If you’re new to astronomy, you might be wondering what size telescope to see planets. It’s the most common question beginners have, and the answer is more nuanced than a single number. The right telescope size depends on your goals, budget, and viewing conditions. This guide will help you understand the key factors so you can make a great choice.
What Size Telescope To See Planets
So, what does “size” really mean? In telescopes, size refers to the aperture—the diameter of the main lens or mirror. Aperture is king. It determines how much light your telescope gathers. More light means brighter, clearer, and more detailed views. For planets, which are small but bright, aperture is crucial for seeing details like Jupiter’s cloud bands or Saturn’s rings.
Minimum Aperture for Planetary Viewing
You can start seeing planets with a very small telescope. Even a 60mm (2.4-inch) refractor will show you Jupiter’s moons and Saturn’s rings as tiny “ears.” However, for satisfying views that reveal actual details, a larger aperture is recommended.
- 70mm to 80mm (3-inch) Refractors: A decent starting point. You’ll see Saturn’s rings, Jupiter’s main cloud bands, and the phases of Venus.
- 90mm to 114mm (3.5 to 4.5-inch) Telescopes: This is a much better starting range. Views become more stable and detailed. The Great Red Spot on Jupiter becomes a possible target under good conditions.
- 130mm to 150mm (5 to 6-inch) Telescopes: The ideal beginner range for serious planetary observation. These scopes collect significantly more light, revealing finer details and allowing for higher magnification.
The Sweet Spot: 6-inch to 8-inch Aperture
For many amateur astronomers, a 6-inch or 8-inch Dobsonian reflector is the perfect planetary telescope. It offers excellent detail without being too bulky or expensive. Here’s what you can realistically expect to see with a telescope in this range:
- Jupiter: Clear view of two main cloud belts, the Great Red Spot (when it’s facing Earth), and up to four Galilean moons as distinct disks.
- Saturn: Clearly defined rings, the Cassini Division (a dark gap in the rings), and several of its moons like Titan.
- Mars: During its close approaches, you can see dark surface markings and its white polar ice caps.
- Venus: Obvious phases, similar to the Moon’s.
- Moon: Incredible, crater-filled views with dramatic shadows along the terminator line.
Beyond Aperture: Other Critical Factors
Aperture isn’t everything. A large, poor-quality telescope will perform worse than a smaller, well-made one. You must consider these other elements.
Focal Length and Focal Ratio
The focal length is the distance light travels inside the telescope to reach focus. It’s usually marked on the tube. The focal ratio (f/number) is the focal length divided by the aperture. A longer focal length and higher focal ratio (e.g., f/8 to f/15) are generally better for planets. They provide higher magnification more easily and often have fewer optical aberrations.
Optical Quality and Telescope Type
There are three main types. Each has pros and cons for planets.
- Refractors (Lens-based): Offer high-contrast, sharp views. A good 4-inch apochromatic refractor can outperform a cheaper 6-inch reflector on planets, but they are more expensive per inch of aperture.
- Reflectors (Mirror-based): Best value for aperture. A 6-inch Newtonian reflector on a Dobsonian mount provides stunning planetary views for a reasonable price. They require occasional collimation (mirror alignment).
- Compound Telescopes (Catadioptric): Like Schmidt-Cassegrains. They pack a long focal length into a short tube. Excellent for planets and very portable. They sit in a middle price range.
The Importance of Eyepieces
Your telescope’s view is only as good as your eyepiece. The eyepiece determines magnification. Magnification is calculated as Telescope Focal Length / Eyepiece Focal Length. A 1000mm telescope with a 10mm eyepiece gives 100x magnification. For planets, you’ll want a selection of eyepieces that give you a range of magnifications.
- Start with a low-power eyepiece (e.g., 25mm) to find and center the planet.
- Then switch to a high-power eyepiece (e.g., 10mm, 6mm) to zoom in. The maximum useful magnification is usually about 50x per inch of aperture (2x per mm).
Mount Stability
A wobbly mount ruins planetary viewing. The image will shake every time you touch the telescope or if the wind blows. A solid, stable mount is essential. Equatorial mounts are great for tracking planets smoothly, but a simple Dobsonian base is famously stable and easy to use.
Step-by-Step: Choosing Your First Planetary Telescope
- Set a Realistic Budget: Include money for a couple of additional eyepieces and maybe a planetary filter.
- Prioritize Aperture Within Your Budget: Aim for at least a 4.5-inch reflector or a 3.5-inch refractor. A 6-inch or 8-inch Dobsonian is the top recommendation if you can manage its size.
- Research Specific Models: Read reviews and ask on astronomy forums. Brands like Sky-Watcher, Celestron, and Orion have reliable beginner options.
- Consider Portability: Where will you store it? How far will you carry it? A huge telescope you never use is worse than a smaller one you use often.
- Check the Included Accessories: Does it come with a finderscope and decent eyepieces? Factor in the cost of upgrading poor-quality ones.
What You Won’t See (Managing Expectations)
It’s important to have realistic expectations. Even with a large telescope, planets will appear small. You will not see Hubble-like, color-saturated, fill-the-eyepiece views. Instead, you’ll see small, bright disks with subtle details. Seeing these details is a skill—called “seeing”—that improves with time and patience. Atmospheric turbulence often makes the image look like it’s under running water.
Tips for Getting the Best Planetary Views
- Let Your Telescope Acclimate: Bring your telescope outside at least 30 minutes before you observe. This allows it to cool to the outdoor temperature, preventing shaky air inside the tube.
- Observe on Nights of “Good Seeing”: Steady, calm nights provide clearer views than windy or turbulent ones, even if the later night is clearer.
- Use High Magnification Wisely: Only use the highest power when the atmosphere is steady. Often, a medium power (150x-200x) provides a sharper, brighter view.
- Look for Moments of Clarity: Watch patiently. The atmospheric blurring will briefly settle, giving you a split-second of stunning clarity.
- Try Planetary Filters: A yellow or blue filter can enhance contrast on Jupiter. A red or orange filter can help bring out details on Mars.
Common Mistakes to Avoid
- Buying Based on Magnification Claims: Ignore box labels that say “600x power!” The useful magnification is limited by aperture and atmosphere.
- Starting Too Big: A massive, complex telescope can be frustrating and may end up in the closet. Simplicity is key for beginners.
- Observing Over Roofs or Pavement: Heat rising from houses and asphalt creates terrible air turbulence. Observe over grass if possible.
- Giving Up Too Soon: Your first view might be disappointing. Take time to learn your equipment and observe on different nights.
Recommended Telescopes by Aperture
Here are some typical recommendations for different apertures. Remember, prices and models change, so use this as a guideline.
Under 4-inch Aperture
Good for very beginners or as a highly portable option. The Celestron AstroMaster 90mm refractor or similar. Views are decent, but you’ll likely want to upgrade soon if you get serious.
4-inch to 5-inch Aperture
A solid step up. The Sky-Watcher Heritage 130P (5-inch) tabletop Dobsonian offers incredible value and performance in a compact package.
6-inch to 8-inch Aperture
The classic recommendations. The Orion SkyQuest XT6 or XT8 Dobsonian reflectors. These provide lifetime-quality views without breaking the bank. They are the most often suggested “first real telescope.”
Over 8-inch Aperture
For the committed enthusiast. A 10-inch or 12-inch Dobsonian will show even more detail but is large, heavy, and requires darker skies to reach its full potential on planets.
FAQ Section
Can I see planets with a 70mm telescope?
Yes, you can. You will see Saturn’s rings, Jupiter’s moons and bands, and the phases of Venus. However, the views will be small and lack fine detail. It’s a good start but has limitations.
Is a refractor or reflector better for viewing planets?
High-quality refractors often give slightly sharper, higher-contrast views, but they are expensive per inch of aperture. Reflectors offer more aperture for your money, which usually translates to more detail. For most beginners, a reflector like a Dobsonian is the best balance of performance and cost.
What is the best telescope for seeing Jupiter and Saturn?
An 8-inch Dobsonian reflector is often considered the best all-around telescope for viewing Jupiter, Saturn, and other planets. It collects plenty of light for detailed views, remains relatively portable, and is affordable compared to other types of similar size.
How much magnification do I need to see planets?
You can see planets as disks at about 50x magnification. To see good detail, you’ll want 150x to 250x. On nights of exceptional stability, you might use 300x or more with a large aperture scope. Remember, magnification is empty without good aperture and steady skies.
Can I use a telescope for planets in the city?
Absolutely. Planets are bright, so light pollution isn’t a major problem for them. The main issue in the city is atmospheric turbulence from heat and buildings. Finding a stable seeing conditions is more important than dark skies for planetary observation.
Do I need a computerized GoTo mount for planets?
No, you don’t. Planets are bright and easy to find manually. A simple manual mount is often more stable and less hassle for planetary viewing. Computerized mounts are helpful for finding faint deep-sky objects, but they add cost and complexity that isn’t necessary for the planets.
Final Thoughts
Choosing what size telescope to see planets is about finding a balance. Aim for the largest aperture you can comfortably afford, store, and carry, while ensuring it has good optics and a stable mount. For the vast majority of beginners, a 6-inch or 8-inch Dobsonian reflector hits that balance perfectly. It will provide years of rewarding views of the solar system and beyond. Remember, the best telescope is the one you use regularly, so choose one that fits your life. Start observing, be patient with the conditions and your own learning curve, and you’ll be rewarded with breathtaking glimpses of other worlds.