Telescope Field of View Calculator
How much sky a telescope shows — the true field of view through an eyepiece, or the field and pixel scale with a camera — and whether the Moon or a chosen object fits.
Through an eyepiece, magnification is the telescope’s focal length over the eyepiece’s, and the true field is the eyepiece’s apparent field divided by that magnification.
How the telescope field of view calculator works
Through an eyepiece, magnification is the telescope’s focal length over the eyepiece’s, and the true field is the eyepiece’s apparent field divided by that magnification. With a camera the field is set by the sensor size: 2 arctan(width ÷ 2f), and each pixel covers 206.265 × pixel size ÷ focal length arcseconds. The full Moon is about half a degree; the Pleiades two degrees; the Andromeda galaxy three.
Formula: eyepiece: TFOV = AFOV ÷ (f_tel ÷ f_ep); camera: FOV = 2 arctan(w ÷ 2f); scale = 206.265 × pixel (µm) ÷ f (mm) arcsec
Worked examples
| Inputs | True field of view | Note |
|---|---|---|
| 1,200 mm scope, 25 mm 52° eyepiece | 1.08° (65′) | 48×, 1.08° — the Moon fits |
| Camera only: APS-C on 1,200 mm | 1.12° (67′) | 1.12°; 0.65″ per pixel — the Pleiades do not fit |
| A 400 mm refractor with a 32 mm 70° eyepiece | 5.6° (336′) | 12.5×, 5.6° — Andromeda fits |
FAQFrequently asked questions
Why do wide-field eyepieces matter?
The true field is the apparent field divided by magnification: at 48× a 52° Plössl shows 1.08°, an 82° wide-field eyepiece 1.7° — enough to frame the Pleiades. Long focal-length telescopes need wide eyepieces or a focal reducer for large objects.
What pixel scale should I aim for?
About 1–2 arcseconds per pixel for deep-sky imaging under typical seeing, 0.3–0.5 for planets with a fast camera. Much finer wastes resolution on blur; much coarser makes stars square.
How do I widen a camera’s field?
A focal reducer shortens the effective focal length; a larger sensor sees more at the same scale. Cropping does the opposite. Dividing the sensor’s diagonal by the focal length approximates the diagonal field in radians.
Does a Barlow change the field?
A 2× Barlow doubles the effective focal length, halving the field and doubling magnification and pixel scale. Enter the combined focal length.
Where these figures come from
- NIST — CODATA 2018 fundamental physical constants — G and the speed of light
- IAU 2015 Resolution B3 — nominal solar and planetary conversion constants — the astronomical unit, solar mass and planetary radii
- Royal Observatory Greenwich — the historic home of the prime meridian
Last checked: September 2026. Constants are CODATA 2018 (G, c) and IAU 2015 nominal values (solar and planetary parameters).