PhysicsCalcs
Optics

Telescope Field of View Calculator

Calculate how much sky your telescope can see with any eyepiece.

Interactive calculator

Telescope Field of View Calculator

Calculate true field of view using AFOV or field stop method. Plan eyepiece setups to frame celestial objects.

Try an example

Your result will appear here.

Choose a calculation mode, fill in the known values, and click Calculate.

Quick Guide

  • Enter telescope and eyepiece focal lengths + AFOV.
  • Result shows degrees of sky visible.
  • Moon ≈ 0.52° — a good reference for framing.

Table of Contents

Key Takeaways

  • True FOV = AFOV / Magnification (simple method).
  • True FOV = (field stop / focal length) × 57.3° (accurate method).
  • The Moon is about 0.52° across — a good reference for FOV.
  • Wide-angle eyepieces (82-100° AFOV) give dramatically more sky per view.
  • Higher magnification = narrower FOV; lower magnification = wider FOV.

What Is Field of View?

Field of view (FOV) describes how much sky you can see through your telescope at once. A wider true FOV lets you view larger objects like star clusters, while a narrow FOV is better for resolving fine details on planets. The FOV depends on your telescope’s focal length, eyepiece focal length, and the eyepiece’s apparent field of view.

Formulas

TFOV=AFOVM=AFOV⋅FepFtelTFOV = \frac{AFOV}{M} = \frac{AFOV \cdot F_{ep}}{F_{tel}}
TFOV=dfsFtel×180°πTFOV = \frac{d_{fs}}{F_{tel}} \times \frac{180°}{\pi}

Celestial Object Angular Sizes

ObjectAngular Size
Moon / Sun0.52°
Orion Nebula (M42)~1°
Pleiades (M45)~2°
Andromeda Galaxy (M31)~3°
Jupiter (disk)~0.008° (30″)
Saturn (with rings)~0.012° (43″)

How to Use

  1. Enter your telescope focal length.
  2. Enter eyepiece focal length and AFOV (or field stop diameter).
  3. Click Calculate to see your true FOV and what objects fit.

Examples

8" Dob + 25mm Plössl (52° AFOV)

M = 1200/25 = 48×; TFOV = 52/48 = 1.08° (just fits M42)

SCT + 10mm 82° UWA

M = 2032/10 = 203×; TFOV = 82/203 = 0.40° (planetary detail)

FAQ

What is the difference between AFOV and TFOV?›

Apparent field of view (AFOV) is a property of the eyepiece design — how wide the view appears. True field of view (TFOV) is how much sky you actually see through the telescope+eyepiece combination. TFOV = AFOV / magnification.

Which method is more accurate — AFOV or field stop?›

The field stop method (TFOV = field stop diameter / telescope focal length × 57.3°) is more accurate because it doesn't depend on the manufacturer's AFOV specification, which can be measured differently. However, not all eyepiece specs include field stop diameter.

How do I know my eyepiece's AFOV?›

Check the eyepiece barrel or packaging. Common values: Kellner/Huygens ~40°, Plössl ~52°, wide-angle ~68°, ultra-wide ~82°, Ethos-style ~100°. If unmarked, estimate at 50° as a starting point.

Can I see the entire Orion Nebula in my telescope?›

The Orion Nebula (M42) is about 1° across. You need a TFOV of at least 1° to see it all. With a 1200mm telescope and 25mm Plössl (52° AFOV), TFOV ≈ 1.08° — just barely fits. A wider AFOV eyepiece or shorter focal length telescope helps.

Sources

  1. Tele Vue – Telescope Formulas
  2. Sky & Telescope – Eyepiece Basics
  3. Wikipedia – Field of View (optics)
Manish Kumar

Author & technical reviewer

Manish Kumar

PhysicsCalcs tools are reviewed with an educational focus: clear formulas, transparent assumptions, and practical context for students and science learners.

Learn more about Manish