Telescope magnification calculator
Enter your telescope's aperture and focal length and the eyepiece's focal length and apparent field of view. You get the magnification, the size of the exit pupil, the true field of view in the sky and the focal ratio, along with the highest and lowest magnifications by common rules of thumb.
Runs in your browser: nothing you enter is sent anywhere.
How to use it
- Enter the aperture (the diameter of the main mirror or lens) and the focal length of the telescope, both in millimetres.
- Enter the eyepiece focal length and its apparent field of view, usually printed on the eyepiece or in its specifications.
- If you use a Barlow lens, enter its factor (2 for a 2×). For a focal reducer, enter a number below 1, such as 0.63.
How it works
Magnification = telescope focal length × Barlow factor ÷ eyepiece focal length. Exit pupil = aperture ÷ magnification. True field = apparent field ÷ magnification. Focal ratio = effective focal length ÷ aperture. The highest and lowest powers are rules of thumb: 2 × the aperture in mm and aperture ÷ 7.
Worked example
A 200 mm telescope with 1200 mm focal length and a 25 mm eyepiece of 52 degrees: 1200 ÷ 25 = 48 times. Exit pupil 200 ÷ 48 = 4.17 mm. True field 52 ÷ 48 = 1.08 degrees. Focal ratio 1200 ÷ 200 = f/6.
Tips
- Start with a low-power eyepiece to find an object, then switch to a higher one.
- A bigger magnification dims the image and magnifies atmospheric turbulence too.
- For a Barlow, the field of view in the eyepiece’s specification stays the same, so the true field shrinks with the power.
Frequently asked questions
How is telescope magnification calculated?
Divide the focal length of the telescope by the focal length of the eyepiece. A 1200 mm telescope with a 25 mm eyepiece gives 1200 ÷ 25 = 48 times. A Barlow lens multiplies the telescope's effective focal length, so a 2× Barlow doubles the power.
What is the exit pupil?
The width of the beam of light leaving the eyepiece, in millimetres. It equals the aperture divided by the magnification, or the eyepiece focal length divided by the focal ratio. Small pupils give dim, sharp images, and a pupil wider than your eye's own opening wastes light.
How is the true field of view found?
Divide the eyepiece's apparent field of view by the magnification. A 52 degree eyepiece at 48 times shows about 1.08 degrees of sky. This is the common approximation and is accurate enough for planning.
What is the maximum useful magnification?
A widely used rule of thumb is about 2 times the aperture in millimetres, so 400 times for a 200 mm telescope. It is not a hard limit: steady air and the quality of the optics decide what you can really use, and most nights the sky allows less.
What is the lowest useful magnification?
The calculator shows the aperture divided by 7 mm, the pupil of a fully dark-adapted young eye. Below that the exit pupil is larger than your eye can take in. Older eyes open less, so your own minimum is likely a little higher.