Telescope Buying Guide

The beauty of a dark night sky and the desire to view its treasures invisible to the naked eye can compel a budding astronomer to consider purchasing a telescope. However, such a purchase requires important decisions. Poor telescopes often have inexpensive optics and a feeble mount; this combined with simple inexperience can frustrate a growing interest in amateur astronomy. Care should be taken, therefore, in selecting equipment that does not discourage a burgeoning passion for the science.

Before committing to a telescope, stargazers should consider purchasing of a pair of binoculars. Although not often thought of as an astronomical tool, a good pair of 7x50 binoculars will reveal thousands of stars and some of the brightest deep sky objects. Binoculars have a much wider field of view than do telescopes, showing a larger area of the sky. Because of this, binoculars are useful for sweeping over large regions of the sky, especially within the Milky Way galaxy, enabling observers to take in many more stars than are visible to the naked eye. By panning the sky with binoculars, one will also become more familiar with the night sky, an invaluable skill if a telescope is later purchased. Binoculars do, however, have significant drawbacks: their magnification cannot be changed, and without a mount it can be difficult to steady one’s arms long enough to find deep sky objects and to study them in detail.

An upgrade from binoculars to a telescope can be a worthwhile investment to a dedicated amateur astronomer. The telescopes typically available in department stores for $100 to $200 are often of poor quality, and will not provide the views of the night sky required to transform budding stargazers into lifelong amateur astronomers. The optics are often made of plastic, and the mount is often much too weak to provide a stable, jitter-free view through the telescope. Additional clues that indicate a poor telescope, other than the place of purchase, are a small 5x24 finderscope, 0.965” eyepieces, and any claims of available magnification. Larger 6x30 finderscopes are much better, as are standard 1.25” eyepieces. The magnification of a telescope is not an important feature to consider when looking at a new telescope; rather, the aperture is much more relevant to the telescope’s performance. Aperture is the name for the opening, at the front end of a telescope, which determines the amount of light entering the telescope. The larger the aperture, the brighter the views and the greater the resolution and clarity of a telescope. Even the most expensive telescopes can rarely be pushed beyond a magnification of about 300x, as a result of the Earth’s atmosphere causing images in any telescope to waiver. Magnifications beyond this limit simply produce images too faint and unsteady to be useful. Rather than bargain hunting for a telescope at a local department store, telescopes should be purchased from a planetarium or store specializing in telescopes and accessories; in the absence of such retailers, there are several online sources for nearly every make and size of telescope and accessory imaginable.

There are several different types of telescopes, each with specific advantages and disadvantages; unfortunately, there is no single “best” telescope. The three main types of telescopes for beginners are achromatic refractors, Newtonians and Dobsonians. The most common first scope is a 70mm or 80mm achromatic refractor. Refractors are the traditional long and slender telescopes, and use a single lens in the front of the tube to focus the incoming light to an eyepiece at the base of the tube. Achromatic refractors are relatively inexpensive, and provide reasonably clear views in greater focal ratios of about f/10. They are easily transportable, and give good views of the planets and the Moon. For their aperture, however, achromatic refractors are still rather expensive.

Newtonian - which use a mirror at the base of the telescopic tube and reflect the light towards an angled mirror and then into the eyepiece - are cheaper per inch of aperture than a refractor. An average beginner Newtonian telescope is between 4” and 6”, and is mounted on an equatorial mount. The two axes of movement on such a mount are oriented to the celestial sphere, and therefore track the movement of the sky more easily than a regular tripod mount. These telescopes, with their additional light gathering power, will provide better views of faint deep sky objects. Their smaller focal ratios (usually f/4 to f/6), however, limit their effectiveness in providing the high powers necessary for planetary viewing.

Dobsonian telescopes (usually Newtonians) are based on a simple mount that swivels both up and down and in circles. Because they employ a minimal mount, these types of telescopes are relatively inexpensive and offer the largest aperture for the money. A good beginner Dobsonian ranges from 6” to 8”, and can be purchased for approximately $500.

Arguably the most popular amateur telescopes are the Schmidt-Cassegrains for their small size, portability and wide range of available accessories. They are, however, significantly more expensive as a first telescope, usually starting at about $1000.

In terms of accessories, a good set of eyepieces is necessary to maximize the performance of a telescope. Eyepieces of various focal lengths will provide an observer with different magnifications. Two or three different eyepieces will suffice for a beginning observer: one to provide low power, wide field views; one to provide medium power, and one for high power planetary observing. A good beginner set of eyepieces will cost around $300.

Clearly, amateur astronomy can become an expensive hobby quickly, so before purchasing a telescope, it is imperative that starting observers be sure they are truly interested. Finding deep sky objects is not always an easy task, and without perseverance, observing sessions can easily become a frustrating event. This is why a good pair of binoculars is highly recommended before the purchase of a telescope is considered. “The Backyard Astronomer’s Guide,” by Terence Dickinson and Alan Dyer, is an extremely valuable resource for anyone wishing to progress in their interest in observational astronomy. It contains extensive information regarding all aspects of observing the night sky.

• Sky and Telescope - Binoculars and Telescopes
• Introduction to Observing
• Observing the night sky
• Observing the Sun
• Planetary data

Used, with permission, from the Canadian Space Agency
"Grade 9 Astronomy Curriculum Resources" (2003)