A Helpful Telescope Glossary

Telescope Terms and Glossary

Telescope Terminology Confusion When you enter into any new area of science, you almost always find yourself with a baffling new language of technical terms to learn before you can converse with the experts. This is certainly true in astronomy both in terms of terms that refer to the cosmos and terms that describe the tools of the trade, the most prevalent being the telescope. So to get us off of first base, let’s define some of the key terms that pertain to telescopes to help you be able to talk to them more intelligently.

The first area of specialization in telescopes has to do with the types of telescopes people use.

The three designs a telescope may have are the Refractor, the Reflector and the Schmidt Cassegrain telescope.

The refractor telescope uses a convex lens to focus the light on the eyepiece.
The reflector telescope has a concave lens which means it bends in. It uses mirrors to focus the image that you eventually see.
The Schmidt Cassegrain telescope uses an involved system of mirrors to capture the image you want to see.

$Refractor Telescope$
Refractor	Reflector	Schmidt-Cassegrain
3 Basic Types of Telescopes

Beyond the basic types, other terms refer to parts of the telescope

Collimation is a term for how well tuned the telescope is to give you a good clear image of what you are looking at. You want your telescope to have good collimation so you are not getting a false image of the celestial body.
Aperture is a fancy word for how big the lens of your telescope is. But it’s an important word because the aperture of the lens is the key to how powerful your telescope is. Magnification has nothing to do with it, its all in the aperture.
Focuser is the housing that keeps the eyepiece of the telescope, or what you will look through, in place. The focuser has to be stable and in good repair for you to have an image you can rely on.
Mount and Wedge. Both of these terms refer to the tripod your telescope sits on. The mount is the actual tripod and the wedge is the device that lets you attach the telescope to the mount. The mount and the wedge are there to assist you with a superior viewing session and to keep your expensive telescope safe from a fall.
An Altazimuth Mount refers to the tripod of the telescope that holds the device in place and makes it useful during a star gazing session. The altazimuth mouth allows the telescope to move both horizontally (which is the azimuth) and vertically. In this way you have full range to look at things close to the horizon or directly overhead.
Coma has a different meaning than the one we are used to, and that’s a good thing. The coma is the blurry area on the outer rims of your view through the telescope. How big the coma is and to what extent it interferes with your viewing will have is important to the effectiveness of your telesscope.
Planisphere. A fancy word for a star chart. It is nothing less or more than a detailed map of where everything is in the cosmos and how to find the star you wish to study by keying off of known stars.
Barlow. This refers to a specialized type of lens that you can buy to enhance the magnification of your telescope.

The ability of a telescope to focus light is related to the optical features you use.

Considerations to make when purchasing optical equipment – Aperture, Magnification, Focal Length, Focal ratio, Wave Number, and Resolution. If you purchase low quality merchandise, you won't be able to see what you are looking for as well.

The most important consideration is the Aperture. This is the ability to focus light and is related to the size of the lens or mirror that is used. In other words, the bigger the lens or mirror, the brighter the final image. The biggest telescope, however, is not always the best.
Magnification is dependent upon the lenses used and the distance to the eyepiece. A general rule of thumb to follow is 40-60x per 1 inch of aperture. Usually low magnification gives the best light.
Focal length is defined as the optical length of the telescope lens. Long focal lengths usually mean higher magnification. Don't mistake the length of the telescope for focal length though. Some compound telescopes light paths are folded and become longer in a short tube.
The focal ratio tells us about he brightness and width of the field of view. They are divided into three sections. F/10 or higher is good for viewing the moon, planets, or stars. F/8 is great for all around viewing. F/6 and lower is meant for viewing deep sky objects.
The wave number is also known as the wave error. It tells us how good the mirror or lens was ground to an almost perfect surface. The smaller the number, the better the lens or mirror. The minimum number that is acceptable is one fourth. The performance is in the accumulation of the numbers of each optical piece.
Resolution is the ability to find the details in the moon or a planet. It is dependent upon how well the telescope can separate two close objects. It also rests on the aperture and the observing conditions that are present.

Aperture

Bringing light to the eyepiece is the most important quality of a telescope, and light gathering ability is limited by the aperture or diameter of a telescope's mirror or lenses. A telescope with a 70mm objective lens collects 100 times as much light as the human eye, and an 8 inch telescope boosts the eye's light gathering ability by nearly 1000 times. Simply put, a larger telescope produces images that are bigger, brighter, and sharper.

Refractors

The refractor telescope with a lens in front and an eyepiece in back was invented by Galileo 400 years ago. Small refractors with objective lenses from 60mm to 90mm are popular with beginning observers because they offer crisp, high contrast views of bright objects. A 70mm refractor on an Alt-Azimuth mount is a good first telescope for a student because it is inexpensive, easy to use, and offers pleasing detail on the Moon and planets. Larger refractors can be troubled by "chromatic aberration" also known as false color or blue fringing unless the objective lens is made of exotic and expensive ED or Fluorite glass.

Reflectors

Using mirrors instead of lenses, the reflector telescope was invented by Sir Isaac Newton. Reflector telescopes offer more light gathering power per dollar than any other telescope design, but they require a little more patience to use because mirrors need to cool to the temperature of the night air before they produce their best images. The mirror may also need to be aligned or "collimated" occasionally. Dobsonian reflectors up to eight or ten inch aperture can be handled easily by one person and are large enough to show hundreds of galaxies and resolve globular clusters into thousands of individual stars.

Catadioptric

Compound or catadioptric telescopes are a modern invention. Using a combination of mirrors and lenses these compound telescopes "fold" a long optical path into a compact body. Catadioptric telescopes are well suited for use with high-tech computerized mounts. These computerized mounts will help find thousands of stars, galaxies, and galactic nebula, but they need a power source such as a rechargeable battery because internal AA batteries only last a few hours. Computerized telescopes featuring 90mm to 8 inch Catadioptric designs are popular because they offer a terrific combination of power and portability.

Eyepieces

Most telescopes today come with one or two eyepieces in the 1.25 inch size. Changing the eyepiece changes the magnification of the telescope because the magnifying power is equal to the focal length of the telescope divided by the focal length of the eyepiece. A telescope with a 900mm focal length and a 10mm eyepiece will magnify the image by 90 times. (900mm/10mm = 90x magnification). There are many eyepiece designs, but plossl eyepieces have become the most popular because they offer very good performance for a reasonable price. Exotic wide field eyepieces often sell for hundreds of dollars each but have many fans. Avoid telescopes that use the older style 0.96" size eyepieces because they often use H type or Huygenian eyepieces which are notorious for fuzzy images and a narrow field of view.

Magnification

One specification that manufacturers sometimes list is "maximum useful magnification" which is usually about 50 times the telescope aperture in inches or 2 times the aperture in millimeters. A 70mm refractor will show good lunar detail up to 140x magnification; pushing the magnification beyond 140x may show an image that is larger but fuzzier. Beware of small telescopes that claim 500x magnification, it may be impossible to see anything at that power.

Optical Coatings

Plain glass reflects light. Telescope lenses and eyepieces may lose a significant amount light due to reflection at each optical surface. Optical designers overcome this problem by adding anti-reflection coatings to glass surfaces. Multi-layer coatings can be more efficient than single layer coatings. The term "multi-coated" often means that one surface has multi-layer coatings while internal surfaces may have single layer coatings. "Fully multi-coated" means that every air-to-glass surface has multi-layer anti-reflection coatings.

Finder Scopes

Telescopes need a finder scope with a wide field of view because the main scope often shows a field of view so narrow that objects can be hard to find. Red-dot finders are best suited for finding bright objects like the moon and planets. A 6×30 or a 9×50 optical finder scope is a small scope with a cross hair that allows the telescope to be aimed precisely.

Alt-Azimuth Mounts

A sturdy telescope mount is essential for getting a good view. The Alt-Azimuth mount is light and easy to use because it moves up and down (altitude) or side to side (azimuth). The drawback is that an Alt-Azimuth mount cannot track stars and planets unless it has a built in computer. As soon as you get the Moon or a planet centered in the eyepiece, the Earth's rotation causes it to drift toward the edge. For this reason manual Alt-Azimuth mounts work best for smaller telescopes and lower magnification.

Equatorial Mounts

The equatorial mount is sometimes known as the German Equatorial Mount (GEM) because it was invented in Germany. An equatorial mount uses counterweights to balance the telescope, and allows the telescope to track stars and planets with a single slow-motion knob or with a motor drive. A good equatorial mount can cost more that the optical parts of a telescope, but it will keep a planet centered in the eyepiece at high power or even allow photography through the telescope.

Astrophotography

Once you have a telescope you may want to start taking pictures. It‘s only fair to warn you: those pictures you see in magazines took literally years of practice to achieve. The Moon is a good place to start, simply holding a point-and-shoot camera up to the eyepiece can produce a nice picture of the Moon. For close-up views of the planets a modified web-cam like Celestron's NexImage is a good choice. The combination of a web-cam and image processing software can produce amazing images of the planets, but it still takes a lot of practice to get the best results!

Now for an explanation of terms referred to in Meade telescope specs..

Eyepieces

Telescope Eyepieces With the telescope's primary optics (objective lens, primary mirror, or a combination of lenses and mirrors) having formed an image at the telescope's focus, the purpose of the eyepiece (consisting of two or more small lenses mounted in a metal barrel) is to magnify this image. Eyepieces are available in a wide range of optical configurations, barrel diameters, and focal lengths. It is the focal length of the eyepiece, in conjunction with the focal length of the main telescope, that determines the operating power of the eyepiece. (See How to Calculate Power)

Eyepieces: Meade telescopes include eyepieces to magnify objects seen through the telescope. Wide field eyepieces permit finding and observing objects with great clarity and ease. High power eyepieces may be used under favorable conditions to increase image size for more detailed observations.

Telescope Barlow Lens The Barlow Lens: Inserted into the telescope in front of the eyepiece, the Barlow lens effectively multiplies the focal length of the main telescope

A 2X Barlow lens doubles the main telescope's effective focal length, thereby doubling the power of each eyepiece used with the Barlow.
Telescope Diagonal Mirror
Diagonal Mirrors, Erecting Prisms, and Viewfinders: A variety of telescope accessories are either supplied as standard equipment or available optionally, depending on the telescope model.

Diagonal Mirrors: When observing objects nearly overhead through refracting or mirror-lens telescopes, the diagonal mirror (or in some cases, diagonal prism) permits a comfortable observing position. The diagonal mirror diverts light out to a right-angle to the telescope's main tube. All Meade refractors and mirror-lens telescopes include a diagonal mirror or prism for this purpose.

Viewfinders: Most telescopes have rather narrow fields of view. As a result, finding and centering an object in the telescopic field can be difficult unless a viewfinder is used. The viewfinder is a small, low-power, wide-field telescope, usually equipped with internal crosshairs for easy object-sighting. With the viewfinder aligned parallel to the main telescope, objects first located in the viewfinder are then also in the main telescope's field.

The Red Dot Viewfinder: Because the main telescope has a fairly narrow field of view, locating objects directly in the main telescope can sometimes be difficult. The red dot viewfinder projects a small red dot that permits you to more easily locate objects. When the red dot viewfinder and optical tube are aligned to each other, both point to the same position in the sky. An object located in the viewfinder is therefore in the eyepiece of the main telescope.

Tripod: A rigid, full-length field tripod is used to support the telescope and is adjustable to allow standing or seated observations through the telescope.

Astronomy Software: Meade Telescopes include AutoStar®Suite Astronomer Edition Software on DVD AutoStar Suite Astronomer Edition software will help you learn the night sky. It displays more than 10,000 celestial objects including planets, stars, galaxies and nebulas. You can print out star charts and even plan your observing sessions. It operates on any Windows®based PC.

Telescope Mountings

Once an object, whether terrestrial or astronomical, is located and centered in the telescope's field of view, the telescope's mechanical mounting permits the observer to track, or follow, the object as it moves across the landscape or sky. Types of telescope mountings include the following:

Altazimuth Mountings: The simplest type of telescope mount allows the telescope to be moved up-and-down (in vertical, or altitude) and left-to-right (in horizontal, or azimuth). The altitude-azimuth (altazimuth) mounting thus permits the observer to follow objects by simple motions of the telescope in vertical and horizontal. Slow-motion controls, can facilitate these motions. The altazimuth mount, owing to its simplicity and relatively lower cost, is widely used with telescopes in both land-viewing and astronomical applications. Example: Meade NG70-SM and 90AZ – ADR.

Equatorial Mountings on a Telescope Equatorial Mountings: Although celestial objects are essentially fixed in their positions in the sky (on the celestial sphere, the imaginary spherical surface on which all astronomical objects are located), they appear to move in an arc across the sky, as the earth rotates underneath the sky once every 24 hours. From an astronomical point of view, therefore, the task of the telescope mounting is to compensate for the Earth's rotation and allow the observer to track the Moon, planets, and stars. This task is made vastly easier by the equatorial mounting, the type of mounting incorporated into most larger or more advanced telescopes.

By aligning one axis of the equatorial mount to the Earth's rotational axis (a simple process which involves pointing one telescope axis to the North Star), the observer can track astronomical objects by turning one control cable, instead of the two simultaneous motions required with the altazimuth mount. If a small motor is attached to the equatorial mount, this tracking can be performed automatically. These motor drives are available for most Meade equatorially mounted telescopes.