Telescope Magnification Calculator (with imperial system support)

Below you will find two calculators that will help you know the capabilities of a telescope in terms of magnification.

The magnification of a telescope refers to the number of times the image is being magnified so you can see far-away objects with a greater level of detail. It is also sometimes referred to as “power “.

Magnification is not only determined by a telescope but by the whole optic chain of the system, meaning the telescope + accessories. Most of the time it is going to be determined by the telescope and the eyepiece you are using, but other accessories like a Barlow lens, a focal extender, or a focal reducer can also play a role.

Telescope Magnification Calculator

To calculate the magnification of a telescope you will need to know two things. The focal length of a telescope and the focal length of the eyepiece you are going to be using with it.

The focal length of a telescope is the length of the path light has to travel to get from the mirror or lens at the front of a telescope to the exit pupil where you are going to place your eye to watch. If you don’t know the FL of your telescope, it can be usually found simply by googling “[model] focal length”. It is usually given in millimeters (mm) although the calculator below also supports inches. It will usually go from 400mm to 1600mm for home telescopes.

The focal length of eyepieces is the same concept but is obviously a lot smaller due to their size. It will usually go from 2.3mm to 60mm and can usually be found printed on the side. It is also given in millimeters (mm).

Telescope Highest Useful Magnification Calculator (with support for inches)

There is a limit to how much magnification can a telescope reach before you don’t get any more detail. In fact, if you cross that limit the image now only will not get any better, but it will get worse. Blurriness and optical aberrations will start to show up.

This limit is called the highest useful magnification and it is going to be determined by the telescope’s aperture. You can use the calculator above to determine how much power can a telescope reach and then use the one in the previous section to try a few different eyepiece focal lengths and establish what eyepiece size is the highest power one can use. Remember that shorter focal lengths = more magnification. The limit in eyepiece sizes is going to be between 6mm and 2mm for most home telescopes.