If you’ve ever looked through a telescope and wondered how much bigger things appear, you need to know how to find magnification of a telescope. It’s a fundamental concept that helps you understand your instrument’s power and choose the right eyepiece for any target.
Magnification, or power, tells you how much closer an object looks compared to your naked eye. It’s not fixed; it changes based on a simple combination of two parts of your telescope setup. Let’s break it down in a clear, simple way.
How To Find Magnification Of A Telescope
The core formula for calculating telescope magnification is straightforward. You only need two numbers, which are usually printed on your telescope equipment.
The Formula: Magnification = Telescope Focal Length ÷ Eyepiece Focal Length
That’s it. You divide the focal length of your telescope by the focal length of the eyepiece you are using. The result is the magnification power.
Step 1: Locate Your Telescope’s Focal Length
Your telescope’s focal length is a key specification. It’s often printed on a label near the front of the tube or on the box/manual. It’s measured in millimeters (mm) or sometimes in inches.
- Look for a number like “1200mm”, “650mm”, or “2000mm”.
- It might be listed as “f/5” or “f/10”. This is the focal ratio. To get focal length, multiply this number by the aperture (the diameter of the main lens or mirror). For example, an f/5 telescope with a 150mm aperture has a focal length of 750mm (5 x 150 = 750).
Step 2: Identify Your Eyepiece Focal Length
This is easier. Every eyepiece has its focal length engraved or printed on its side. It’s also in millimeters.
- Common eyepiece sizes are 25mm, 10mm, 6mm, and 32mm.
- The smaller the number on the eyepiece, the higher the potential magnification it provides.
Step 3: Do the Simple Math
Now, just plug the numbers into the formula. Let’s do a real example.
- Your telescope has a focal length of 1000mm.
- You are using a 25mm eyepiece.
- Calculation: 1000 ÷ 25 = 40.
- Your magnification is 40x. The object appears 40 times closer than it does to your unaided eye.
Another example: Same telescope (1000mm) with a 10mm eyepiece. 1000 ÷ 10 = 100x magnification.
Why Understanding Magnification Matters
More power isn’t always better. Knowing how to calculate it helps you make smart choices.
Practical Limits and the “Useful Maximum”
Every telescope has a limit to how much it can magnify effectively. A common rule of thumb is a maximum of about 50x per inch of aperture (or 2x per millimeter).
- A 100mm (4-inch) telescope has a useful max around 200x.
- Exceeding this limit gives you a bigger, but dimmer and fuzzier, image. Atmospheric conditions often limit you to much lower powers anyway.
Choosing the Right Power for the Target
- Low Power (20x – 50x): Best for wide views of star clusters, large nebulae, and sweeping the Milky Way. It gives a bright, wide field of view and is easier to aim.
- Medium Power (50x – 100x): Excellent for general lunar and planetary viewing, as well as smaller star clusters.
- High Power (100x and above): Reserved for planets, the Moon’s fine details, and splitting close double stars. Requires good telescope optics and steady atmospheric “seeing.”
Factors That Affect Your View Beyond Magnification
Magnification is just one piece of the puzzle. These other factors are just as important.
Aperture: The Most Important Spec
Aperture is the diameter of the main lens or mirror. It determines how much light the telescope gathers.
- Bigger aperture = brighter, sharper images at any given magnification.
- A large-aperture telescope at 200x will show a much better image than a small-aperture scope at the same 200x.
Eyepiece Quality and Design
Not all 10mm eyepieces are created equal. Better eyepieces with multi-coatings and advanced optical designs provide sharper views edge-to-edge, better contrast, and more comfortable eye relief (the distance your eye can be from the lens).
Atmospheric Conditions (“Seeing”)
On a night when the air is turbulent, high magnification will just magnify the blur. Stars twinkle violently, and planets look like they’re under running water. Low to medium power often provides a better view on such nights.
Using a Barlow Lens to Increase Magnification
A Barlow lens is a cost-effective way to double or triple your eyepiece collection. It’s a tube with a lens that you insert before the eyepiece.
- A 2x Barlow lens doubles the magnification of any eyepiece.
- Formula with Barlow: Magnification = (Telescope FL ÷ Eyepiece FL) x Barlow Factor.
- Example: 1000mm scope, 25mm eyepiece, 2x Barlow. (1000 ÷ 25) x 2 = 40 x 2 = 80x.
This means your 25mm effectively acts like a 12.5mm eyepiece, and your 10mm acts like a 5mm. It’s a very handy tool.
Common Mistakes and Misconceptions
Let’s clear up some frequent errors people make when thinking about telescope power.
- Mistake 1: Believing the “max power” advertised on cheap telescopes. A 600x power claim on a small department store scope is meaningless; the image will be terrible.
- Mistake 2: Always using the highest magnification eyepiece. Start low to find and center the object, then switch to higher power if the view supports it.
- Mistake 3: Forgetting that exit pupil shrinks with higher power. Exit pupil is the beam of light hitting your eye. If it gets to small (below about 0.5mm), the view becomes dim and uncomfortable.
- Mistake 4: Ignoring field of view. Higher magnification means you see a smaller patch of sky. You might not be able to fit the entire Moon or a large nebula in the view.
Practical Exercise: Calculate Your Own Setups
Grab your telescope and eyepieces. Write down your telescope’s focal length. Then, for each eyepiece, calculate the magnification.
- Telescope Focal Length: _______ mm
- Eyepiece 1: _______ mm = _______ x
- Eyepiece 2: _______ mm = _______ x
- With 2x Barlow, Eyepiece 1 becomes: _______ x
Now you have a practical chart for your gear. Keep this with your telescope case for quick reference at night.
Advanced Consideration: Calculating Field of View
Once you know the magnification, you can estimate how much sky you see. You need the eyepiece’s Apparent Field of View (AFOV), often listed as 50°, 68°, or 82° on the eyepiece.
Formula: True Field of View = Eyepiece AFOV ÷ Magnification
Example: A 25mm eyepiece with a 52° AFOV at 40x magnification gives a True Field of View of 52° ÷ 40 = 1.3 degrees. (The full Moon is about 0.5 degrees, so you could fit over two Moons side-by-side in your view).
FAQ Section
What is the formula for telescope magnification?
The formula is Magnification = Focal Length of Telescope ÷ Focal Length of Eyepiece. It’s the primary method for how to find magnification of a telescope.
Where do I find my telescope’s focal length?
Check the telescope tube itself, often on a decal near the front or focuser. It’s also always in the user manual and original product specifications. It’s usually listed in millimeters (mm).
Is higher magnification always better?
No, absolutely not. Higher magnification makes the image dimmer, shakier, and more susceptible to blur from telescope limits and bad atmosphere. The best view is often at a moderate power.
What does a Barlow lens do to magnification?
A Barlow lens multiplies the magnification produced by your eyepiece. A 2x Barlow will double it, and a 3x Barlow will triple it. It effectively gives you additional eyepiece focal lengths without buying new ones.
What is the maximum useful magnification for my telescope?
A general practical maximum is 50 times the aperture in inches, or 2 times the aperture in millimeters. For a 4-inch (102mm) scope, that’s about 200x. Many nights, the atmosphere won’t allow for even this much.
Why does my high-power view look so blurry?
Blur at high power is usually caused by one of three things: 1) Exceeding the telescope’s optical limits, 2) Poor atmospheric “seeing” conditions, or 3) The telescope not being properly cooled to the outside air temperature or collimated (aligned).
Can I use any eyepiece with my telescope?
Most modern telescopes use either 1.25-inch or 2-inch diameter eyepiece barrels. As long as the physical barrel size fits your focuser, you can use it. The resulting magnification will depend on it’s focal length as per the formula.
Putting It All Together
Knowing how to find magnification of a telescope empowers you to use your instrument effectively. It’s not about chasing the biggest number, but about selecting the right tool for the celestial object you’re observing.
Remember the simple math: Telescope FL ÷ Eyepiece FL. Start your observing sessions with your lowest power (longest focal length eyepiece) to find targets easily. Then, experiment with higher powers if the object and conditions allow it. Your telescope’s aperture and the night’s “seeing” are the real bosses; magnification just lets you zoom in on what they provide.
With this knowledge, you can plan your observations, understand your equipment’s capabilities, and avoid the frustration of using inappropriate power. Clear skies and happy viewing!