How Do You Calculate A Telescope?

While using a telescope is fairly straightforward on the whole, when it comes to the calculations you need to work out specific magnifications and distances, this can be quite confusing. 

If you are someone that is great at math and numbers, then this is something you will not likely struggle with. However, if you are not as confident with numbers, seeing all of the calculations may be quite daunting to you. 

However, when you break down all of the information you need to use to calculate a telescope, it is not as difficult as it first seems. In this article, to help you along, we have talked you through all of the calculations you will need to take into consideration to help make the process easier for you. 

How Do You Calculate A Telescope?

How A Telescope Works?

To get a better understanding of how to calculate a telescope, first you will need to have a good understanding of how a telescope works. This will then give you a better idea of why you are using the information in the calculations.

There are two aspects to a telescope - the eyepiece and the objective. The objective is typically a lense or a mirror that reflects and directs the light into the eyepiece. The eyepiece features a magnified lens that reacts to this light and allows you to see the images created more clearly. 

The objective uses all of the light from the aperture and then transfers it to the lens which is far smaller. As a result of this, the image that is seen through the telescope is magnified, and is also brighter thanks to the increased light that the objective has taken in, compared to the smaller lens. 

How Do You Calculate A Telescope?

When we refer to calculating a telescope, this is directly linked to the magnification equation of the telescope. This is the equation you will need to become accustomed to working out.

While this equation may seem quite confusing at first, it is fairly straightforward when you know what you need to look for. However, it is worth noting that you can use online magnification calculators that will calculate these numbers for you if it is something you are struggling with. 

Magnification Equation

The magnification equation for a telescope is: M = Fo / Fe

To break this equation down, here is what each aspect means:  M - M is the amount of magnification Fo - This is the amount of focal length of the objective, this is more commonly known as the telescope focal length)

Fe - This is the focal length of the eyepiece

All of these figures are fairly easy to find, and when you break down the equation, it is actually fairly easy to figure out. Once you have the magnification of the telescope, you will then be able to carry out other calculations based on this.

Typically, the larger the telescope and the objective are, the better and clearer the magnification will be. 

F-Ratio Calculation:

Here is how to work out the f-ratio calculation, which is useful to know that will help you to work out other aspects of the telescope's specifications:

F-Ratio: Fr  = Fo / Fd

The F-ratio is useful to know as this will help you to work out the ratio between the objective diameter, and the focal length. 

Some other things you will want to take into consideration is the Do which is the objective’s diameter, and the FOVe, which is the eyepiece field of view. These will help you to work out other aspects of the telescope.

Other Telescope Calculations:

  • Minimum Magnification: Mmin = Do / 7
  • Star Magnitude Limit:  Lm = 2 + 5*log (Do)
  • Surface Brightness: SB = 2 * Dep²
  • Exit pupil Diameter: Dep = Do / M
  • Field of View Scope: FOVs = FOVe / M
  • Resolving Power: Pr = 115.8” / Do

It is worth noting that when you are using these calculations, the calculations may vary ever so slightly because they are rough estimates, rather than being exact. They work on specific assumptions, rather than facts.

As a result of this, if there is a change in the lighting, or there are disturbances in general, this can affect how well a telescope can work and perform on the whole, which is worth keeping in mind.

Why Do you Need to Calculate A Telescope?

It is always useful to work out the calculations of your telescope because it will give you a better idea of how well the telescope performs. It will allow you to work things out such as how much magnification it has, and what the focal length is. 

When you are calculating your telescope, here are the things it will allow you to know the data for, linking to your telescope specifically:

  • Focal Length (omm)
  • Magnification (NaNx)
  • True Field of View (NaN degrees)
  • Exit Pupil (NaN mm)
  • Theoretical Resolving Power (Infinity arcseconds)
  • Approximate Limiting Magnitude if Telescope (+ NaN - dark, moonless skies)

Once you know all of this information, you will be able to have a better understanding of how well your telescope performs. It is worth noting that a lot of this information is not super necessary if you are someone that is just using a telescope as a light hobby.

However, if stargazing is something you want to get into on a bigger scale, it is worth knowing, especially if you are looking to purchase a more expensive and high-tech telescope. 


We hope that you have found this article useful. As you can see, there are a number of different calculations you will want to take into consideration when looking at your telescope.

While you do not need to work these calculations out, it is useful if you are looking to take star gazing more seriously. 

In addition to this, it will help you to have a better understanding of how well your telescope performs, too.

If the calculations are still too difficult for you to work out, alternatively, you can choose to use an online telescope calculator instead that can help you to work out these equations.

Gordon Watts