Telescope “Power”

“How powerful is that telescope?” is a question that’s often asked by those considering purchasing one (or seeing various telescopes at an astronomy club star party). And it’s certainly understandable in that power has always been associated with these optical marvels. But in reality, a telescope actually has three kinds of power—two of which are much more important than its ability to simply magnify what it sees.
The most important function of a telescope—and the reason astronomers continue to build ever-larger ones—is its light-gathering power. This is the amount of light its objective lens or primary mirror collects. And the bigger the scope, the brighter are the images seen through it. Now here’s a fact about this that surprises most people and runs contrary to what they expect: when you double the size of the telescope, you don’t double the amount of light it collects but quadruple it! That’s because the area of a circle increases as the square of the diameter, which means there’s four times as many square inches of surface area on, for example, a 6-inch glass as on a 3-inch. There are now observatory telescopes on the drawing boards or under construction that will dwarf the famed 200-inch Hale reflector on Palomar Mountain. Can you imagine the wonders they will show of the universe?

Second in importance is resolving power. This is a telescope’s ability to reveal fine
features in an object. The bigger the scope, the more detail it will show—for example, on the Moon and planets, and more clearly resolve close double stars and stars in dense clusters. And here, when you double the size of a telescope, you double its resolution since that depends on the diameter of the lens or mirror and not its surface area.

Finally, we come to magnifying power—how much a telescope enlarges objects compared to what the unaided eye shows. This involves many factors, including type, size and optical quality of the instrument, stability of the mounting, state of the atmosphere, and the kind of object being observed. The Moon and planets typically take higher magnifications than do star clusters, nebulae and galaxies, which are best seen using low powers and wide fields of view.

An important word of advice: never purchase a telescope on the basis of magnification alone! As a rule of thumb, a good lower power is 10X per inch of aperture and high power upper limit 50X per inch. This means a range of 40X to 200X for a 4-inch aperture. Many observers find 25X per inch (100X for a 4-inch glass) to be an optimum magnification for most objects and average sky conditions. My own experience (having logged over 20,000 hours of stargazing time) is that just 30X to 50X on a good quality scope will show pretty much everything you could want to see in the sky!

— James Mullaney
Former assistant editor at Sky & Telescope magazine and author of 10 books on
stargazing. His latest, Celebrating the Universe!, is available from