What Can You Expect From A 100mm Telescope? (With Photos)

A 100 mm telescope is a great way to get started in astronomy. The focal length of a telescope refers to its size.

The larger the focal length, the longer the lens and the wider the field of view.

For example, a 100 mm aperture telescope has a much wider field of view compared to a 70 mm telescope.

What Can You Expect From A 100mm Telescope (With Photos)

You can see more detail in the sky with a larger aperture.

This means that you can spot celestial objects such as planets, faint stars, double stars, star clusters, galaxies, nebulae, and comets.

With a 100mm telescope, you can expect to experience awesome optical quality even for far-off objects.

In this article, we go through exactly how far these telescopes will take you.

What Exactly Is Aperture

Aperture is simply the diameter of the hole at the front of the telescope. It’s measured in millimeters or inches.

The bigger the number, the bigger the hole. The smallest aperture available today is 1 inch.

You can find out what aperture you need by looking up your desired magnification. Magnification is just another word for zoom.

When you look at the night sky, you’ll notice that the further away an object is, the dimmer it appears.

That’s because light travels faster when traveling farther. So if you have a small telescope, you won’t be able to see very distant objects.

To compensate for this effect, astronomers use a technique called “magnification”.

They magnify the image so that they can see things that are farther away. For instance, let’s say you’re using a 20x telescope.

If you were to look at Jupiter, which is over 500 million miles away, you would not be able to make out any details.

But if you used a 200x telescope, you could easily see Jupiter’s moons.

What Is The Maximum Magnitude Of A 100mm Telescope?

If you want to know what magnitude an object is, it’s basically a measure of brightness.

Astronomers use a scale called the magnitude system—the lower the number, the brighter the object.

The brightest object in our solar system is Venus. Its magnitude is -4.7.

The faintest object we’ve ever seen is Antares, which is also known as the red giant star. It has a magnitude of +6.2.

The maximum magnitude of a 100mm telescope is 13.6.

What Is Focal Ratio?

Focal ratio is a measurement of how sharp a telescope is. Sharpness is determined by the amount of light that falls into each pixel.

The higher the focal ratio, the sharper the images. There are three different focal ratios: f/5, f/10, and f/20.

These numbers refer to the distance between the center of the primary mirror and the edge of the secondary mirror.

Now, here’s where it gets tricky. Most people don’t realize that there are actually two types of focal ratios.

One type is the physical focal ratio. This is the actual distance between the center of one mirror and the other.

The second type is the apparent focal ratio. This is how big the image looks to you.

What Is Focal Length?

The focal length is defined as the distance from the telescope eyepiece to the last lens in a telescope.

It is essentially the length of the optical tubes. For example, if you have a 50mm telescope, then its focal length is 50mm.

A shorter focal length telescope will produce a larger field of view than a longer focal length telescope.

For instance, a 10-inch Dobsonian telescope with a 400mm focal length will give you a much wider field of view than a 12-inch Dobsonian telescope that has a 300mm focal length.

What Can You Expect To See Through A 100mm Telescope?

You might ask yourself, “Well, I’m going to buy a 100mm telescope, but what do I get?”.

Here’s a list of some of the most common things you can see through a 100mm telescope:

The Moon

The Moon

The Moon can be seen in a great deal of detail through a 100mm telescope.

You will be able to see the craters and textures on the lunar surface on a clear night.



As you can see from this picture, you can see mars more clearly through a 100mm telescope than a 70mm telescope.

It is possible to see details and shadows on the surface of mars on a clear night.

A cheap telescope will afford you less distinction in the image.



Although Venus is the closest planet to earth, it is not all that distinct through a 100mm telescope because it is very bright.

It appears more like a bright star than a detailed planet, and, as such, it is pretty tricky to locate.



Jupiter is the farthest planet that you will be able to see through a 100mm telescope.

It is visible, despite its incredibly far distance from earth, thanks to its distinct orange and brown bands and the way that it reflects light.

On a really clear night, you may even be able to detect some of its many moons!



Stars with a limiting magnitude that is under 13.6 will be visible through a 100mm telescope.

Constellations, star parties, and star fields that are not visible to the naked eye can be enjoyed through amateur telescopes.

Galaxies And Nebulas

Galaxies And Nebulas

Globular star clusters and galaxies like the Orion Nebula and The Milky Way are awesome targets for novice astronomers using 100mm telescopes.

What Can’t You See Through A 100mm Telescope?

There are several things that you cannot see through a 100mm scope. They include:

  • Neutron Stars
  • Black Holes
  • Planets smaller than Mercury
  • Mercury itself – due to its proximity to the sun.
  • Saturn and Uranus – due to their distance from earth
  • Pluto and other dwarf planets – due to their extreme distance and small size

If you’re interested in seeing these objects, you’ll need a bigger scope.

How Much Does A 100mm Telescope Cost?

Telescopes are available at many different prices. You can find telescopes ranging anywhere between $100 and thousands of dollars.

However, there are some things that you need to keep in mind before buying one.

Different Types Of Telescope

First, make sure that you know what type of telescope you want.

There are three types of telescopes: the refractor telescope, reflector telescopes, and catadioptric telescopes.

Refractor Telescope

Refractors have lenses made out of glass.

They are usually used by amateur astronomers because they are inexpensive and easy to maintain.

Reflector Telescopes

Reflectors have mirrors instead of lenses. They are generally used by professionals because they produce sharper images than refractors.

Catadioptric Telescopes

A catadioptric telescope combines both lenses and mirrors. They are the most expensive and complicated telescopes to build.

The Hubble telescope is a catadioptric telescope.

Types Of Mount

Second, choose the right mount. Mounts are the devices that hold the telescope onto your tripod.

Some mounts are designed for specific models of telescopes, while others are universal.

Universal Mounts

Universal mounts are easier to use and are the least expensive.

Computerized Mounts

Computerized mounts are more complex and hi-tech and can be programmed to pinpoint certain areas of the sky.

Equatorial Mount

Equatorial mounts allow you to point the telescope at any object in the sky.

Alt-azimuth Mounts

Alt-azimuth mounts are fixed so you must move the telescope to look at other objects.

Double Vs. Single Mirror Telescopes

Finally, determine if you want a single mirror or double mirror telescope. Single mirror telescopes have just one mirror.

Double mirror telescopes have a secondary mirror. The second mirror allows you to correct incoming light coming through the first mirror.

They tend to be more expensive than single mirror telescopes but result in a brighter image and better optical quality.

Final Thoughts

A 100mm telescope is a great mid-way telescope for stargazing enthusiasts.

It offers the clarity and range that will enable you to see distant stars and spot details on the moon and Mars.

It is cheaper and more portable than a more powerful / larger aperture telescope.

Gordon Watts