So you would like to buy your first telescope, but aren't sure where to go to buy one or which style to purchase. You saw one in the toy or hobby store, but the clerk there could only tell you it costs $99.99 and knew nothing about how it works. So you took the next step and went to a camera store and the sales person asked if you would prefer a reflector, refractor or catadioptric style; and, if he or she really knew their stuff, they began introducing terms like, focal length, focal ratio, primary mirror or objective lens. Although you kept your poise and nodded your head in an affirming manner, you really had no idea what they were talking about.

To buy a relatively good telescope at a reasonable price you really need to understand some telescope terminology so you have a feeling about what is on the market, how it might fit your needs and how to understand sales personnel who are describing the features of the scope.

The EMDSO astronomy club presents this information in the four articles on our website under the heading entitled "Your First Telescope" We hope this information will help you in understanding telescope basics and some of the more popular designs on the market today.

One should note that all of the information presented in this document is accessible on the internet and in many publications in book stores. Nothing within this document is proprietary, but it is all gathered in one document that is meant to provide the first time telescope buyer a synopsis of what is available on the market without the pressure that some sales people might put on you as a customer.

Telescope Basics

To understand telescopes one must first understand that the purpose of a telescope is to gather light from the object being viewed, focus that light, and magnify it so the object’s details can be seen.  Notice that the first component is to gather light.  If you don’t gather enough light, high magnification won’t matter.  In fact higher magnification darkens the object every time, so your telescope needs to have enough light gathering capability for you to magnify the object.

The ability of a telescope to gather light is determined by its aperture.  The aperture is the diameter of the primary lens or primary mirror that captures the light of the object being viewed.  The captured light from the aperture is focused inside the optical tube. Since the optical tube is built around the primary lens or primary mirror, one can generally deduce that the larger telescope diameters gather more objective light than smaller telescope diameters. So users have to decide on the practicality of the physical size and weight of their ideal telescope.

The magnification of an object being viewed through a telescope is done through the telescope’s eyepiece lens and is determined by the ratio of the telescope’s focal length (F.L.) divided by the focal length of the eyepiece lens (f.l.).

The focal length of the telescope is determined by the light bending design of the objective lens or the primary mirror and is measured from the point where light bending or reflecting begins and ends at the focal plane where the light rays cross. These concepts are discussed in more detail under the different styles of telescopes presented later.

So a typical 4"(101 mm) telescope with a focal length of 24" (610 mm) and viewed through a 25 mm eyepiece will have a magnification (Mag) = 610 mm/25 mm = 24.4 X or, rounded off, 24 power. This means the object is 24 times larger through the telescope than what your eyes can see without the telescope. If the eyepiece is changed to a 15 mm lens, the magnification will be 610 mm/15 mm = 40.6 X or rounded off to 40 X, which is 40 times larger than what your eyes alone can see. But remember, the higher magnification darkens the object, so if you continue to reduce the eyepiece focal length you will eventually get to a point where the object is too dark to be seen through the telescope.

Increasing magnification also fills up the aperture window with the object being studied. This can be a serious problem in small aperture telescopes, since the turning earth tends to move the object out of the viewing window, more commonly called the Field of View (FOV), as fast or faster, than one can adjust the telescope to keep the object in the FOV. Also the higher magnification forces the scope to be looking at a much smaller section of sky, which further exacerbates the effort to keep the object in the scope's FOV.

A final problem with high magnification is in focusing the object. Even if you splurge and buy a telescope with tracking capability, you still have to adjust the eyepiece to bring the object into focus. Every time you touch the focus knob, you impart a small disturbance to the telescope, which generates vibrations that blur the objects appearance. The smaller the telescope the more pronounced is this vibration and high magnification only serves to amplify this problem. So it is often very difficult to manually focus an object with a high magnification lens.

When a manufacturer designs a telescope's focal length they do so by creating curvature in the objective lens or the primary mirror. This curvature is defined by the focal ratio, which is the ratio of the telescopes focal length to its aperture. The aperture of the telescope is the diameter of the objective lens or the primary mirror and is generally close to the inside diameter of the telescope barrel. It is generally displayed as (f/#) and read as (f number). An f/5 design means that the focal ratio is equal to 5. An f/10 means that the focal ratio is equal to 10.

With Equation 2, one can calculate the focal length of a telescope if he or she knows the focal ratio and the aperture size.

When viewing the skies through any telescope, it is highly advantageous to have the scope mounted on a tripod for stability. Most telescopes today are mounted on a variation of the Alt-Azimuth tripod, which is a tripod that allows the user to adjust both the horizontal (altitude) axis and the vertical (azimuth) axis with hand dials, electronic controls or just pushing the telescope barrel.

More sophisticated telescopes may also come with Equatorial Mount tripods, which allow the observer to set the telescope at the latitude of the observing location so the scope only has to be adjusted from east to west once a celestial object has been positioned in the telescopes viewing window (FOV). These mounts can be expensive since they are frequently accompanied by small DC synchronous motors that automatically move the telescope's optical tube across the sky at the same speed the earth is turning, virtually locking the object in the telescopes FOV. These mounts are called Tracking Mounts since they track the celestial object across the sky once they are found.

More expensive tracking mounts include a computerized hand control that features 40,000 or more celestial objects programmed onto the celestial grid. Once properly aligned, telescopes on mounts with these programmed hand controllers will move the optical tube to the sky location where the object, requested through the control, resides. These mounts are called Go-To mounts since they go to the object you request through the hand control.

A very popular mount used by many amateur astronomers for reflector telescopes is the Dobsonian Mount. This mount is actually an Alt-Azimuth mount that consists of a swivel table that sits on the ground with an easily movable telescope barrel. Most of these mounts are manually operated but some can be rigged up as a "Tracking" or "Go-To" mount.

The telescope mount that accompanies a beginner telescope is generally the Alt-Azimuth tripod mount. The adjustment knobs that move the altitude and azimuth on beginner models are generally stiff and of poor design. As the quality of the telescope increases, however, the quality of the mount generally follows. Some of the more expensive telescopes require that you buy the mount separately. Separate mounts can vary in cost from as low as $200.00 to $10,000 or more.

Now that we know a bit about telescope basics, it is time to look at some of the more popular designs in the market today. So check out the Refractors, Reflectors and Catadioptric explanations on our website.