Telescope Field of View Calculator

Calculate the true field of view (TFOV) of your telescope to know how much of the sky you are seeing.

Use the simple method with eyepiece AFOV and magnification, or the advanced method with focal lengths for a more precise calculation.

Practical Examples

See how the calculator works with common telescope and eyepiece combinations.

Orion Nebula with a Beginner Telescope

Simple

Calculating the TFOV for a standard 52° AFOV Plössl eyepiece at 75x magnification.

Method: Simple (AFOV & Magnification)

AFOV: 52 °

Magnification: 75x

Andromeda Galaxy View

Simple

Using a wide-angle eyepiece (68° AFOV) with a telescope providing 120x magnification to frame the galaxy.

Method: Simple (AFOV & Magnification)

AFOV: 68 °

Magnification: 120x

Precise Calculation for a Schmidt-Cassegrain

Advanced

Using the advanced method for a telescope with a 2032mm focal length and an eyepiece with a 35mm field stop.

Method: Advanced (Focal Lengths)

Field Stop: 35 mm

Focal Length: 2032 mm

Lunar Observation with a Refractor

Advanced

Calculating the exact TFOV for a 900mm focal length refractor and an eyepiece with a 22mm field stop to see if the entire moon fits.

Method: Advanced (Focal Lengths)

Field Stop: 22 mm

Focal Length: 900 mm

Other Titles
Understanding the Telescope Field of View Calculator
A comprehensive guide to understanding what you see through your telescope and how to calculate it.

What is Telescope Field of View?

  • The Basics of AFOV and TFOV
  • Why It Matters for Stargazing
  • Factors That Influence Your Field of View
The Telescope Field of View (FOV) determines how much of the sky you can see when you look through your eyepiece. It's a critical concept in astronomy for framing celestial objects. There are two types of FOV: the Apparent Field of View (AFOV), which is a fixed property of the eyepiece, and the True Field of View (TFOV), which is what you actually see. This calculator helps you determine the TFOV based on your specific telescope and eyepiece combination.
Apparent vs. True Field of View
AFOV is the angular width of the sky that your eyepiece would show you if you held it up to your eye like a magnifying glass. It's measured in degrees and typically ranges from 40° to over 100°. TFOV, on the other hand, is the actual patch of sky visible when that eyepiece is attached to your telescope. It's always smaller than the AFOV and depends heavily on the telescope's magnification.

Example Scenario

  • Imagine trying to view the entire Pleiades star cluster. If your TFOV is too narrow, you might only see a few stars at a time, missing the full spectacle. Knowing your TFOV allows you to select the right eyepiece for the job.

Step-by-Step Guide to Using the Calculator

  • Choosing Your Calculation Method
  • Entering Your Equipment Specifications
  • Interpreting the Results
This calculator offers two methods to find your TFOV. The 'Simple' method is quick and easy, while the 'Advanced' method provides a more precise result if you know the technical specifications of your gear.
Simple Method (AFOV & Magnification)
  1. Apparent Field of View (AFOV): Find the AFOV of your eyepiece. This is usually printed on the eyepiece itself or listed in its manual. Enter this value in degrees.
  2. Telescope Magnification: Determine the magnification. If you don't know it, you can calculate it by dividing your telescope's focal length by your eyepiece's focal length. Enter this value.
  3. Calculate: Click the 'Calculate' button to see your TFOV.
Advanced Method (Focal Lengths)
  1. Eyepiece Field Stop Diameter: This is the diameter in millimeters of the metal ring inside the eyepiece that physically limits the view. You can find this in the eyepiece specifications.
  2. Telescope Focal Length: Find the focal length of your telescope, also in millimeters. This is a primary specification of your telescope.
  3. Calculate: The calculator will use these values to compute a very accurate TFOV.

Units of Measurement

  • The primary result is given in degrees (°), which is standard. For finer detail, especially in high-magnification planetary viewing, the results are also provided in arcminutes (') and arcseconds ('). One degree contains 60 arcminutes, and one arcminute contains 60 arcseconds.

Real-World Applications of TFOV

  • Planning Your Observations
  • Star Hopping and Navigation
  • Astrophotography Framing
Knowing your TFOV is not just an academic exercise; it has practical benefits for every observing session.
Finding and Framing Objects
Large objects like the Andromeda Galaxy or the Orion Nebula can be larger than your TFOV with a high-power eyepiece. Calculating your TFOV beforehand helps you choose an eyepiece that will fit the entire object in the view. For example, the full Moon is about 0.5 degrees in diameter. If your calculated TFOV is 1 degree, you'll see the Moon with plenty of surrounding space.
Star Hopping
Star hopping is a technique for finding faint objects by using bright, nearby stars as waypoints. Knowing your TFOV is essential for this, as star charts often show circles representing a standard 1-degree or 0.5-degree field of view. You can match your TFOV to these charts to gauge distances and navigate the night sky effectively.

Astrophotography Tip

  • In astrophotography, the TFOV of your camera sensor combined with your telescope is crucial for framing your shot. While this calculator is for visual astronomy, the same principles apply. A larger TFOV is often desired for wide-field shots of nebulae and galaxies.

Common Misconceptions and Correct Methods

  • AFOV vs. TFOV Confusion
  • Is Bigger Always Better?
  • The Myth of 'Maximum Magnification'
There are several common misunderstandings related to telescope optics that can lead to a frustrating experience.
Confusing Apparent and True Field of View
The most common mistake is assuming the AFOV of the eyepiece is what you'll see. Remember, the telescope's magnification always reduces the AFOV to the TFOV. A 100° AFOV eyepiece will not show you a 100° patch of sky; its TFOV might only be 1° or less at high magnification.
The Quest for High Magnification
Many beginners believe that more magnification is always better. However, increasing magnification shrinks your TFOV, makes the image dimmer, and amplifies atmospheric instability. Often, a lower magnification with a wider TFOV provides a much more pleasing and detailed view, especially for large deep-sky objects.

A Note on Barlow Lenses

  • A Barlow lens increases magnification, which in turn decreases your TFOV. A 2x Barlow will double the magnification but cut your TFOV in half. This is great for planetary viewing but not for wide-field vistas.

Mathematical Derivation and Formulas

  • The Simple Formula Explained
  • The Advanced Formula for Precision
  • Converting Degrees to Arcminutes and Arcseconds
The calculations are straightforward and based on fundamental optical principles.
Simple Formula

True Field of View (TFOV) = Apparent Field of View (AFOV) / Magnification This formula is a direct relationship. If you double the magnification, you halve the true field of view. It's a very reliable approximation for most eyepieces.

Advanced Formula

TFOV (degrees) = (Eyepiece Field Stop Diameter / Telescope Focal Length) * (180 / π) This formula is more accurate because it's based on the physical dimensions of the equipment. The (180 / π) part, which is approximately 57.3, is a conversion factor from radians to degrees, as the initial ratio gives the field of view in radians.

Conversion Example

  • If the calculator gives a TFOV of 0.75°:\n- TFOV in Arcminutes = 0.75 * 60 = 45'\n- TFOV in Arcseconds = 45' * 60 = 2700'