Telescope FOV Calculator
Enter your telescope and eyepiece to see the magnification, how much sky fits in the view, and whether the combination makes sense for what you want to observe.
- Magnification
- 48×
- True field of view
- 1° 5′
- 1.08° · Apparent-field approximation
- Exit pupil
- 4.2 mm
- General deep-sky: galaxies, clusters, most nebulae.
- Focal ratio
- f/5.9
- Max useful magnification
- ≈ 406×
- Min useful magnification
- ≈ 29×
- Resolution (Dawes limit)
- 0.57″
- Limiting magnitude
- ≈ 14.2
The formulas
- Magnification
M = F_telescope × Barlow ÷ F_eyepiece- Focal lengths in the same unit (mm).
- True field (apparent-field method)
TFOV ≈ AFOV ÷ M- Quick and good to a few percent; the eyepiece's advertised apparent field is itself rounded.
- True field (field-stop method)
TFOV = field stop ÷ (F_telescope × Barlow) × 57.3°- More exact when the field-stop diameter is known.
- Exit pupil
EP = D ÷ M (= F_eyepiece ÷ focal ratio)- Diameter of the beam leaving the eyepiece. Over ~7 mm, light is wasted.
- Useful magnification range
≈ D ÷ 7 to ≈ 2 × D (D in mm)- The 2×D (50× per inch) rule of thumb assumes steady air; most nights top out lower.
- Dawes limit
R ≈ 116 ÷ D (mm) arcseconds- Closest pair of equal stars a telescope can split.
- Limiting magnitude
m ≈ 7.7 + 5 × log10(D in cm)- Faintest stars under a dark sky; real results vary ±1 with sky and experience.
Sources
Choosing magnification
More magnification is not always better. Every doubling of magnification spreads the same light over four times the area, so the image gets dimmer, and the atmosphere's turbulence (seeing) is magnified too.
Exit pupil is the best single guide: 4–7 mm for large, faint nebulae and star fields; 2–3 mm for most galaxies and clusters; 0.5–1.5 mm for planets, the Moon and double stars on steady nights.
Apparent vs true field of view
The apparent field is how wide the eyepiece's view looks to your eye — 50° feels like looking through a tube, 82° like looking through a window. The true field is the patch of actual sky you see, and it shrinks as magnification grows.
For reference, the full Moon is about 0.5° across, the Pleiades about 2°, and the Andromeda Galaxy's bright core about 1° (its full extent over 3°).