The short answer
The best magnification for planets is the highest one that still looks sharp on that night, and that’s usually less than people expect. A common rule of thumb puts a telescope’s maximum useful magnification at about 50× per inch of aperture, or 2× per millimeter. On most nights, though, turbulence in the atmosphere limits planetary views to roughly 200–250× whatever the telescope, and often less. Many observers spend most of their planetary time between about 100× and 200×.
How magnification works
Magnification is the telescope’s focal length divided by the eyepiece’s focal length. A telescope with a 1,000 mm focal length gives 40× with a 25 mm eyepiece and 100× with a 10 mm eyepiece. Swap eyepieces to change power. A Barlow lens multiplies the power of any eyepiece, typically by two.
The magnification calculator does the math for your telescope and eyepieces, and shows the exit pupil and field of view as well. How to choose an eyepiece explains those terms.
The two limits
Aperture sets the ceiling. The diameter of the main lens or mirror decides how much fine detail the telescope can resolve. Beyond roughly 2× per millimeter, the image just gets bigger and dimmer without showing anything new. Experienced observers call this “empty magnification”.
| Aperture | Rule-of-thumb maximum | Notes |
|---|---|---|
| 70 mm (2.8 in) | About 140× | The aperture, not the air, is usually the limit |
| 90 mm (3.5 in) | About 180× | Often reachable on steady nights |
| 130 mm (5.1 in) | About 260× | The top end needs a steady night |
| 200 mm (8 in) | About 400× | Usually limited by the air, not the telescope |
The atmosphere sets the daily limit. Astronomers call the steadiness of the air “seeing”. On a poor night, planets shimmer and boil, and anything above 100× looks soft. On an excellent night, a large telescope can be pushed well past 250×. Seeing changes from night to night and even minute to minute, and you can’t predict it perfectly. The stargazing conditions tool gives a guide.
Planet by planet
- Jupiter. Its four large moons show at any power. Cloud belts appear at modest magnification; finer detail, such as the Great Red Spot and festoons, needs steady air and moderate to high power. Jupiter is large, so it often looks best a step below your maximum.
- Saturn. The rings are visible even at low power. Higher power, on a steady night, shows the gap in the rings called the Cassini Division and bands on the planet.
- Mars. Small, even near opposition. It rewards the highest power your seeing allows, and it’s worth waiting for moments of steady air. Away from opposition, it’s a tiny disk.
- Venus. Shows phases, like the Moon, at low to moderate power. Its glare can be reduced by observing in twilight, when the sky is not fully dark.
- Uranus and Neptune. Tiny disks at moderate to high power. Their color is the main reward.
A method that works
- Wait until the planet is as high as it gets; low planets shimmer more. Check planets visible tonight.
- Find and center it with your lowest-power eyepiece.
- Step up in stages, refocusing each time.
- When the image stops getting more detailed and starts getting softer, step back down one stage. That’s the right power for that night.
- Stay at the eyepiece. Moments of steady air come and go, and the finest detail appears in those moments.
A sharp, smaller image always shows more than a large, blurry one. Your eye picks out detail better when the image is crisp, even if it seems small at first.