Telescope FOV Calculator

Eyepiece Mode

When using an eyepiece, the magnification, true field of view, and exit pupil are calculated from your telescope and eyepiece parameters.

A Barlow lens multiplies the effective focal length, increasing magnification. A focal reducer decreases it, giving a wider field.

Camera Sensor Mode

For astrophotography, the field of view depends on the physical sensor size and telescope focal length.

Width and height FOV are calculated independently. The diagonal FOV uses the sensor diagonal.

Object Fit Preview

The preview circle represents your true field of view. Celestial objects are drawn at their approximate angular size so you can see how they fit in the eyepiece or on the sensor. This helps with planning observations and choosing the right eyepiece or focal length for a target.

Key Terms

• Focal Ratio (f/) — Telescope focal length divided by aperture. Lower f-numbers mean faster optics and wider fields.
• Exit Pupil — The diameter of the light cone leaving the eyepiece. Should be between ~0.5 mm and 7 mm for comfortable viewing.
• Apparent FOV — The angular width of the view as seen through the eyepiece. Common values range from 40° to 100°+.
• True FOV — The actual angular extent of sky visible through the telescope.
• Dawes Limit — The theoretical angular resolution of the telescope, determined by aperture.
• Drift Time — How long an object takes to cross the field of view with the drive off, at the sidereal rate of 15.04 arcminutes per minute.
• Pixel Scale — Arcseconds of sky per camera pixel (206.265 × pixel size in µm ÷ focal length in mm). Around 1–2″/px is well matched to typical 2–4″ seeing.

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