Telescope Field of View Calculator: Magnification, Exit Pupil & Image Scale

Your details

The focal length of your telescope tube, printed on the label or in its spec sheet.
mm
The diameter of the main lens or mirror. Used for focal ratio, exit pupil, and limiting magnitude.
mm
The focal length of the eyepiece, printed on its barrel. Common values: 4-40 mm.
mm
The angular diameter of the sky circle you see when holding the eyepiece to your eye alone. Check the eyepiece box: Plossl ~50 deg, wide-angle 65-82 deg, ultra-wide 100+ deg.
deg
Informational only. The AFOV input above is what the calculator uses.
Enter 2 for a 2x Barlow, 0.63 for a focal reducer, 1 for no accessory.
Width of your camera sensor in mm. Used to compute the sensor field of view. APS-C is about 23.5 mm, full-frame is 36 mm.
mm
Pixel pitch in micrometres (microns). Found in the camera spec sheet.
micron
True field of viewGood exit pupil for most viewing
1.3deg

Degrees of sky visible through the eyepiece-telescope combination

True FOV in arcminutes78arcmin
Magnification40x
Focal ratio (f/number)10
Exit pupil2.5mm
Visual limiting magnitude12.1
Image scale0.78arcsec/px
Sensor field of view (width)1.346deg
FOV vs Moon diameter2.6
True FOV (deg)1.3
Exit pupil (mm)2.5

1.30 deg true FOV at 40x magnification

  • Your true FOV of 1.30 deg (78 arcmin) is 2.6x the Moon diameter - excellent for wide star fields and large nebulae.
  • 40x is a low magnification ideal for sweeping star fields, large open clusters, and the Milky Way.
  • Exit pupil of 2.5 mm gives a good balance of brightness and detail for general-purpose viewing.
  • Your aperture can theoretically reveal stars down to magnitude 12.1 in ideal conditions - about tens of thousands of stars.

Next stepTo find your best low-power eyepiece, target an exit pupil of 5-7 mm: ideal eyepiece focal length = 5 x f/10.0 to 7 x f/10.0, giving about 50-70 mm.

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