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How powerful is your telescope?
Wednesday, September 10, 2008 • Posted September 9, 2008

At star parties we're often asked: "How powerful is your telescope?" It's a good question, but one we have to answer with a brief explanation about telescopes. And we often start by talking about binoculars which many people have and already know something about.

Unlike telescopes, binoculars usually have information about power printed on them, expressed in two numbers, like 6x30 or 7x40.

The first number, which gives the instrument's power (also called magnification), tells how much larger and nearer an object will seem to appear. With 7x40s, for example, 7x means seven-power, thus these binoculars make things look seven times larger and nearer than when viewed with the naked eye.

For most binoculars, the power is fixed and cannot be changed. So, the question about power can be easily answered for binoculars. (Zoom binoculars are an exception as their power is changeable over a range.)

The second number gives the aperture which tells the diameter (in millimeters) of the lens at the big end. So the aperture of 7x40 binoculars is 40mm.

The big lens, however, doesn't do the magnifying. It gathers light from the object being viewed and focuses an image of the object on the lens nearest your eyes. That lens, called the eyepiece, does the magnifying.

Now let's look at telescopes. They too have a lens (or mirror) to gather and focus light. (Refracting scopes have a lens at the front end while reflecting scopes have a mirror at the back end--both serving the same purpose.) The diameter of the lens or mirror is the telescope's aperture. So my 8-inch reflecting telescope has an 8-inch mirror to gather and focus light.

And as in binoculars, the light is focused on the eyepiece which magnifies the image. But here's where telescopes have an advantage over binoculars. While binocular eyepieces are fixed, and thus so is their power, telescopes are made so eyepieces, and therefore power, can be easily and quickly changed.

Inserting a more powerful eyepiece yields a higher power than a less powerful eyepiece. For example, I have several eyepieces which give me a choice of powers from 77x to nearly 300x.

So the next time you're at a star party looking through a telescope, impress your friends by asking the star party host: What power are we seeing with this eyepiece? The host will be impressed too.

Sky Calendar.

  • 9 Tue. evening: The Moon is below Jupiter in the south.
  • 11 Thu. very early evening: Venus is less than a moonwidth above Mars with Mercury to their lower left low in the west.
  • 15 Mon.: The full Moon is this year's Harvest Moon, being the full Moon nearest the fall equinox.
  • 19 Fri. very early evening: Venus, Mars, Mercury and the star Spica are grouped near the western horizon; later the Moon is three moonwidths below the Pleiades star cluster when they rise by 10:30 p.m.
  • 22 Mon.: Fall (autumn) equinox, the first day of fall when the day and night are of equal length; the Moon is at 3rd quarter.
  • 24 Wed. morning: The crescent Moon is above the Beehive cluster, and tomorrow morning is below the cluster.
  • 26 Fri. morning: The crescent Moon is to the right of the star Regulus low in the east.

Naked-eye Planets. (The Sun, Moon and planets rise in the east and set in the west due to Earth's west-to-east rotation.) Evening: Venus, Mercury and Mars are low in the west with Jupiter dominating the south. Morning: None.

Stargazer appears every other week. Paul Derrick is an amateur astronomer who lives in Waco. Contact him at 918 N. 30th, Waco, 76707, (254) 753-6920 or See the Stargazer Web site at

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