Lesson 4: Magnification math

Work out how much your telescope magnifies, and learn when more power helps and when it hurts.

Indoors with a pencil, then outside to test it.

Steps for your telescope

Power comes from the eyepiece

Magnification tells you how many times bigger something looks through your telescope. At 20x, a tree looks 20 times bigger, as if it were 20 times closer. You change the magnification by changing the eyepiece.

The formula

Magnification = telescope focal length ÷ eyepiece focal length

Focal length is the distance from the lens or mirror to the point where the light comes to a sharp focus. Your telescope's focal length is in your manual, and it's often printed on the tube. Each eyepiece has its focal length printed on it in millimeters.

Travel Scope 80

Your Travel Scope 80 has a focal length of 400mm.

Eyepiece	Math
20mm	400 ÷ 20 = 20x
10mm	400 ÷ 10 = 40x

StarSense Explorer LT 80AZ

Your StarSense Explorer LT 80AZ has a focal length of 900mm. The 2x Barlow lens doubles the power of either eyepiece.

Eyepiece	Math
25mm	900 ÷ 25 = 36x
10mm	900 ÷ 10 = 90x
25mm with the Barlow	36 × 2 = 72x
10mm with the Barlow	90 × 2 = 180x

Another telescope

Find your telescope's focal length, then divide it by the number on each eyepiece. Try your own numbers in the viewer below.

Notice the pattern. Smaller eyepiece numbers give more power.

A student looks through a telescope at night — A student looks at Saturn through a telescope at the University of Wisconsin-Madison. Image: Brainandforce, CC BY 4.0

More power isn't always better

Higher power spreads the same light over a bigger picture, so the view gets dimmer. It also magnifies the shimmer of the air above you, so the view gets blurrier.

Travel Scope 80

Celestron lists 189x as the highest useful magnification for your telescope. That's the most it can handle on a perfect night. Your two eyepieces stay well under it, and they give bright, steady views.

StarSense Explorer LT 80AZ

Celestron lists 189x as the highest useful magnification for your telescope. That's the most it can handle on a perfect night, and your 10mm eyepiece with the Barlow comes close. Save it for the Moon on very steady nights.

Another telescope

A common rule says a telescope can handle about 60x for each inch of aperture on a perfect night. On most nights the air sets a lower limit.

How astronomers use power

Find your target with your lowest power eyepiece.
Center it in the view.
Switch to a higher power eyepiece if you want a closer look.
If the view turns dim or mushy, go back to lower power.

Big targets like star clusters look best at low power, because they spread across a lot of sky.

Worth a closer look

A half-lit first quarter Moon — A first quarter Moon. Craters look sharpest along the shadow line.

Close-up of a crater rim on the Moon — The rim of Copernicus crater, photographed by NASA's Lunar Reconnaissance Orbiter.

Saturn photographed through a small telescope — Saturn photographed through a backyard telescope.

The Pleiades star cluster — The Pleiades, also called the Seven Sisters.

Albireo, a gold star and a blue star — Albireo through an 8-inch telescope. One star glows gold and the other blue.

Swipe for more. Tap a photo to see it big. Photo credits

Sky challenge

Look at the same faraway object with each of your eyepieces. Use the formula to predict how many times bigger it should look with the higher power, then check it with your own eyes.

Check what you learned

Answer all three to finish the lesson and light its star.

1A telescope has a focal length of 400mm. What is the magnification with a 20mm eyepiece?

2On a telescope with a 900mm focal length, which eyepiece gives more magnification?

3At high power, the view looks dim and shaky. What should you try?