the primary mirror.

A concave surface with a three-dimensional figure. ( We will try to get a nice graphic up explaining the differences between spherical, parabolic and paraboloidal surfaces.)

Reflex Sight:
A reflex sight is a zero power finder that greatly simplifies aiming your telescope. It works by projecting an illuminated reticule (pattern) on to a flat piece of glass. This is very reminiscent of a heads up display. This reticule is often shaped like a bullseye. You simply look through the glass and move the bullseye over the object in the sky that you want to look at with your telescope. The Telrad Reflex Sight is the most popular of these types of finders and in our opinion, should be installed on every telescope as it takes ALL of the frustration out of finding objects in the sky. At $40.00 to $50.00, it is very economical and probably the most cost effective and helpful accessory one can add to a telescope.

Seeing is the term used by astronomers to describe the quality (transparency) of the atmosphere on any given night. Our atmosphere is extremely dynamic. Most nights, the stars twinkle. Twinkling is caused by the light of the stars bending through our turbulent atmosphere. Poor seeing results in poor image quality at the eyepiece. Imagine looking at a quarter at the bottom of a swimming pool. Now imagine that the swimming pool has big waves. Get the idea? If you have a night of good seeing, grab that higher power eyepiece and use it. Otherwise, be happy with those lovely, wide angle lower power views.

The point in space directly above you.


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