Telescope Magnification Calculator
Calculate the magnification of a telescope with a given eyepiece
Focal length of the telescope objective
Focal length of the eyepiece
How to Use This Calculator
Enter Telescope Focal Length
Input the focal length of your telescope in millimeters. This is typically specified by the manufacturer. Common values: 400-600 mm (small telescopes), 800-1200 mm (medium), 1500-2000 mm (large).
Enter Eyepiece Focal Length
Input the focal length of your eyepiece in millimeters. Common eyepiece focal lengths: 40 mm (low power), 25 mm (medium), 10 mm (high power), 5 mm (very high power).
Calculate
Click the "Calculate Magnification" button to get the magnification power. The result shows how many times larger objects will appear through this telescope and eyepiece combination.
Formula
M = F_telescope / F_eyepiece
Where:
- M = Magnification (dimensionless, often written as "50×")
- F_telescope = Focal length of telescope objective (in mm)
- F_eyepiece = Focal length of eyepiece (in mm)
Example Calculation:
For a telescope with 1000 mm focal length and 20 mm eyepiece:
F_telescope = 1000 mm
F_eyepiece = 20 mm
M = 1000 / 20 = 50×
Objects appear 50 times larger through this combination.
Another Example:
Same telescope with 10 mm eyepiece:
M = 1000 / 10 = 100×
Higher magnification, but smaller field of view and dimmer image.
About Telescope Magnification Calculator
Telescope magnification determines how much larger objects appear when viewed through the telescope compared to the naked eye. It's calculated by dividing the telescope's focal length by the eyepiece's focal length. Understanding magnification is crucial for choosing the right eyepieces for different observing tasks—higher magnification for planets and details, lower magnification for wide-field views and finding objects.
When to Use This Calculator
- Eyepiece Selection: Choose eyepieces to achieve desired magnification
- Observing Planning: Determine what magnification you'll get with different eyepieces
- Telescope Comparison: Compare magnification capabilities of different telescopes
- Education: Understand how focal lengths affect magnification
- Astronomy: Plan observations knowing expected magnification
Why Use Our Calculator?
- ✅ Instant Results: Get accurate magnification calculations immediately
- ✅ Easy to Use: Simple interface requiring only focal lengths
- ✅ Educational: Includes formula explanations and worked examples
- ✅ Practical: Helps choose appropriate eyepieces for your needs
- ✅ 100% Free: No registration required
Common Applications
Planetary Observing: For detailed views of planets, high magnification (150-300×) is often used. This calculator helps you determine which eyepiece will provide the desired magnification for your telescope.
Deep Sky Observing: For large objects like nebulae and star clusters, lower magnification (50-100×) with wider fields is preferred. This calculator helps you select eyepieces that provide appropriate magnification for extended objects.
Lunar Observing: The Moon can be viewed at a wide range of magnifications. This calculator helps you explore different magnification options for various lunar features.
Tips for Best Results
- Use consistent units (millimeters for both focal lengths)
- Shorter eyepiece focal lengths provide higher magnification
- Maximum useful magnification is typically 2× per mm of aperture (e.g., 200× for 100 mm telescope)
- Higher magnification reduces brightness and field of view
- Start with lower magnification to find objects, then increase for detail
- Atmospheric conditions often limit usable magnification to 200-300×
Frequently Asked Questions
What's the maximum useful magnification?
The maximum useful magnification is typically about 2× per millimeter of telescope aperture. For example, a 100 mm (4-inch) telescope can effectively use up to about 200× magnification. Beyond this, images become dim and blurry without additional detail. Atmospheric conditions often limit practical magnification to 200-300× even for large telescopes.
Why does higher magnification make images dimmer?
Higher magnification spreads the same amount of light over a larger apparent area. Since the light comes from the same aperture, the light per unit area (brightness) decreases. This is why high-power views appear dimmer, especially for extended objects like nebulae.
Can I get unlimited magnification?
No, there are practical limits. Beyond the maximum useful magnification (about 2× per mm aperture), images become dim and blurry without showing more detail. Atmospheric turbulence also limits usable magnification. Most observations are best at 50-200× depending on the object and conditions.
What's a good magnification for different objects?
Planets: 150-300×. Moon: 50-200×. Double stars: 100-300×. Star clusters: 50-150×. Nebulae and galaxies: 50-100× (lower is often better for large objects). This calculator helps you achieve these magnifications with different eyepiece combinations.
How do I increase magnification?
You can increase magnification by using eyepieces with shorter focal lengths or by adding a Barlow lens (which multiplies magnification, typically 2× or 3×). For example, a 2× Barlow lens doubles the magnification of any eyepiece. This calculator helps you determine the base magnification, which you can then multiply by the Barlow factor.