Telescope calculator

Fill in the values below of your telescope and eyepiece to get the results. Real values can differ slightly.

Hover your cursor over an icon/letter to view an explanation, or view the full explanation at the bottom of this page.

1. Objective values
2. Eyepiece values
3. Other (optional)
Objective results
Eyepiece results
Simulation (approximation of reality)


Diameter (Ø)

The diameter of the primary objective (in millimeters or inches).

Focal length (f)

The distance (from the center of) the objective to the point where the light comes together (in milimeters).

Focal ratio (f/)

The ratio between the focal length and the diameter (in millimeters).

Calculation: Ø / f telescope.

Focal length eyepiece (f)

The focal length of the eyepiece.

AFOV (apparent field of wiew)

The apparent field of view of the eyepiece. This is the field of view of the eyepiece in degrees (°) at a magnification of 1.

Barlow/reducer (x)

Is used to alter the focal length of the eyepiece and can thus be used to lengthen or shorten it.

Limiting magnitude location (m)

The weakest visible star from the location in the zenith.

Exit pupil (mm)

The maximum diameter of the pupil in diameters. This differs per age and person, but is usually between the 6mm and 7mm.

Maximum magnification (x)

The theoretical maximum magnification (with excellent optics and circumstances).

Calculation: Ø in milimeters * 2

Resolving power (′)

The theoretical resolving power of a telescope is the distance in arcminutes (&prima;) between 2 points that can be split by the telescope.

Calculation: 116 / Ø (Dawes' limit)

Limiting magnitude (m)

The theoretical magnitude of the weakest object that can be seen with this telescope at the given location and with the given maximum pupil diameter.

Calculation: 5 * log10(objective diameter / pupil diameter)

Surface area objective (cm2)

The total surface area of the objective in square centimers.

Calculation: Pi * (Ø / 2) * (Ø / 2)

Magnification (x)

The amount at which the object is magnifide in apparent diameter.

Calculation: f telescope / f eyepiece

Exit pupil (mm)

Diameter (in milimeters) of the sharp image that is given by the telescope and eyepiece.

Calculation: f eyepiece / magnification

FOV (field of view)

The (true) field of view is the diameter of the view in degrees (°) or arcminutes (′).

Calculation: afov eyepiece / magnification

Transition (min/sec)

The duration in minutes or seconds that one point in the ecliptic needs to travel the full width of the field of view in a telescope that is not following.

Calculation: 240 x tfov