If you want to calculate the magnification of your telescope, you’re in luck: It’s a surprisingly simple formula. In fact, as we’ll see in a moment, your telescope is actually capable of many different magnifications, by combining it with different eyepieces.
I’ve created the tool below to make it super easy to calculate telescope magnifications. It will also give you an idea of what types of objects you could view with each magnification.
Keep in mind that these are very rough guidelines – most objects can be viewed at a variety of magnifications, and many stargazers will try several eyepieces to get different views of the same object.
There are also other factors that can limit high magnification views, such as your telescope’s aperture and the Earth’s atmosphere. We’ll talk more about the maximum usable magnification later in this article.
To calculate the magnification, we just need two numbers:
- The telescope’s focal length (for example, 1200mm)
- The eyepiece’s focal length (for example, 25mm)
To find the magnification, we’ll simply divide the numbers:
1200mm / 25mm = 48x magnification
If we wanted more magnification, we could use an eyepiece with a shorter focal length, such as 10mm:
1200mm/10mm = 120x magnification
We could also use these eyepieces with a different telescope (say, with a 600mm focal length) to get different magnifications:
600mm/25mm = 24x magnification
600mm/10mm = 60x magnification
As you can see, the 1200mm telescope gives a higher magnification with these eyepieces – and the 10mm eyepiece gives a higher magnification with both telescopes.
So the telescope and eyepiece affect the magnification in different ways:
- Telescopes with longer focal lengths will give a higher magnification (for a given eyepiece)
- Eyepieces with shorter focal lengths will give a higher magnification (for a given telescope)
How to find an eyepiece’s focal length
Finding the focal length of an eyepiece is easy since it’s common to refer to eyepieces by their focal length. So a 17mm eyepiece has a focal length of 17mm. Most eyepieces are clearly marked with their focal length.
How to find a telescope’s focal length
Finding the focal length of a telescope is slightly trickier because the most prominent number you see will usually be the aperture. So if you see an 8-inch Dobsonian or a 70mm refractor, that is referring to the aperture, not the focal length.
If you’re shopping for a telescope online, the focal length should be listed somewhere in the specs (if not the description). On Amazon you can also search the Customer questions & answers, since other customers may have asked about the focal length.
If you already have a telescope, the focal length can often be found on the telescope itself. If not, it should be in the manual.
Changing the magnification with a Barlow lens
A Barlow lens is an accessory that multiplies the magnification. For example, a 2x Barlow doubles the magnification, and a 3x Barlow triples it.
A Barlow isn’t actually an eyepiece – rather, it fits in between the eyepiece and the telescope focuser.
Going back to our 600mm telescope:
600mm/10mm = 60x magnification (without Barlow)
We could double this magnification with a Barlow lens:
(600mm/10mm)*2 = 120x magnification (with 2x Barlow)
So a Barlow lens basically just gives you additional magnification options. In a way, it doubles your eyepiece collection because each eyepiece now has two possible magnifications: with a Barlow and without.
Calculating the maximum usable magnification
Every telescope has a maximum usable magnification. Beyond this magnification, objects will continue to look bigger, but they won’t look better.
In fact, they’ll start to look much worse.
To estimate the maximum usable magnification, multiply the aperture (in inches) by 50. If you’re using millimeters, multiply the aperture by 2.
For example, if your telescope has an 8-inch aperture, the maximum usable magnification will be 400x. A small refractor with a 60mm aperture would only go to 120x before the view starts to deteriorate.
So if your telescope has a huge 20-inch aperture, would you be able to comfortably view the skies at 1,000x magnification?
Probably not. At very high magnifications, the Earth’s atmosphere will cause the view to break up even when using a large-aperture telescope. Turbulence in the atmosphere causes details to become blurred – and as you increase the magnification, this blurring only becomes more noticeable.
A final thought
Although it’s useful to calculate magnifications, keep in mind that higher magnifications aren’t inherently better or more powerful. There are lots of good reasons to use low magnification – the views tend to be brighter, and you can see a wider swath of sky.
Also remember that there are other factors that can affect views. Depending on the quality and aperture of your telescope, as well as atmospheric conditions, higher magnifications may not look as good.