Telescope Calculator for Your Telescope’s Performance

You'll love this handy telescope calculator for your telescope's performance when changing eyepieces and accessories from Sky and Telescope.

S&T: Craig Michael Utter

Welcome to's Telescope Calculator. This interactive tool is designed to give you answers to your commonly asked questions about your telescope's performance when changing eyepieces and accessories.

To use this telescope calculator tool properly, you'll need to know a few basic details about your telescope and eyepieces. Most of these numbers can be found on your eyepiece barrel, on your telescope's tube or mount, or in the equipment's instruction manual. Failing that, you can always visit the website of your telescope or eyepiece manufacturer.

Telescope Calculator Inputs:

  1. Scope Aperture: The diameter of a telescope’s main lens or mirror — and the scope’s most important attribute.
  2. Scope Focal Ratio (f/number): A lens or mirror’s focal length divided by its aperture. For instance, a telescope with an 80-mm-wide lens and a 400-mm focal length has a focal ratio of f/5.
  3. Eyepiece Focal Length: Eyepiece focal lengths are nearly always printed on the eyepiece itself and are labeled in millimeters. A telescope’s magnification can be changed by using eyepieces with different focal lengths; shorter focal lengths yield higher magnifications.
  4. Eyepiece Apparent Field: For most eyepieces (Kellners, Orthoscopics, or Plössls), this value is about 50 degrees. For specialty oculars, check the barrel of the eyepiece first.
  5. Barlow Lens: A lens that's placed into the focusing tube to effectively increase a telescope's focal length and, in turn, the magnification of any eyepiece used with it.
  6. Focal Reducer: A device that shrinks the effective focal length of the telescope.


  1. Focal Length: The distance (usually expressed in millimeters) from a mirror or lens to the image that it forms. In most telescopes the focal length is roughly equal to the length of the tube. Some telescopes use extra lenses and/or mirrors to create a long effective focal length in a short tube.
  2. Magnification (power): The amount that a telescope enlarges its subject. It’s equal to the telescope’s focal length divided by the eyepiece’s focal length. As a rule of thumb, a telescope’s maximum useful magnification is 50 times its aperture in inches (or twice its aperture in millimeters).
  3. True Field of View: The circle of sky that you see when you look through a telescope or binoculars. Generally, the lower the magnification, the wider the field of view.
  4. Exit Pupil: The size of the circle of light that hits your eye when you look through the eyepiece. If the exit pupil is larger than 7mm, it means your eye probably won't see all the light coming from the eyepiece. (That's because, for an average adult, the pupil of the dark-adapted human eye is only 7 mm wide.) So in that case you should switch to a higher magnification.
  5. Theoretical Resolving Power: The angular size of the smallest detail you can see at high magnification. This tells you the closest double star you can expect to separate.
  6. Approximate Limiting Magnitude of Telescope: A number denoting the faintest star you can expect to see. The higher the magnitude, the fainter the star. For example, a 1st-magnitude star is 100 times brighter than a 6th-magnitude star.

    Telescope aperture = mm
    Telescope focal ratio=
    Eyepiece focal length = mm
    Eyepiece apparent field =
    Barlowlens =
    Focal Reducer =
    Here is an analysis of your telescope's performance using the parameters selected above:

    If you see any problems with this telescope calculator tool, or any of our interactive tools, please send an email to

11 thoughts on “Telescope Calculator for Your Telescope’s Performance

  1. Ruben

    not only is the calculator not working,but
    the drop down windows disappear before you make your selection!
    does anybody have a pencil and eraser I can barrow ?

  2. Dobsonian1

    Magnification calculation is wrong. Here are my inputs:

    1200 mm aperture
    5.91mm f/ratio
    20mm eyepiece focal length
    52 degree apparent field
    no barlow
    no focal reducer

    magnification should be 60x, not 355x. checked against two other online calculators.

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