How to Calculate Telescope Magnification

Magnification is one of the most important factors of a telescope, no doubt. But then again, we all seem to hesitate while calculating it.

And why won’t you feel like that? It is not like you and I are any kind of mathematician. 

Of course, a few details are given with it. You can find these in the instruction manual or even on their official website. But how are you supposed to process that information if you don’t know the method? 

Well, there are some easy solutions to that. And yes, you don’t have to be any mathematician to pull those out. 

So, pay close attention as I am going to guide you on how to calculate telescope magnification in this article.

What is the Magnification of a Telescope?

Magnification refers to the quality of enlarging an object. If anybody says that the magnification of a device is 2, it means that it is capable of presenting the object 2 times larger than its original size.

Similarly, the telescope magnification also indicates the capability of magnifying an image. In general cases, the maximum amount of magnification is almost 50x than the diameter of the aperture. 

The eyepiece that you see in your telescope is responsible for the magnification. However, it is a tricky job to adjust an eyepiece correctly. 

Why Calculate Telescope Magnification?

We got the point that magnification plays an important role in the performance of a telescope. But what is the point of calculating it?

Well, things are quite different from what they seem to be. When you are handling a telescope, you will need tons of alterations with its magnification ability. 

The constant focusing and defocusing is not something that comes by default. Therefore, for each alteration, you need to have a clear concept about the calculations and its accuracy.

Again, other important factors like true field view and exit pupil also depend on the magnification. Therefore, it is important for you to learn how to calculate magnification to properly operate a telescope.

How to Calculate Telescope Magnification

So, now that you have learned what a magnification means and also the importance of calculating it correctly, it is time for you to master the method of the calculation.

The process is quite easy. Some related factors might also prove helpful for you to know it better. So, here are a few easy steps that are discussed below to help you out.

Finding Focal Length

The focal length denotes the distance between the lens and its newly formed image. Although all of it sounds a bit difficult to figure out, it is quite easy.

In general, the length of your telescope’s tube is considered as its focal length. But, what if the size of the tube is too short?

Well, in such a situation, additional lenses or mirrors are used to alter the usual fact and provide a longer focal length.

You should keep in mind that the focal length directly affects the image size. So, it is better to check out the focal length before purchasing it. An ideal telescope usually has a focal length of almost 1000 to 1200 millimeters. 

Therefore, note down the focal length of your telescope that you will find on its label and continue the further procedure.

Finding Eyepiece-Size

The eyepiece-length of a telescope also affects the magnification directly. You can say that the relation between them is inversely proportional.

Yes, you guessed it right. The smaller your eyepiece will be, the larger amount of magnification you will get.

In most cases, the length of the eyepiece is given just over it. It is printed in smaller fonts so, look closer and you will eventually find it.

The length is measured in millimeters in general. However, while calculating the magnification, alter the units if needed as you are going to need all the factors in the same unit.

Now, there is another important thing to keep in mind. And this is about keeping the eyepiece-length limited to its maximum length capacity. 

There is a thing called Newtonian scope to help you out. Simply multiply your telescope’s focal ratio by seven and you will find your largest or maximum eyepiece-length.

So, note down the eyepiece-length of your telescope and convert the unit according to the unit that you had used for your focal length.

Calculating Magnification 

The idea behind calculating a telescope’s magnification is very simple. All you need to do is to divide your telescope’s focal length by its eyepiece-length.

So, now that you have found out both of the parameters, let’s begin calculating the magnification.

As you know, magnification does not have any unit. So, keep the units of these parameters exactly the same so that you don’t have to face any sort of confusion afterward.

Let us take an example where the focal length of your telescope is 800 mm. Again, the eyepiece-length is given 10 mm. Therefore, the magnification will be,

Focal Length / Eyepiece-Length

= 800/10

= 80

Therefore, you got your magnification as 80x. While dealing with magnification, it is better to follow the traditional thumb rule. The rule provides a 50x for each inch of aperture. Confused? 

Well, let me explain it this way. Suppose that you have a telescope of 6 inches. So, according to the rule of thumb, the magnification would be multiplied by 50x which gives you a result of 300x.

However, increasing the magnification does not necessarily make your observation better. Therefore, always remember that you should always start with the least magnification. 


So now, have any issue that how does telescope magnification work? If yes, then I must say the answer is no longer any mystery for you.

Although it is not a must to learn all those related information, I highly recommend you do so. This way, you can have a clearer concept about the method.

Of course, there are factors that are more important than magnification. Therefore, check the manual along with the packaging very carefully so that you don’t miss any detail.

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