Tools

True Field of View Calculator

What This Calculator Tells You

The true field of view (TFOV) is the actual amount of sky you can see through your eyepiece, measured in degrees. This determines whether an object will fit in your view and helps you navigate by knowing how much sky you're seeing.

Calculator

The eyepiece's AFOV - printed on the eyepiece or in specs. Common values: 52° (Plössl), 68° (wide), 82° (ultra-wide), 100°+ (extreme).

Your current magnification (telescope FL / eyepiece FL).

1.30°
(78 arcminutes)
Good general-use

Most galaxies, globular clusters, planetary nebulae

How to Interpret the Result

  • 1° = 2 full Moon diameters - The full Moon is about 0.5° across
  • Wider field = easier finding - More sky visible makes locating objects easier
  • Object should fit comfortably - Ideally, the target occupies 25-75% of your field

Some Common Objects

ObjectSize
Andromeda Galaxy (M31)3.0°
Pleiades (M45)1.8°
Orion Nebula (M42)1.0°
Full Moon0.5°
Ring Nebula (M57)1.4′
Jupiter (at opposition)50″
Saturn (with rings)42″

Eyepiece Types and Typical AFOVs

Eyepiece TypeTypical AFOVCharacteristics
Plössl50-52°Budget-friendly, good eye relief
Wide-field65-72°More immersive, popular choice
Ultra-wide80-84°Spacewalk effect, premium price
Extreme wide100-120°Maximum immersion, highest cost

Rules of Thumb

  • For star-hopping: Use at least 1° TFOV to see guide star patterns
  • For most deep sky: 0.5° to 1.5° works well for the majority of objects
  • For planets: TFOV matters less; focus on magnification instead
  • Double AFOV, double TFOV: An 82° eyepiece shows 60% more sky than a 52°

Common Mistakes & Misconceptions

  • Confusing AFOV and TFOV - AFOV is fixed for the eyepiece; TFOV depends on magnification.
  • "I need the widest field possible" - Very wide fields at high power may have edge distortion and are expensive.
  • Ignoring magnification - A 100° AFOV eyepiece at high power may still give a narrower TFOV than a 52° at low power.

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