If you’ve ever looked through a telescope, you’ve probably wondered just how much bigger everything appears. Knowing how to find the magnification of a telescope is the first step to understanding its power. It’s a simple calculation that every stargazer should know. This guide will show you exactly how to do it, step by step.
We’ll also explain why magnification isn’t the most important spec. You’ll learn how to choose the right power for different objects in the sky. Let’s get started.
How to Find the Magnification of a Telescope
Finding your telescope’s magnification is straightforward. You only need two numbers. These numbers are usually printed right on your telescope’s hardware. The formula is simple: divide the telescope’s focal length by the eyepiece’s focal length. The result is your magnification, also called power.
For example, if your telescope has a 1000mm focal length and you use a 20mm eyepiece, your magnification is 50x. That means the object appears 50 times larger than it does with your naked eye. It really is that easy to figure out.
The Two Numbers You Need: Focal Length and Eyepiece
First, locate your telescope’s focal length. This is often printed on a sticker on the telescope tube or in the manual. It’s measured in millimeters (mm). Common focal lengths are 650mm, 1000mm, or 1200mm. It represents the distance light travels inside the scope to bring an image to focus.
Second, check your eyepiece. The focal length of the eyepiece is almost always printed on its side. You’ll see numbers like 25mm, 10mm, or 6mm. A smaller number on the eyepiece means higher magnification when used with your scope.
Step-by-Step Calculation
- Identify your telescope’s focal length (e.g., 1000mm).
- Identify your eyepiece’s focal length (e.g., 25mm).
- Divide the telescope focal length by the eyepiece focal length: 1000 ÷ 25 = 40.
- Your magnification is 40x.
Why Magnification Isn’t Everything
Beginners often think higher magnification is always better. This is a common mistake. Too much magnification can make an image dim, fuzzy, and shaky. Every telescope has a practical limit, usually around 50x per inch of aperture. For a 4-inch telescope, that’s about 200x maximum under perfect conditions.
The atmosphere itself often limits useful magnification. On a night with poor “seeing,” high power just magnifies a blurry, wobbly image. Sometimes, a lower power view is actually more detailed and pleasing to look at.
The Role of Aperture
Aperture is the diameter of the telescope’s main lens or mirror. It’s the most important spec. A larger aperture gathers more light. This means you can see fainter objects and get sharper images. Aperture determines the maximum useful magnification your telescope can handle. A small scope can’t support a very high power effectively.
Choosing the Right Magnification for the Job
Different celestial objects require different magnifications. Here’s a simple guide:
- Low Power (20x to 50x): Best for large star clusters, big nebulae, and sweeping the Milky Way. It gives a wide, bright field of view. It’s also easiest for finding objects initially.
- Medium Power (50x to 100x): Excellent for viewing the moon, planets, and smaller star clusters. This is a great all-around range for many nights.
- High Power (100x and above): Used for planets (to see cloud bands on Jupiter or the rings of Saturn), small planetary nebulae, and splitting close double stars. This requires good atmospheric conditions.
Using a Barlow Lens to Increase Magnification
A Barlow lens is a special accessory that multiplies your power. It fits between the telescope and the eyepiece. Common Barlows double (2x) or triple (3x) the magnification. So, that 25mm eyepiece that gave 40x becomes, with a 2x Barlow, effectively a 12.5mm eyepiece giving 80x.
It’s a cost-effective way to expand your eyepiece collection. However, remember the quality limits. A cheap Barlow lens can degrade image quality, so invest in a decent one if you use it often.
How to Calculate Magnification with a Barlow
- Calculate the normal magnification: Telescope FL ÷ Eyepiece FL.
- Multiply that result by the Barlow’s factor (e.g., 2x).
- Example: (1000mm ÷ 25mm) = 40x. Then, 40x x 2 = 80x magnification.
Practical Limits and the “Empty Magnification” Problem
When you push magnification beyond what your telescope’s aperture or the night’s atmosphere can support, you get “empty magnification.” The image gets bigger but not clearer. It becomes dimmer and loses contrast. Details don’t improve; they just become a larger blur.
If you’re view looks worse when you switch to a higher-power eyepiece, you’ve hit the limit. Just switch back to a lower power. The sweet spot is usually well below the theoretical maximum.
Field of View: The Other Side of the Coin
Magnification directly affects your field of view—how much sky you can see at once. Higher magnification shows a smaller patch of sky. This is like looking through a narrow pipe. Lower magnification gives a wider view, like a window.
An eyepiece’s apparent field of view also matters. Two eyepieces with the same focal length can show different widths of sky. This spec is usually listed in degrees on the eyepiece packaging.
Estimating True Field of View
You can estimate your true field of view. Divide the eyepiece’s apparent field by the magnification. If an eyepiece has a 50° apparent field and you’re at 50x magnification, your true field is about 1 degree of sky (roughly two full-moon widths).
Tips for Getting the Best View at Any Magnification
- Let your telescope cool down. If it’s stored inside, it needs time to match the outside air temperature. This prevents wobbly images from tube currents.
- Collimate your telescope regularly. This means aligning the mirrors or lenses. A misaligned scope cannot produce sharp images at any power.
- Use a stable mount. A wobbly tripod makes high magnification useless. Make sure everything is tightened securely.
- Be patient. Wait for moments of steady air (“good seeing”) to use high power on planets. The view will sharpen for a few seconds at a time.
Common Mistakes to Avoid
One big mistake is using too high a magnification right away. Always start with your lowest-power eyepiece to find and center the object. Then you can work your way up to higher powers if the view warrants it.
Another error is ignoring eye placement. With high power, you need to position your eye very carefully at the eyepiece. Take your time to get comfortable. Also, avoid touching the scope while looking through it, as even a small nudge can jiggle the view.
Recording Your Observations
It’s helpful to note which magnifications you use for different objects. Keep a simple log. Write down the date, object, telescope and eyepiece used, magnification, and a note on the view. Over time, you’ll learn exactly which eyepiece to grab for Saturn or the Orion Nebula without even thinking about the math.
Conclusion: Power in Your Hands
Now you know exactly how to find the magnification of a telescope. Remember the simple formula: Telescope Focal Length divided by Eyepiece Focal Length. But more importantly, you understand that magnification is just one tool. The best views come from matching the power to the object, the conditions, and your telescope’s capabilities.
Start with your lowest power eyepiece to find and frame your target. Then experiment. See how the view changes as you increase magnification. You’ll quickly develop an intuition for what works. Clear skies are ahead for your observing sessions.
FAQ Section
What is the formula for telescope magnification?
The formula is: Magnification = Telescope Focal Length ÷ Eyepiece Focal Length. For instance, a 1200mm telescope with a 12mm eyepiece gives 100x power.
Can a telescope have too much magnification?
Absolutely. Excessive magnification makes images dim, fuzzy, and shaky. It’s limited by aperture, optical quality, and atmospheric conditions. There’s a practical maximum for every telescope and every night.
How do I find my telescope’s focal length?
Check the telescope’s tube, the front cap, or the user manual. It’s usually printed there in millimeters (mm). If not, you can sometimes find the model online to look up its specifications.
What does a 50x magnification mean?
It means the object appears 50 times larger in angular size than it does when viewed with your naked eye. The Moon, for example, would look 50 times wider across.
Is a 3mm eyepiece good?
A 3mm eyepiece provides very high magnification. It’s only useful in telescopes with long focal lengths or on nights with exceptionally steady air. For many scopes, it often provides too much power for regular use.
What magnification is best for viewing planets?
A good starting point is 150x to 200x for planets like Jupiter and Saturn. You may be able to go higher on nights of excellent “seeing.” For Mars during its close approach, even higher magnifications around 250x-300x can reveal surface details.
How does a Barlow lens affect magnification?
A Barlow lens increases the effective focal length of your telescope. A 2x Barlow doubles the magnification of any eyepiece you use with it. So a 20mm eyepiece acts like a 10mm eyepiece when the Barlow is inserted.