If you’re looking at buying your first telescope or upgrading your current one, you probably have a big question. What is a good aperture for a telescope? This single number is the most important specification for any telescope, and getting it right makes all the difference between a frustrating experience and years of enjoyment.
Aperture simply means the diameter of the telescope’s main light-gathering lens or mirror. It’s usually measured in millimeters or inches. A bigger aperture lets in more light, which allows you to see fainter objects and finer details. But bigger isn’t always better for every person. The best aperture for you depends on what you want to see, where you’ll use it, your budget, and how portable you need it to be.
What Is A Good Aperture For A Telescope
There is no one perfect aperture that works for everyone. A good aperture is one that balances light-gathering power with practical considerations like portability, cost, and your typical observing conditions. For most beginners, a good starting aperture is in the range of 70mm to 130mm (about 3 to 5 inches) for refractors and 114mm to 200mm (4.5 to 8 inches) for reflectors.
Why Aperture is King in Astronomy
Before we look at specific numbers, let’s understand why aperture matters so much. Your telescope is essentially a light bucket. Every photon of light from a distant galaxy, nebula, or planet has to travel across vast distances of space to reach you. The aperture is the opening that collects those precious photons.
- Brightness: A larger aperture collects more light, making images brighter. This is crucial for seeing faint galaxies and nebulae.
- Resolution: A larger aperture can resolve finer detail. This means you can see more cloud bands on Jupiter, the Cassini Division in Saturn’s rings, or tighter double stars.
- Magnification Potential: While eyepieces control magnification, a larger aperture supports higher useful magnification before the image becomes too dim and fuzzy.
Aperture Recommendations by Telescope Type
Different telescope designs have different strengths, and that influences what a good aperture looks like for each.
Refractor Telescopes (Lens-based)
These use lenses at the front of the tube. They are low-maintenance and give sharp, high-contrast views. However, good quality refractors get expensive at larger apertures.
- Good Starter Aperture: 70mm to 90mm. Perfect for the Moon, planets, and bright star clusters.
- Excellent All-Around Aperture: 100mm to 130mm (4 to 5 inches). A fantastic choice for a serious beginner or intermediate. Offers great planetary and wide-field views.
- Advanced/Portable Aperture: 150mm+ (6 inches). These are premium instruments, often used for astrophotography or high-end visual observing.
Reflector Telescopes (Mirror-based)
These use a primary mirror at the bottom of the tube. They offer the most aperture for your money. The most common design for beginners is the Newtonian reflector.
- Good Starter Aperture: 114mm to 130mm (4.5 to 5 inches). A very popular and affordable entry point with real power.
- Sweet Spot Aperture: 150mm to 200mm (6 to 8 inches). Often considered the best balance of power, portability, and price. You can see a huge amount of the universe with an 8-inch scope.
- Serious Observer Aperture: 250mm to 400mm+ (10 to 16 inches). These are large, often requiring a permanent or semi-permanent site, but the views are breathtaking.
Compound Telescopes (Catadioptric)
These use a combination of mirrors and lenses, like Schmidt-Cassegrains (SCTs). They pack a long focal length into a compact tube.
- Good Starter Aperture: 127mm (5 inches). A very portable yet capable package.
- Versatile Workhorse Aperture: 200mm to 235mm (8 to 9.25 inches). The classic SCT size, excellent for both visual observing and astrophotography.
- Advanced Aperture: 280mm+ (11 inches). Powerful instruments for dedicated observers.
Choosing Based on What You Want to See
Your goals are the biggest factor in choosing a good aperture.
For Planets, the Moon, and Double Stars (Planetary Observing)
While aperture helps, stable air conditions (good “seeing”) and optical quality are often more critical. You don’t need a huge scope, but a good one.
- Minimum Recommended: 70mm refractor or 114mm reflector.
- Ideal Range: 100mm to 150mm refractor or 130mm to 200mm reflector. This provides enough detail to keep you engaged for years.
For Galaxies, Nebulae, and Star Clusters (Deep-Sky Observing)
Here, aperture is your best friend. More light gathering is essential to see these faint fuzzies.
- Minimum Recommended: 130mm (5 inches). You’ll see the brightest deep-sky objects.
- Ideal Range: 200mm to 300mm (8 to 12 inches). This is where countless galaxies and nebulae become visible with structure and detail.
For a Bit of Everything (General Purpose)
Most people want to look at it all. This requires a balance.
- The Best Balance: A 150mm to 200mm (6 to 8 inch) reflector or a 100mm to 130mm (4 to 5 inch) refractor. These apertures show pleasing views of both planets and many deep-sky objects.
The Practical Downsides of Big Apertures
It’s tempting to just get the biggest scope you can afford. But you must consider these practical factors, which often define what a truly “good” aperture is for your situation.
Portability and Storage
A telescope you don’t use is a bad telescope. If a scope is too heavy or bulky to setup easily, it will gather dust in the closet.
- Small (under 100mm refractor, under 130mm reflector): Very portable. Easy to take outside in one trip.
- Medium (100-150mm refractor, 150-200mm reflector): Moderately portable. The tube and mount may require two trips. Manageable for most adults.
- Large (200mm+ reflector, 150mm+ refractor): Heavy and bulky. Often requires a permanent observatory or a dedicated observing site in the yard. Setup time is significant.
Cost
Aperture cost increases quickly. The scope itself gets bigger, but you also need a heavier, more stable mount to hold it steady. A flimsy mount under a large scope is worse than a small scope on a solid mount.
Local Observing Conditions (Your “Sky”)
If you observe from a city or suburb with significant light pollution, a very large aperture will mostly gather more light pollution, making the sky background brighter. A moderate aperture (e.g., 8 inches) from a dark sky site will outperform a giant 16-inch scope from the city for most deep-sky objects. Sometimes, a smaller, quality scope you can easily take to a dark site is the better choice.
Step-by-Step Guide to Choosing Your Aperture
- Be Honest About Your Budget: Set a total budget for the telescope, mount, and a couple of eyepieces. Don’t spend it all on aperture alone.
- Assess Your Storage & Transportation: Where will you store it? Can you lift it? Do you have a car to take it to dark skies?
- Define Your Primary Targets: Are planets your main interest, or are you dreaming of faint galaxies?
- Check Your Typical Viewing Location: Backyard, balcony, or require travel? Light pollution is a key factor.
- Match Aperture to Type & Goals: Use the recommendations above. When in doubt, an 8-inch (200mm) Dobsonian reflector is often the most recommended “best all-around” aperture for visual observers.
- Don’t Forget the Mount: Ensure the mount is rock-solid for the telescope’s size. A wobbly view is unusable.
Common Aperture Myths Debunked
Let’s clear up some confusion you might encounter.
- Myth: Aperture is the same as magnification. Truth: Magnification is determined by the eyepiece. Aperture determines brightness and detail, which support magnification.
- Myth: You need a huge aperture to start. Truth: A smaller, easy-to-use scope you actually use is infinitely better than a complex giant that never leaves the house.
- Myth: More aperture always means a better image. Truth: In poor atmospheric conditions, a smaller scope often provides a sharper, steadier image of planets than a large one struggling with “bad seeing.”
FAQs About Telescope Aperture
Is a 70mm telescope aperture good?
Yes, for specific purposes. A 70mm refractor is a good starter aperture for viewing the Moon, planets like Jupiter and Saturn, and bright star clusters. It’s portable and affordable. It’s not ideal for faint deep-sky nebulae and galaxies.
What is a good aperture size for a beginner telescope?
A good beginner aperture is between 70mm and 130mm for a refractor, or 114mm and 150mm for a reflector (like a Dobsonian). These sizes offer a great mix of capability, ease of use, and affordability, letting you learn the sky without being overwhelmed.
Is 5 inches a good aperture for a telescope?
Absolutely. A 5-inch (127mm) aperture is an excellent mid-range size. It shows good detail on planets and reveals a significant number of deep-sky objects. It’s often a very portable size, making it a fantastic all-around choice for many hobbyists.
Is there such a thing as too much aperture?
For an individual, yes. If the scope is so large and heavy that you rarely set it up, or if it requires a special vehicle to transport, it’s too much aperture for you. The best telescope is the one you use regularly.
Can I see galaxies with a small aperture?
You can see the brightest galaxies, like the Andromeda Galaxy (M31), with even a small 70mm telescope from a dark site. However, they will appear as faint, fuzzy patches. To see spiral structure or more distant galaxies, you generally need an aperture of 8 inches or more.
How does aperture affect astrophotography?
For deep-sky astrophotography, the lens or telescope’s focal ratio (f-number) is often more critical than raw aperture for determining exposure time. Fast focal ratios (like f/4 to f/7) are preferred. Many dedicated astrophotography telescopes have moderate apertures (70-130mm) but very fast optics.
Final Thoughts on Finding Your Perfect Aperture
Choosing a good aperture is about finding your personal sweet spot. It’s the intersection of your astronomical dreams and your real-world constraints. Remember that the community often gives a specific piece of advice for a reason: the classic 8-inch Dobsonian reflector is so frequently recommended because it offers a massive amount of aperture (200mm) at a very reasonable price and size. It brings thousands of celestial objects within reach while still being manageable for one person to carry and set up.
Start by being realistic. A smaller telescope that gets used weekly will bring more joy than a larger one that becomes a furniture piece. You can always upgrade later, and your first scope often becomes a trusted portable friend even if you get a bigger one later. The key is to get outside under the stars. Any aperture that lets you do that consistently is a good aperture for you.