Choosing a Good Telescope for Astronomy

Picking a telescope is a big decision.  Before we get into the pros and cons of each type of telescope, here are some rules of thumb to keep in mind so you get the best equipment to fit your needs.

First of all, don’t buy a cheap “trash” telescope that advertises amazingly-high magnification (>300x or 400x or more), and which is mounted on a spidery tripod that causes the image of any object to shake and wobble until you feel woozy.  These trash scopes are sold in many department stores and some sporting good stores or as an afterthought in some camera stores.  Do not EVER buy such a telescope.  You will regret it… guaranteed. 

You can spend a few hundred dollars on a good scope, or a few thousand on a great scope.  But as a guideline, you shouldn’t buy a telescope that costs less than $250.  You’ll be disappointed with the results.  If you can’t afford that much, save a few more dollars and use binoculars for now.


An example of a cheap “department store” telescope. Stay away.

Don’t worry about magnification.  As mentioned above, a telescope’s most important feature is aperture… the diameter of its light-collecting lens or mirror.  A telescope with a larger aperture collects more light, gives you a brighter image, and lets you see finer detail.  Your first scope should have an aperture of at least 80-90 mm.  Otherwise, the images of anything other than the Moon and perhaps Jupiter will be too dim and fuzzy.  You can, for example, see dozens of galaxies beyond our own Milky Way through a scope with 80 mm aperture from a dark location… they will be faint, but you will see them.  A 6-inch or 8-inch telescope (150 mm to 200 mm) will pull in fainter objects much more clearly, especially if you have to battle  light pollution. 

(Note: When astronomers refer to a 150 mm telescope, for example, they are referring to the diameter of the objective lens or mirror).

The view through a large-aperture telescope is almost always more impressive than the view of the same object through a smaller scope. The drawbacks of aperture?  Bigger aperture means higher cost and a bulkier telescope.  I’ll leave budgeting up to you.  But keep in mind the size and weight of a telescope, and how far you have to move it to your main observing site.  Some excellent beginner scopes might be 4-5 feet long, 8-9 inches wide, and have two main parts, each of which weighs 20 to 30 lbs or so.  Can you store and move all that?  If not, you will have to consider a more compact telescope.  You might give up some aperture, or buy a more expensive but compact telescope.  Just remember: a big scope isn’t any good if you can’t use it.

A Few Telescope Terms…

We’ve already mentioned the most critical optical characteristic of a telescope… its aperture, the diameter of the main lens or mirror of the telescope.  More aperture makes for a brighter image.  A good backyard telescope has an aperture of 80 mm to 300 mm (3.15” to 12”) or more.  Some big billion-dollar professional telescopes have an aperture of 10 m (400 inches), about the size of a small trout pond.

The objective lens or mirror collects light from a distant object and brings it to a focus.  The length over which this happens is called the focal length.

A second lens, called an eyepiece, is placed near the point where the light from the objective comes to a focus.  The eyepiece magnifies the image from the objective.  The eyepiece also has a focal length.  The magnification of a telescope and eyepiece is very simple to calculate.  If the focal length of the objective is “F” and the focal length of the eyepiece is “f”, then the magnification of the telescope/eyepiece combination is F/f.  For example, if a telescope has an objective lens with focal length of 1200 mm (about 48”) and it has an eyepiece of focal length 25 mm (about 1”), then it will have a magnification of 1200/25=48x.  Nearly all telescopes allow you to change eyepieces to get different magnifications.  If you want to get a magnification of 100x with this example, you use an eyepiece with 12 mm focal length.

The aperture of the objective lens of this simple telescope is D.  The focal length of the objective lens if F.  The focal length of the eyepiece is f.  So the magnification is F/f.  The focal ratio is F/D.

The aperture of the objective lens of this simple telescope is D. The focal length of the objective lens if F. The focal length of the eyepiece is f. So the magnification is F/f. The focal ratio is F/D.

Another rule of thumb… the maximum useful magnification of a telescope is about 50x the aperture in inches.  Any higher and the image gets too dim and fuzzy to be useful.  So a 4-inch scope can get you about 200x before the image gets too fuzzy and dim, a 6-inch scope gets you 300x, and so on.  This is not a hard-and-fast rule.  Sometimes, when the atmosphere is unsteady, you can only get to 20x or 30x per inch of aperture.  With high-quality optics and steady seeing, you might get to 70x or even 100x per inch of aperture, so for example, up to 400x with a 4-inch scope.  But this is rare.

The third key specification of a telescope is the focal ratio, which is the focal length divided by the objective diameter. A long focal ratio implies higher magnification and narrower field of view with a given eyepiece, which is great for observing the moon and planets and double stars. For such objects, a focal ratio of f/10 or more is ideal. But if you want to see wide views of star clusters, galaxies, and the Milky Way, a lower focal ratio is better. You get less magnification, but you see more of the sky. Wide field telescopes have a focal ratio of f/7 or less.