Does Telescope Aperture Really Matter? YES — Here’s Why!

How much does your telescope’s aperture really matter? Is bigger always better?

These are questions I get all the time. And the short answer is: yes, aperture matters—a lot. But there’s more nuance to it than just “bigger equals better.” In this post, we’ll explore what aperture actually is, why it’s such a crucial spec on any telescope, and how it affects your ability to see faint objects, resolve fine detail, and capture stunning astrophotography images.

I’ll also share real examples from my own imaging sessions with different telescopes, and give you a few free tools that can help you understand how aperture affects your setup. And at the end, I’ll help you decide whether a larger aperture is really worth the investment—or if a smaller scope might serve you just as well.

What Is Telescope Aperture?

A telescope’s aperture refers to the size of its main lens or mirror—the part that gathers light. It’s the most important factor in determining how much light your telescope can collect.

Over the years, I’ve come to think of aperture as the heart of your telescope. Why? Because the more light your scope can gather, the more you’ll be able to see: fainter stars, more nebulosity, richer galaxies, and finer details on planets or the Moon.

To borrow a line from Queen’s Bohemian Rhapsody:

“Open your eyes, look up to the skies and see…”
Well, that’s exactly what a bigger aperture helps you do—see more of the universe, more clearly.

Buckets in the Rain: A Simple Analogy

Think of aperture like putting buckets out in a rainstorm. A small bucket collects some water. A large bucket collects a lot more. Telescopes work the same way, but instead of water, they collect photons—tiny particles of light that carry all the visual information from stars, nebulae, and galaxies. This is where some people start to get lost. You hear telescope enthusiasts throwing around terms like:

• Light-gathering power
• Limiting magnitude
• Resolving power

All of these are directly tied to aperture, and all of them impact what you can see and photograph with your telescope. In the next sections, I’ll break each one down in simple terms—starting with light-gathering power—so you can really understand how aperture affects your stargazing or astrophotography experience.

Light-Gathering Power: Why Size Really Matters

Most telescope mirrors and lenses are round—unless you’re talking about something exotic like the hexagonal segments on the James Webb Space Telescope. Because of that shape, a telescope’s light-gathering power is calculated using the surface area of a circle. Remember this formula from school?

Light-Gathering Power = π × (Aperture diameter ÷ 2)²

Here’s what that looks like in action:

Let’s take a telescope with a 50mm aperture, like the Seestar S50. The radius is half the diameter—25mm. Square that (625), multiply by π (around 3.14), and you get roughly 1,964. That number on its own doesn’t say much—until you compare it to a larger telescope.

Now consider a 100mm aperture scope. That’s twice the diameter. Using the same formula, you get a light-gathering power of about 7,854. Divide that by the 1,964 of the 50mm scope and you’ll see the 100mm collects 4 times more light.

Want a shortcut? Just divide the two aperture sizes and square the result:

(100 ÷ 50)² = 2² = 4

It gets even more dramatic with bigger scopes. Take my Celestron Edge HD 8″, which has a 200mm aperture. Compared to a 50mm scope:

(200 ÷ 50)² = 4² = 16

So even though the diameter is four times larger, the telescope collects 16 times more light.

That’s the key takeaway here: light-gathering power increases exponentially with aperture size. And this matters not just for visual observing, but especially for astrophotography—where more light means better signal, faster exposures, and fainter targets.

And hey, if math’s not your thing—no worries. Here’s an easy-to-use calculator to help you compare different aperture sizes and its consequences for light gathering power.

Comparing Telescopes to the Naked Eye

Sometimes you’ll see telescope specs that mention “light-gathering capacity” instead of light-gathering power. It sounds fancy, but what it really means is: how much more light a telescope collects compared to your unaided eyes.

The average fully dilated human pupil has a diameter of about 7mm. So if you’re looking through a telescope with a 200mm aperture, you can calculate the difference like this:

(200 ÷ 7)² = 816

That means a 200mm telescope gathers 816 times more light than your eye alone. No wonder you can suddenly see stars, galaxies, and nebulae that were completely invisible without optical aid!

Want to try this with your own telescope? You can use the calculator above to compare any aperture size against the human eye—just enter 7mm for the eye and your telescope’s aperture, and it’ll show you the multiplier.

How Aperture Helps You See Faint Objects

So, how does aperture actually help you observe or photograph objects in space?

The key lies in understanding how bright or dim these objects are.

This idea goes back to ancient times—Greek astronomer Claudius Ptolemy ranked stars by brightness, from 1st magnitude (the brightest) to 6th magnitude (the faintest visible to the naked eye). Then in 1856, astronomer Norman Pogson turned this into a mathematical scale that’s still used today: the logarithmic magnitude scale.

Here’s the formula:

Brightness Ratio = 2.512^(m2 - m1)

In simple terms, each step in magnitude means a brightness change by a factor of 2.512. So, a 6th magnitude star is about 100× dimmer than a 1st magnitude one. An 11th magnitude star? That’s 10,000× dimmer than a 1st magnitude star. As you can imagine, the faintest objects in the night sky require far more light-gathering power to see—and that’s where aperture comes in.

We still use apparent magnitude to describe how bright an object appears from Earth. Apps like Stellarium show this value for each celestial object, which helps you figure out how hard something will be to see.

Some of the brightest objects have negative magnitudes because they’re brighter than the original scale, most of these objects are within our solar system.

Famous Celestial Objects Ordered from Brightest to Dimmest

The Sun has an apparent magnitude of –26.7, making it by far the brightest object in the sky (but always remember to use proper solar filters when observing!). The Full Moon follows with a magnitude of –12.7. Venus, at its brightest, shines at –4.7, though it can dim to –4.4 at its faintest. Jupiter, another prominent object in our sky, has an apparent magnitude of –2.9, though this can vary between –2.4 and –2.0 depending on its position. Sirius, the brightest star in our night sky, has a magnitude of –1.46. Despite its brilliance, Sirius is actually 13 billion times dimmer than the Sun when observed from Earth, as it lies 8.6 light-years away—about 80 trillion kilometers.

Looking beyond our solar system, some deep-sky objects are still visible to the naked eye, like the Andromeda Galaxy and the Orion Nebula, which are bright enough to catch without a telescope. However, once you get beyond magnitude 6, telescopes become essential for observing or photographing these faint targets. The larger your telescope's aperture, the more light it can gather, allowing you to reveal even the faintest objects that would otherwise remain hidden.

If you're curious about comparing brightness levels, I’ve added a magnitude calculator below for you to explore the differences in brightness of two objects yourself, depending on their apparent magnitude.

Limiting Magnitude

Now that we understand how apparent magnitude works, let's dive into something you'll often encounter in telescope specifications: limiting magnitude. This term indicates the faintest object a telescope can still detect, and it all comes down to one factor—the aperture of your telescope. The formula for calculating it is:

Limiting Magnitude ≈ 2 + 5 × log₁₀(aperture in mm)

For instance, with a 50mm aperture telescope, your limiting magnitude would be around 10.5. This is enough to observe most faint celestial objects we’ve discussed. But if you’re using a 200mm aperture telescope, like an 8-inch reflector, your limiting magnitude increases to 13.5. And with a 300mm scope? You can reach an impressive 14.35—allowing you to detect some incredibly faint objects.

Of course, these numbers assume ideal conditions, such as perfect dark skies and top-quality equipment. But, as most of us know, very few people live under those perfect Bortle Class 1 skies.

To help you get a more realistic estimate, I've added a limiting magnitude calculator on my blog, which takes into account the light-pollution level of your skies. You can select your Bortle class, and the tool adjusts the calculation based on your specific sky conditions. For example, the difference in visibility between a pristine Bortle Class 1 sky and a heavily light-polluted Bortle 9 sky is significant—about 3 magnitudes, to be exact. In a city sky, you might only see a few dozen stars, but under dark, clear skies, thousands of stars will light up the night.

Limiting Magnitude for Astrophotography

It's important to note that the limiting magnitudes provided by telescope manufacturers are typically based on visual observation. However, for astrophotographers like me, things are a bit different. When we attach a camera to the telescope and use a tracking mount to follow the sky, we can push that limiting magnitude much further by taking long exposures. Unlike our eyes, which can only capture light in real time, a camera sensor can gather light over extended periods—ranging from several seconds to even hours. So, how does this affect the limiting magnitude?

For example, astrophotographers often take exposures ranging from 10 to 300 seconds, depending on the brightness of the object I'm targeting. Then, we stack dozens—or even hundreds—of those images together to create a final picture. This "integration time" significantly increases the telescope’s sensitivity, allowing us to capture much fainter objects.

To help astrophotographers like myself, I’ve developed a limiting magnitude calculator on my website. This tool estimates your telescope’s limiting magnitude based on its aperture, your sky conditions (Bortle class), and your total exposure time. It’s easier to use than some of the more complex calculators out there. While it makes a few assumptions about your equipment, it’s fairly accurate and even a little conservative in its estimates.

For instance, last month I captured 70 exposures of the Whirlpool Galaxy, each lasting 300 seconds, from my Bortle Class 7 skies using my Celestron Edge HD 8" telescope. When I plugged these values into the calculator, it estimated a limiting magnitude of around 20.2. But when I cross-checked this with ASTAP (a software that can analyze stacked images and calculate limiting magnitude), it showed a value of 21.65. I also tested the calculator with my image of the Tadpoles Nebula. Using my 80mm refractor, I took 70 exposures of 300 seconds each. The calculator estimated a limiting magnitude of 18.53, while ASTAP indicated a value of 19.36—again, a little better than expected based on the calculatpr. Feel free to give the limiting magnitude calculator below a try!

Resolving Power and Aperture

Your telescope’s aperture not only determines how much light it can gather, but also its resolving power — the ability to distinguish fine details in the night sky. This includes the telescope’s ability to separate two closely spaced objects, like binary stars, as well as its capacity to resolve intricate features within a single object, such as the cloud bands on Jupiter or the fine details of a nebula. The resolving power is typically measured in angular resolution, which is the smallest angle at which your telescope can separate details. But what exactly does "angular resolution" mean?

The Sky in Angles

Angular resolution relates to how we measure the size of objects in the sky. The sky is like a vast sphere surrounding Earth, and astronomers divide it into 360 degrees, just like a circle. However, because most celestial objects appear very small, we use smaller units of angular measurement, such as:

• 1 degree = 60 arcminutes (′)
• 1 arcminute = 60 arcseconds (″)

Therefore, 1 degree = 3,600 arcseconds.

These angular measurements help us describe how large or close together objects appear in the sky, regardless of their true size or distance. Here are a few familiar objects and their apparent sizes in the sky:

• 🌕 Full Moon: ~0.5 degrees wide, or 30 arcminutes (1,800 arcseconds)
• 🌌 Andromeda Galaxy: ~3 degrees across (six times the width of the Moon)
• 🌠 Orion Nebula (M42): ~1 degree wide (about the width of two full Moons side by side)
• 🪐 Jupiter: ~30–50 arcseconds, depending on its distance from Earth
• ♂️ Mars: Ranges from ~4 to 25 arcseconds
• ♀️ Venus: Between ~10 and 60 arcseconds
• 🌟 Albireo (a double star system): The two stars are separated by ~35 arcseconds

These examples help illustrate what your telescope might be able to resolve. For instance, if your telescope has a resolving limit of 1 arcsecond, it could theoretically separate two stars that are 1 arcsecond apart, or show planetary features that span just a few arcseconds.

Rayleigh vs. Dawes Limit

To estimate the resolving power of a telescope, astronomers often use the Rayleigh criterion or the Dawes limit. These two formulas provide a theoretical resolution based on the telescope's aperture. Both assume perfect optics and excellent seeing conditions:

• Rayleigh Criterion: This more conservative method is based on diffraction theory and defines two objects as "resolved" when the central peak of one diffraction pattern overlaps with the first minimum of another.
• Dawes Limit: This method is slightly more optimistic and is based on visual experiments with double stars. It defines resolution as the ability to distinguish two stars just before they merge into one.

To help you better understand your telescope’s resolving power, I’ve created a calculator for both. This tool allows you to input your telescope's aperture and instantly calculate both the Rayleigh and Dawes resolution limits in arcseconds, which represent your telescope’s smallest resolvable angle. Keep in mind that real-world resolving power also depends on the quality of your telescope’s optics and the atmospheric seeing conditions on any given night.

Obstruction Loss (by Area)

Thanks for sticking with me through all the details! Now, let’s dive into a couple of extra tips related to aperture. When comparing telescopes with different designs, it’s important to consider that certain types — like Newtonians and Schmidt-Cassegrain telescopes — have a secondary mirror that blocks some of the incoming light. This phenomenon is known as obstruction loss.

For example, my Celestron Edge HD 8" has a 203mm aperture, but it also features a secondary mirror with a 68mm diameter. This means the secondary mirror blocks about 33.5% of the aperture’s diameter. At first glance, that seems like a substantial loss of light. However, the actual impact is less severe than it might seem. As we discussed earlier, it’s not just the diameter that matters, but the total surface area of the aperture.

To calculate the obstruction loss by area, we square the diameter of the secondary mirror (68mm), which gives us 4,624 square millimeters, and then divide that by the square of the total aperture (203mm), which gives us 41,209 square millimeters. The result shows that the obstruction loss by area is only about 11.2% of the total light-gathering capacity. This means that even though the secondary mirror is blocking some light, the telescope is still capturing most of it. If you’d rather skip the math, you can use the calculator below to clculate the actual light loss for your own telescope.

When a Larger Aperture is Worth It… and When It Might Not Be

Now, let’s get into some practical tips about when upgrading to a larger aperture is truly worth it… and when it might not be. As we’ve discussed, a larger aperture means more light and better resolution for faint objects in the night sky. But before we all start imagining a massive 39-meter extremely large telescope in our backyards, let’s consider the trade-offs — like weight, size, and cost.

When it comes to APO refractors, the bigger the aperture, the heavier, longer, and more expensive they tend to be. For example, an 80mm Sky-Watcher Evostar is lightweight at just 3.3 kg, about half a meter long, and priced at around $700. In contrast, a 203mm ASKAR APO refractor weighs over 18 kg, is more than a meter long, and costs about $9,000. That’s a big difference! You’ll need to ask yourself: Do I want to carry that outside every night, or do I even have the space to leave it set up? Refractors are great for beginners, especially with modern smart scopes like the Seestar S50. But once you get beyond a 150mm aperture, refractors become bulky and prohibitively expensive for most people.

If you’re looking for more aperture at a more affordable price, mirror-based telescopes like Dobsonians or Newtonians are excellent alternatives. A 8-10" Newtonian gives you great light-gathering power for around $500–$1000. However, the downside is that they are large and heavy — a 10" Newtonian can weigh around 14 kg and be over 1 meter long. Are you really willing to haul that telescope around every night?

For a more compact solution, Schmidt-Cassegrain telescopes are a solid choice. These have folded optical paths that reduce the overall size of the telescope. My 8" Celestron Edge HD is a great example — it weighs just 6.3 kg, is under half a meter long, and costs about $1500. It’s portable and powerful. But when you move up to the 11" version, the weight jumps to 13 kg and the price climbs to over $4000.

Ultimately, the best telescope is the one you’ll actually use. That’s why smaller, more portable smart scopes like the Seestar S50 — with a 50mm aperture — are so popular. They’re light, wireless, and controlled from your phone.

So, yes, more aperture means better views and images. But don’t forget to factor in the cost, size, and how often you’ll actually want to set it up. I hope this blog post helped shed some light on the topic of aperture. If you found it useful, be sure to leave a like or a comment, or subscribe to my youtube channel!

Clear skies!

Wido.

Some examples of popular telescopes by aperture, weight, size, and approx. cost