Telescope Field of View Calculator

The true field of view (TFOV) is calculated by dividing the eyepiece's apparent field of view (AFOV) by the magnification: TFOV = AFOV ÷ Magnification. Magnification itself equals the telescope focal length divided by the eyepiece focal length. For example, a 1000 mm telescope with a 25 mm eyepiece (52° AFOV) gives 40× magnification and a 1.3° true field of view.

The apparent field of view (AFOV) is printed on the eyepiece box, engraved on the barrel, or listed in the manufacturer's specifications. Common designs have standard AFOVs: Kellner ≈40–45°, Plössl ≈50–52°, wide-angle ≈65–68°, and ultra-wide ≈82–100°. If you cannot find it, 52° is a safe default for a standard Plössl eyepiece.

A Barlow lens multiplies the effective focal length of the telescope, which increases magnification by the Barlow factor (typically 2× or 3×). Higher magnification reduces the true field of view by the same factor. A 2× Barlow halves your TFOV. You can model this by multiplying your telescope's focal length by the Barlow factor in the calculator.

The Moon and Sun span about 0.5° (30 arcminutes), so a TFOV of 0.8–1.5° works well for full-disk views. Large open clusters like the Pleiades need 1–3°. Galaxies like Andromeda (M31) span about 3°, requiring a wide-field setup. Planets are tiny (under 1 arcminute) and benefit from high magnification rather than wide FOV. Globular clusters and most nebulae fit within 20–60 arcminutes.

1 degree = 60 arcminutes (′). 1 arcminute = 60 arcseconds (″). Therefore 1 degree = 3600 arcseconds. To convert degrees to arcminutes, multiply by 60. To convert to arcseconds, multiply by 3600. This calculator displays your true field of view in all three units automatically.

Image scale (also called plate scale) expresses how many arcseconds of sky fit within one pixel of your camera sensor. It is calculated as: Image Scale (″/px) = (Pixel Size in μm × 206.265) ÷ Telescope Focal Length (mm). Smaller values mean finer resolution but require better seeing and tracking. A typical target for good seeing is 1–2 ″/px for high-resolution planetary work, and 1.5–3 ″/px for deep-sky imaging.

Yes — using a focal reducer decreases the telescope's effective focal length, lowering magnification and widening the true field of view. A 0.5× reducer doubles the TFOV. You can also use a shorter telescope, pair a wide-AFOV eyepiece with the same scope, or for imaging, use a larger sensor. Each approach trades field coverage against other optical properties like image scale and f/ratio.

Hubble's Wide Field Camera 3 (WFC3) in its wide-field channel covers approximately 162 × 162 arcseconds (about 0.045° × 0.045°) with a pixel scale of around 0.13 arcseconds per pixel. Its Advanced Camera for Surveys (ACS) Wide Field Channel covers about 202 × 202 arcseconds. These are extremely narrow fields compared to typical amateur telescopes, reflecting Hubble's very long focal length of ~57.6 metres.