How Much Power do I Need?

Small and inexpensive "department store special" scopes are often supplied with cheap two-element 0.965" eyepieces and a Barlow lens that are supposed to give you magnifications of up to 675x. But is that much power really possible – or needed? No!

While any telescope can theoretically magnify any amount, the highest useful power of a telescope under ideal (and therefore rare) seeing conditions is only 50x or 60x per inch of aperture (and turbulence in our atmosphere limits most scopes to 25x or 30x per inch of aperture on an average night). That means that those wobbly 2.4" department store toys are realistically capable of a maximum power of 144x (2.4" times 60x per inch of aperture) on a night of perfect seeing and only 75x on an average night . . . not the 300x to 675X they usually claim. Also, the higher the power, the dimmer the image – and 2.4" refractors don't gather much light to begin with. Push them to 60x per inch, and the images (if they're bright enough to see at all) just get very dim and blurry. It's called "empty magnification."

So, how much power do you really need? It depends on what you want to see. High power is fine within the solar system, although there's a wealth of lunar and planetary detail available at a mere 50x to 100x.

And, except for splitting close binary stars and resolving globular clusters, very high power is usually not needed outside the solar system, either. Stars always look like points of light, no matter how high the power. Many planetary nebulas – like the Ring, M57 – look great (although small) at 100x, but get too dim to see well if you boost the magnification much further in an attempt to see more detail.

And deep space objects? The Andromeda Galaxy is actually over 3o across – more than six times the diameter of the Moon. You don't need a lot of power to see something that big! Except for its small and bright core, however, it's too faint to see with your unaided eye. Binoculars, or a rich field scope at 16x or 20x, gather enough light to let you see Andromeda while still having a wide enough field to show you its full extent. The narrow field of a high power scope, on the other hand, can't capture all of Andromeda and other large objects, such as open clusters, in a single field.

The point is that you don't always need high magnification to see well, but your eyes do need enough light to see at all.



. . . our 34th year