Telescope Field of View Calculator
Calculate the true field of view of a telescope system
Apparent field of view of the eyepiece
Telescope magnification with this eyepiece
How to Use This Calculator
Enter Eyepiece Field of View
Input the apparent field of view of your eyepiece in degrees. This is typically specified by the eyepiece manufacturer. Common values: 50-52° (standard), 68-82° (wide-angle), 100°+ (ultra-wide).
Enter Magnification
Input the magnification of your telescope with this eyepiece. Magnification = focal length of telescope / focal length of eyepiece.
Calculate
Click the "Calculate True Field of View" button to get the actual angular field of view you'll see through the telescope, displayed in degrees, arcminutes, and arcseconds.
Formula
True FOV = Eyepiece FOV / Magnification
Where:
- True FOV = True field of view (in degrees)
- Eyepiece FOV = Apparent field of view of the eyepiece (in degrees)
- Magnification = Telescope magnification = F_telescope / F_eyepiece
Alternative Formula (using focal lengths):
True FOV = (Eyepiece FOV × F_eyepiece) / F_telescope
Example Calculation:
For a 52° eyepiece at 50× magnification:
Eyepiece FOV = 52°
Magnification = 50×
True FOV = 52 / 50 = 1.04°
This is about 62.4 arcminutes, which can show the full Moon (about 0.5°) comfortably.
About Telescope Field of View Calculator
The field of view (FOV) of a telescope is the angular size of the area you can see through the eyepiece. It's determined by the eyepiece's apparent field of view and the telescope's magnification. A wider field of view allows you to see larger areas of the sky, which is great for observing extended objects like nebulae and star clusters, while a narrower field provides higher magnification for detailed views of planets. This calculator helps you determine the true field of view for your telescope and eyepiece combination.
When to Use This Calculator
- Telescope Selection: Choose eyepieces that provide desired field of view
- Astronomy Planning: Determine if an object will fit in your field of view
- Eyepiece Comparison: Compare different eyepiece options for field of view
- Education: Understand the relationship between magnification and field of view
- Observing: Plan observations knowing what objects will be visible
Why Use Our Calculator?
- ✅ Instant Results: Get accurate field of view calculations immediately
- ✅ Easy to Use: Simple interface requiring only eyepiece FOV and magnification
- ✅ Multiple Units: Results in degrees, arcminutes, and arcseconds
- ✅ Educational: Includes formula explanations and worked examples
- ✅ 100% Free: No registration required
Common Applications
Deep Sky Observing: For observing large objects like the Andromeda Galaxy or the Pleiades star cluster, a wide field of view (1-2°) is desirable. This calculator helps you select eyepieces that provide wide enough fields to see entire objects.
Planetary Observing: For detailed views of planets, a narrower field (0.1-0.5°) is often sufficient. This calculator helps you understand how much sky you're seeing at different magnifications.
Astrophotography: Understanding field of view is crucial for framing astrophotography shots. This calculator helps photographers plan compositions and determine what will fit in the frame.
Tips for Best Results
- Higher magnification reduces field of view (trade-off between detail and field size)
- Wide-angle eyepieces (68°+) provide larger fields even at high magnification
- The full Moon is about 0.5° across, which is a good reference for field size
- For finding objects, start with a wide field, then switch to higher magnification
- Field of view decreases with the square of magnification for the same eyepiece
- Remember that true FOV is always smaller than eyepiece apparent FOV
Frequently Asked Questions
What's the difference between apparent and true field of view?
Apparent field of view is the angular size of the view through the eyepiece itself (how wide the view "feels"). True field of view is the actual angular size of the sky you can see. True FOV = Apparent FOV / Magnification. The true FOV is always smaller than the apparent FOV.
Why does higher magnification reduce field of view?
Magnification makes objects appear larger, which means you're seeing a smaller portion of the sky. It's like zooming in with a camera—you get more detail but see less area. The field of view is inversely proportional to magnification.
What's a good field of view for different objects?
For planets: 0.1-0.5° is usually sufficient. For the Moon: 0.5-1° works well. For large deep-sky objects like the Andromeda Galaxy: 1-2° or more. For star clusters and wide-field views: 2-3° or more. The calculator helps you achieve these fields with different eyepiece combinations.
Can I increase field of view without reducing magnification?
You can increase field of view at the same magnification by using eyepieces with larger apparent fields of view. Wide-angle eyepieces (68-100° apparent FOV) provide larger true fields even at high magnification compared to standard eyepieces (50-52° apparent FOV).
How do I measure field of view experimentally?
You can measure field of view by timing how long a star takes to drift across the field (star drift method). If a star takes t seconds to cross, and you know your latitude, the field of view is approximately (t × 15) arcseconds for stars near the celestial equator. Alternatively, use known objects like the Moon (0.5°) as a reference.