Introduction to Telescopes

Important Things to Know About Telescopes

by Susan & Alan French


The single most important factor in what a telescope will show you in the night sky is its aperture – the diameter of the main light collecting optical element. For the refractor, this is the diameter of the lens at the front of the tube. For the Newtonian reflector it is the diameter of the primary mirror at the bottom of the tube. For an SCT or other compound telescope it is generally the diameter of the front optical element.
Aperture determines how much detail you can see in objects. A larger telescope will reveal more detail on the Moon and planets. Aperture also determines how much light the telescope collects and a larger instrument will provide brighter views and show fainter objects. Keep in mind, however, that the steadiness of the air often limits the detail you can see, and you need dark skies to see really faint galaxies and nebulae.

Focal Length

Focal length determines how much of the sky you can see. Telescopes with shorter focal lengths can provide low power, wide field views of the sky. The extreme example is binoculars, which can provide wider views of the night sky than most telescopes and are wonderful for exploring the sky from a lawn chair.

Magnification or Power

If you look at the advertising for telescopes available in some local stores and discount catalogs you may get the idea that magnification is the most important quality of the telescope. As we have said, the most important property of a telescope is its aperture, which limits the magnification. As a general guide the optimum power you can use is about 30 power per inch of aperture, so the common 60mm (2.4″) refractor has an optimum power of 72x. With the highest quality optics and steady skies you can double this, so the highest power would be 144x. This is far less than the 400 power or more that is sometimes advertised for these scopes. In our area powers beyond 250 to 300 power rarely can be used to advantage.

We recommend staying away from any telescope that promotes high powers. Telescopes should be advertised by their aperture and focal length, not by a maximum magnification.

Telescope Mountings

There are many different mountings for telescopes, but they come in two basic flavors. The simplest is the altazimuth mount, which allows the telescope to be moved up and down (altitude) and back and forth (azimuth). This type of mounting was popularized by John Dobson, and is often called a Dobsonian mount, although John points out this type of mounting was used on cannon long before he was born. (Often the term Dobsonian refers to a Newtonian reflector on an altazimuth mount, but any type of telescope can be put on a Dobsonian mount.) To track objects across the sky with an altazimuth mount you need to move it in two directions.

The more traditional telescope mounting is the equatorial mount. This mount has one axis, the polar axis, which is pointed toward Polaris, the North Star. This lines it up with the Earth’s axis. You then can move the telescope north and south and east and west. Since the stars move from east to west you can track stars simply by moving the scope toward the west, or you can add a motor that moves the scope around the polar axis once a day and the scope will automatically track the stars.

In the past amateur astronomers had to find objects in the night sky and point the telescope at the object. With the advent of computerized telescopes, you can now do a very basic set up for the telescope, and then tell the telescope to find objects. Now even an altazimuth mount can track the stars since the computer can figure out the proper drive rates for any part of the sky and “tell” the motors what to do.

Although some amateurs believe finding objects and learning the sky is a fun and interesting part of the hobby, computerized telescopes do have a lot of appeal. The can reduce the initial frustration involved in finding celestial targets and allow more time actually viewing. Also, altazimuth mounts tend to be steadier, and easier to set up and use than equatorial mounts. (If you want to get into long exposure astrophotography, you still need an equatorial mount.)

A Few Cautionary Words

Seeing everything that a telescope can reveal is a learning experience. At first, you may not see much planetary detail or some of the fainter objects at the limit of visibility. As you spend more time at the eyepiece, you will find you can see more and more.

What you can see with a telescope is also dependent on other factors. Our atmosphere often limits the planetary detail you can see. On some nights, little detail may be visible, while on other nights small details may stand out in bold relief. You will see more if you get away from the city lights, and some nights are darker and more transparent than others are, allowing fainter objects to be seen. Finally, telescopes work best when they are at the temperature of the outside air, so take your telescope out well before you start observing.

Evaluating a Telescope

When you look at a telescope in the store or under the sky, here are some things to look for. First of all, the mount should allow the telescope to be moved smoothly and precisely and it should be easy to point it at celestial or terrestrial objects. A telescope that can not easily be pointed at objects in the night sky will prove to be a continual source of frustration. The mount should also hold the telescope steady enough to allow easy and precise focusing to make the image sharp and clear. If you want to do a quick check of the optics, use about 30 power per inch of aperture and see if the telescope “snaps” into focus. Keep in mind that this test may not be valid during the day, when the air is turbulent, or on nights when the air is not steady. You should also consider asking an experienced amateur to evaluate your new telescope or one you are considering for purchase.