What Do Telescope Specifications Mean? (Simple Explanation)

The telescope is one of humanity’s most significant inventions, and ever since its creation in 1608, the telescope went through many changes and upgrades. But what do telescope specifications mean? We must keep up with their ever-evolving nature!

Exactly what your telescope’s specifications mean

Every telescope has five primary specifications that are critical to understand:

  • Aperture determines how much light it gathers
  • Focal length determines magnification
  • Focal ratio (the”f-number”) affects brightness
  • Magnification is how “powerfully” the telescope enlarges objects
  • Resolution is the amount of detail you can observe

There are usually model-specific specs as well, but the above are the most important and universal for choosing the right telescope for your needs.

Most of these are closely related to each other. For instance, focal ratio is simply the ratio of focal length to aperture, and magnification is the ratio of telescope focal length to eyepiece focal length.

But understanding what they mean for your stargazing experience isn’t as obvious. Unless you’re already into photography or other hobbies that revolve around optics, there are a few new concepts to understand. 

This article will help you do exactly that. Read on to learn exactly where these numbers come from and how to understand them as you search for a telescope. 


The aperture is an essential part of your telescope. It is responsible for what you can see through a telescope and how clearly you can see it.

The telescope’s aperture’s size is measured in inches or millimeters and influences your instrument’s “light grasp.” Light gathering is very important for clarity.

More light is collected through a larger aperture, and as a result, the greater the image resolution will be. For example, our eyes have an aperture of 7mm, but an average telescope with an aperture of 60mm can gather 70 times more light. 

You can use a telescope for many types of observations, but there are three major ones: 

  • Astrophotography: you will need at least 6 inches of aperture and a short focal length for this observation type. No satisfying pictures will be produced with less than that.
  • Planetary observation: Weather conditions influence this type of observation so that a large aperture may become a downside. However, an 8 inches aperture and some magnification power may do the job.
  • Deep-sky observation: for this one, you will need a bigger telescope, but it depends on the celestial object’s magnitude.

Many amateurs consider that a small aperture can show more than a larger one if the seeing is low, but in fact, all you will achieve if you mask your telescope is to dimmer your image.

Don’t let these three types of observations discourage you from deciding since some telescopes can be used for all of them. You just need to learn the exact settings! The next telescope specification which we will discuss is the focal length.

Focal Length 

The focal length is the distance traveled by the light inside a telescope, from its aperture (the entry point) to its focuser (the exit point) when the rays converge, making the image.

Basically, the longer the tube, the longer the focal length. But there are some exceptions like the Schmidt-Cassegrain telescope. (It has secondary mirrors that bounce the light before being sent to the focal point.)

This means that even though this telescope is shorter than a Newtonian one, its focal length is longer. Maybe the most useful thing about the focal length is that it determines the magnification and helps you make an idea about the field of view you can expect.

If you want to observe the moon and the planets, a longer focal length will be perfect. It will shorten the field of view, but the magnification will increase.

On the other hand, if you are passionate about astrophotography or just want to gaze at galaxies or other deep-sky objects, a shorter focal length telescope is ideal for you because it will provide a more extensive field view.

One of the more important specifications of a telescope is the focal ratio. Have you ever wondered what the focal ratio is? Let’s find out!

Focal Ratio

There is a relationship between the aperture and the focal length, and it is called the focal ratio, generally known as the f-number. By dividing the focal length with the aperture, you will find out the focal ratio.

For example, if you have a telescope with an aperture of 130mm and a focal length of 900mm, your instrument’s focal ratio will be f/6.92.

The focal ratio is an essential aspect to look into if you want to buy a telescope. This specification, like the focal length, tells you a lot about the telescope.

Larger f-numbers provide a narrower field of view and higher magnification.

If you are passionate about astrophotography, “fast” and “slow” are two fundamental terms you need to understand. You can capture images more quickly with a quick ratio scope (f/5 or below), but the slow ratio telescope offers a better focus.

The focal ratio influences the brightness of large objects or galaxies. A scope with an f/5 ratio will show an image almost four times brighter than an f/10 scope but half as large.

If you want to gaze at the moon or the planets, a telescope with a focal ratio of f/10 is perfect. A focal ratio of f/7 or even less is ideal for wide views as galaxies or star clusters.

A misconception about focal ratio is that many people think faster telescopes show brighter images and come from photography. This statement is true in photography. But for telescopes, equal power and apertures mean the same image brightness, regardless of the objective focal ratio.

Let’s find out what the magnification or the power of the telescope means.


The magnification is the “power” of your telescope. Each eyepiece has its focal length, and the magnification represents the relation between them.

To calculate your telescope’s “power,” you have to do a simple calculation: divide the focal length of the telescope over the focal length of the eyepiece. For example, if the focal length is 1,200mm and the eyepiece is 20mm, your telescope has 60x magnification.

It is essential to adjust your magnification because with more “power” comes less quality. So, in this case, unlike aperture, more isn’t necessarily better.

In most cases, the magnification limit is two times the aperture in millimeters, or 50x in inches. If you go beyond that, the image will be dimmer and fuzzy.

If you gaze at large objects, it will be better to adjust your magnification to low because you will not see the whole object. However, switching to a higher power is perfect for smaller objects and more significant details.

Many people consider that magnification is the most crucial aspect of a telescope, and it is understandable. But the truth is that it’s maybe the least.

More critical than magnification is the optical quality of the mirror or lens. A low-quality image magnified 500x is still a low-quality image, but a very big one.


The resolution, also known as the resolving power of a telescope, tells you how much detail you’ll see. Practically speaking, this might be the most important spec for most people.

The resolving power of a telescope is proportional to the aperture. For example, a 200mm telescope can resolve details as close as 0.58 arcseconds. An arcsecond represents 1/3600 of a degree.

Most of the time, the Earth’s atmosphere limits the practical resolution of any scope to 1″ or more because of its instabilities, depending on the telescope’s location. On top of mountains, telescopes actually have higher resolution because the light reaching the instrument travels through less air. (In case you wondered, that’s why observatories are often located at high elevations.)

Unfortunately, you can’t increase the resolution with a bigger light gathering aperture. The limit of 0.5 arcseconds is linked to a 12 inches (30 cm) aperture. Larger scopes will only gather more light.

The formula for resolution is not that complicated: divide the diameter of your instrument’s objective by the wavelength of the light being observed. You then multiply the result by 250,000.


A telescope can be a fantastic investment for most people. There are many types of telescopes on the market, but they have the same basic specifications. 

The aperture lets the light of the object enter the telescope.

The focal length determines the power of the telescope, which has the role of magnifying the object you gaze at. 

The focal ratio is the “speed” of your scope. 

Most specs are closely related, so it’s important to think of them as a system, not just standalone numbers. That’s key to choosing a telescope that will be fun and easy to use as you explore our stunning universe.