The total magnification of a telescope is incredibly important to know. This is because it dictates how many celestial bodies you can see in the night sky at any time. Just as importantly, it governs how clearly you can observe them.

When star-hopping or focusing on a single planet for long enough, we can quickly become accustomed to the current magnification quality supplied by the telescope and eyepiece. This is despite the fact that the image could be a lot clearer.
Therefore, it’s always a good idea to consider your total magnification. Knowing this parameter will improve your observing experience.
Overview of Telescope Magnification
Telescope magnification is a fundamental concept in astronomy. It determines how much larger objects appear when viewed through your telescope. The magnification is related to what the human eye sees. Understanding magnification is important as it will maximize your observing sessions. It will also help you get the most out of your equipment.
Magnification is something that beginner astronomers tend to focus on. However, higher magnification isn’t always better. The key skill is to find the right balance between magnification, field of view, and image brightness for each observing target and conditions.
Key Terms and Definitions
Before continuing, here are some key terms:
- Focal Length: The distance from the telescope’s primary optical element to the point where light rays converge to form an image.
- Aperture: The diameter of the telescope’s primary light-gathering element (lens or mirror).
- Eyepiece: The optical element you look through, which magnifies the image formed by the telescope.
- Focal Ratio: The telescope’s focal length ratio to its aperture.
- Exit Pupil: The diameter of the beam of light exiting the eyepiece.
- Field of View: The angular size of the sky visible through the telescope.
Understanding these terms will help you understand the following and make informed decisions about your equipment and observing techniques.
Understanding Focal Length
Focal length is critical in determining a telescope’s magnification and field of view. It’s usually measured in millimeters and is typically printed on the telescope tube. It may also be found in the user manual.
The focal length of a telescope is determined by its optical design. For example:
- Refractors: The focal length is roughly the length of the telescope tube.
- Newtonian Reflectors: The focal length is approximately twice the tube length.
- Cassegrain designs: The focal length is much longer than the physical tube length due to the folded light path.
How Telescope Focal Length Affects Magnification
A longer focal length results in higher magnification with the same eyepiece. Conversely, a shorter focal length provides a wider field of view but lower magnification. This relationship is key to understanding how different telescope designs perform for various observing tasks.
For instance, a long focal length telescope (like a Schmidt-Cassegrain) is well-suited for high-magnification views of planets and double stars. In contrast, a short focal length telescope (like a refractor) excels at providing wide views of star fields and large nebulae.
How Eyepiece Focal Length Affects Magnification
Eyepiece focal length determines your telescope’s magnification and field of view. Measured in millimeters, it is the distance from the eyepiece’s field stop to its focal point.
A shorter eyepiece focal length produces higher magnification, and a longer one results in lower magnification. For example, an eyepiece with a focal length of 10mm magnifies more than a 20mm eyepiece.
Calculating Total Telescope Magnification
The formula for calculating magnification is simple:
Total Magnification = Telescope Focal Length ÷ Eyepiece Focal Length
For example, if your telescope has a focal length of 1000mm and you’re using a 25mm eyepiece, the magnification would be 1000 ÷ 25 = 40x (as shown in the calculator below).
This formula is the foundation for understanding how your telescope and eyepieces work together to create different magnifications.
Our Telescope Calculator can determine the magnification of any telescope and eyepiece combination and other parameters. However, here is a calculator to determine the magnification:
Minimum, Optimum, and Maximum Magnification
Here are some guidelines for magnifications that should be considered when using your telescope:
- Minimum magnification: Generally considered to be around 3x per inch of aperture. This provides the widest field of view and the brightest image.
- Optimum magnification: Depending on observing conditions, it usually falls between 15-20x per inch of aperture. This range typically provides the best balance between magnification and image quality.
- Maximum magnification: Typically 50-60x per inch of aperture, but rarely achievable due to atmospheric conditions. This is often referred to as the “maximum useful magnification.” It is the magnification at which image quality is unacceptably degraded.
Please note that these are guidelines, not strict rules. The actual useful magnification on any given night depends on various factors. These factors include atmospheric conditions, telescope quality, and the observer’s experience.
Using Different Eyepieces
As a telescope’s eyepiece determines the magnification, we need to know a little more about them.
How to Find an Eyepiece’s Focal Length
An eyepiece’s focal length is typically engraved on the barrel. Common focal lengths include 25mm, 18mm, 10mm, and 6mm. Some modern eyepieces might also include their apparent field of view, which is useful for calculating the true field of view.
Matching Eyepieces with Telescopes
Choose eyepieces that provide a range of useful magnifications for your telescope. A good starting set might include:
- Low power (wide field): About 5x per inch of aperture
- Medium power: About 15x per inch of aperture
- High power: About 30x per inch of aperture
For example, for an 8-inch telescope, you might choose eyepieces that provide about 40x, 120x, and 240x magnifications.
Adjusting Magnification with Barlow Lenses
A Barlow lens is an accessory that increases the effective focal length of your telescope, thereby increasing magnification. A 2x Barlow, for example, doubles the magnification of any eyepiece used with it.
Barlows are a cost-effective way to expand your magnification range. For instance, a set of three eyepieces can provide six different magnifications when used with a 2x Barlow.
However, be aware that Barlows can reduce image brightness and may introduce some optical aberrations, especially with lower-quality models.
Factors That Influence Magnification Considerations
There are several factors that should be considered when selecting magnification:
Atmospheric Conditions
Turbulence in the atmosphere, often called “seeing,” can limit the effectiveness of high magnifications. This is why astronomers often talk about “seeing conditions.”
On nights with poor seeing, you might find that lower magnifications provide clearer views. Conversely, on nights with excellent seeing, you might be able to use higher magnifications effectively.
For a more in-depth explanation of seeing you may like The Atmosphere and Observing.
Telescope Quality and Design
Higher quality optics and more stable mounts allow for more effective use of high magnifications. Well-collimated mirrors in reflectors, high-quality lenses in refractors, and sturdy mounts all contribute to better high-power performance.
Different telescope designs also have different strengths:
- Refractors often excel at providing crisp, high-contrast views at high magnifications.
- Reflectors typically offer more aperture for the price, allowing for brighter images of faint objects.
- Compound designs like Schmidt-Cassegrains offer a compromise, with good high-power performance in a compact package.
Observer Experience
As you gain experience, you’ll improve at using higher magnifications effectively and interpreting what you see. Experienced observers often:
- Know how to wait for moments of steady seeing to observe fine details.
- Can interpret subtle contrasts and details that beginners might miss.
- Understand how to use averted vision techniques to see faint details.
Practical Examples of Magnification Calculations
Refractor Telescopes
Example: An 80mm f/5 refractor has a focal length of 400mm.
- With a 25mm eyepiece: 400 ÷ 25 = 16x
- With a 10mm eyepiece: 400 ÷ 10 = 40x
- With a 10mm eyepiece and 2x Barlow: (400 ÷ 10) × 2 = 80x
Reflector Telescopes
Example: A 200mm f/6 Newtonian reflector has a focal length of 1200mm.
- With a 40mm eyepiece: 1200 ÷ 40 = 30x
- With a 10mm eyepiece: 1200 ÷ 10 = 120x
- With a 6mm eyepiece: 1200 ÷ 6 = 200x
Compound Telescopes
Example: A 203mm f/10 Schmidt-Cassegrain has a focal length of 2030mm.
- With a 40mm eyepiece: 2030 ÷ 40 = 50.75x
- With a 15mm eyepiece: 2030 ÷ 15 = 135.33x
- With a 15mm eyepiece and 2x Barlow: (2030 ÷ 15) × 2 = 270.66x
How to Optimizing Your Viewing Experience
Here are some tips in regards to selecting magnification for your observing:
- Start with low magnification to quickly locate objects.
- Increase magnification gradually as needed.
- Be patient and allow time for your eyes to adjust.
- Experiment with different magnifications for different objects and conditions.
- Consider using filters to enhance contrast and detail, especially for planetary observation.
- Learn to recognize good seeing conditions and take advantage of them for high-power viewing.
- Practice good dark adaptation techniques to enhance your ability to see faint details.
What You May See at Various Magnifications

A 10x magnification is very low and is usually used for binoculars rather than telescopes. However, a 20x magnification will give you a good view of the Moon and nearby planets. This magnification is often considered the best magnification for basic astronomy.
A magnification of 25x or higher allows you to pinpoint the exact details and features of specific planets and other objects. Saturn’s rings are a good example.
If you want an up-close and personal view of Mars, the Moon, or Jupiter, 50x is a good option. This is sufficient to get a clear view of the planets without the image being too blurry.
You will also be able to clearly see the rings of Saturn in more detail using a 50x magnification. They will be seen as separate structures detached from the planet itself.
A higher magnification is required for those who want to become advanced astronomers and analyze every cloud detail, crater, crevice, and furrow of a planet. A telescope able to deliver a magnification of 100x is a perfect choice. This will give you an excellent image of all the planets and stars in our solar system.
With 100x magnification, you can also see galaxies and nebulae clearly. These are some of the most beautiful sights for an astronomer. An excellent target at this magnification is the Andromeda galaxy. This galaxy is approximately two million light years away from our own galaxy. It is on a collision course with our galaxy.
As indicated earlier, there is a practical limit to the amount you can magnify an image. As you increase the magnification, the image becomes dimmer and blurrier.
The amount of light entering the telescope determines how much magnification can be applied while providing an acceptable image. Therefore, the telescope’s aperture dictates the maximum useful magnification. If you measure the aperture in mm, you multiply it by two to find the maximum magnification. If you use inches, multiply by 50. To calculate this and other values, use the calculator linked to earlier.
Summary
The magnification of a telescope is one of the most important factors to consider when customizing and using a telescope. This is especially so if you want to analyze every last detail of your target. So make sure to calculate your overall magnification so that you know what you’ll be able to see when you next look up into the night sky with your telescope.
While magnification is important, we recommend avoiding any telescope highlighting its magnification. These telescopes are almost invariably poor quality and will ruin your experience.
Frequently Asked Questions
Most telescopes use standard 1.25″ or 2″ eyepieces, but always check your telescope’s specifications.
This could be due to exceeding the maximum useful magnification or poor atmospheric conditions.
Switching between low and high magnifications is common as you observe different objects or details.
Barlow lenses can be a cost-effective way to increase your magnification options, especially for beginners.
While they don’t affect the actual magnification, poor conditions can limit the usefulness of high magnifications.
No, the best magnification depends on the observed object and observing conditions.
As magnification increases, image brightness decreases. This is why larger apertures are beneficial for high magnifications.
Since then, I’ve been an avid stargazer and astronomer, and love nothing more than spending my time charting stars, observing planets, and finding constellations.
This is why I decided to start Telescope Guru. I only wish to share this fun pastime with the world. With this site, I hope to answer all of your questions relating to astronomy, telescopes, and stargazing.
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