How much sky will my telescope show?
This telescope field of view calculator answers two questions at once: how much sky fits in the eyepiece, and how much fits on a camera sensor at prime focus. Enter the telescope’s focal length, the eyepiece focal length and its apparent field (or, better, its field stop), and the sensor size of your camera. The true field of view is drawn to scale against the Moon or a well-known deep-sky object, so you can see at a glance whether the Pleiades fit in one view or whether the Andromeda Galaxy needs a mosaic.
All the arithmetic runs in your browser with the same optics formulas as the telescope magnification calculator. Nothing is uploaded.
How to use the Telescope Field of View Calculator
- Enter the telescope aperture and focal length in millimetres; both are printed on the tube or in the manual.
- Enter the eyepiece focal length and its apparent field of view (tap a preset such as Plössl 52°). If the maker lists a field stop diameter, type it as well — that result is used first.
- Add a Barlow factor if you use one.
- For a camera, pick a sensor (or type its width and height) and a reducer factor if you image through one.
- Pick a target in “Compare with” and read the true field, how many Moons fit, and the drawing.
Telescope Field of View Calculator features
Two eyepiece methods
Apparent field ÷ magnification, and the more accurate field-stop method.
Camera framing
Width, height and diagonal field of a sensor at prime focus, with reducer or Barlow.
To-scale overlay
Eyepiece circle, camera rectangle and target disc drawn to the same angular scale.
Sensor presets
Common camera formats plus astronomy-camera sensors, sized from their pixel count × pixel pitch.
Deep-sky targets
Moon, Sun, Pleiades, Andromeda, Orion Nebula, Beehive, Lagoon, Hercules Cluster, Ring Nebula and Jupiter.
Formulas
Apparent-field method: TFOV = AFOV ÷ M, with M = F × Barlow ÷ fe. A 25 mm Plössl (52°) in a 900 mm telescope gives 36× and 52 ÷ 36 = 1.44°, about 2.8 Moon diameters.
Field-stop method: TFOV = field stop ÷ F × 57.3° (the calculator uses the exact form 2·atan(stop ÷ 2F), which differs by less than 0.01%). The field stop is the physical ring inside the eyepiece barrel that limits the view, so this method avoids the rounding and distortion in quoted apparent fields. A 27 mm stop at 900 mm gives 1.72°.
Camera at prime focus: FOV = 2 × atan(sensor side ÷ 2F). An APS-C sensor 23.5 × 15.6 mm at 900 mm covers 1.50° × 0.99°. A 0.8× reducer multiplies the focal length by 0.8 and widens the frame by about 25%.
Target sizes
The Moon’s apparent diameter varies from about 29.3′ to 34.1′ with its distance, so the calculator uses 31′; the Sun ranges from 31.6′ to 32.7′. Deep-sky sizes are the major-axis values from the SEDS Messier database (for example M31 190′, M45 120′, M42 65′). Faint outer parts of galaxies and nebulae often look smaller visually than these catalogue sizes. Jupiter is shown at about 45″, its size near opposition; it ranges from about 30″ to 50″.
Limits and related tools
- Quoted apparent fields are often rounded, and wide-field eyepieces have distortion, so the two eyepiece methods can differ by a few percent.
- Astronomy-camera sensor sizes are calculated from the pixel count and pixel size makers publish; check your camera’s own sheet and type the exact values if needed.
- For sampling and arcseconds per pixel use the astrophotography pixel scale calculator; for brightness see the exit pupil calculator. Ordinary camera lenses are covered by the camera FOV calculator.
Telescope Field of View Calculator FAQ
How do I calculate the true field of view of an eyepiece?
Divide the eyepiece’s apparent field by the magnification. A 52° eyepiece at 36× shows 1.44° of sky. If you know the field stop, divide it by the telescope focal length and multiply by 57.3°.
Will the whole Moon fit in my eyepiece?
Yes if the true field is above about 0.52°. Pick “Moon” and the calculator says how many Moon diameters fit across the field.
How do I work out the field of view of a camera on a telescope?
Use FOV = 2 × atan(sensor width ÷ 2 × focal length) for each side. Enter the sensor size in the camera section to get width, height and diagonal.
What is a field stop?
The ring inside the eyepiece barrel that sets the edge of the view. Makers such as Tele Vue publish it, and it gives the most accurate true field.
Does a Barlow lens change the field of view?
Yes. A 2× Barlow doubles the magnification and halves the true field. A focal reducer does the opposite for a camera.
Why is the field in my eyepiece different from the result?
Rounded apparent-field figures, eyepiece distortion and a different real focal length all shift it slightly. Timing a star drifting across the field gives the exact value.