What is Focal Ratio on a Telescope? 5 Easy Tips

Chris Klein, Amateur Astronomy Advisor

By Chris Klein

Updated:

Suppose you’re interested in buying your first telescope. You may have come across the term “focal ratio” and wondered, “What is Focal Ratio on a Telescope?”

Focal ratio is the simple formula F/D, where F is the focal length of the telescope and D is the diameter of the telescope’s aperture.

The focal ratio is an important number to consider when choosing a telescope, as it can affect the performance and capabilities of the instrument. In this article, I’ll explain what focal ratio is and how it impacts a telescope’s ability to gather light, provide a wide field of view, and produce high-resolution images. I’ll also provide tips on choosing the proper focal ratio for your observing needs.

Whether you’re a beginner or an experienced astronomer, this article will help you understand the importance of focal ratio in telescopes.

In this article, you get

A thorough understanding of core concepts and definitions: Focal ratio telescope

A comparison of what is meant by fast vs slow telescopes

A how-to guide for choosing the proper telescope focal ratio

A comparison of fast vs slow telescopes for astrophotography vs visual

A look at how eyepieces affect the telescope f ratio and field of view

How to calculate focal ratio of a telescope

By the end of this article, you’ll understand focal ratio and how it impacts your choice of telescopes.

Let’s dive right in.

WHAT IS FOCAL RATIO ON A TELESCOPE? BASIC DEFINITIONS

Here are some basic definitions related to the term “focal ratio” that you should understand when learning about this topic:

FOCAL LENGTH

Focal length is the distance from the center of a lens or mirror to the focal point where it focuses light.

APERTURE

Aperture is the diameter of a telescope’s primary mirror or lens. The aperture size determines how much light the telescope can gather and how much detail and contrast can be seen in the image. Sometimes this is simply referred to as the diameter of a telescope.

FOCAL RATIO

The focal ratio, also known as the f-ratio, is the ratio of the telescope’s focal length to its aperture diameter. In other words, if you take the mm focal length of your telescope and divide it by the mm aperture, you get the f ratio, which is a dimensionless number.

FIELD OF VIEW

The field of view is the sky’s area visible through the eyepiece of a telescope.

MAGNIFICATION

Magnification is the degree to which an object appears larger when viewed through a telescope.

If you’re new to the world of telescopes, read my articles:

WHAT IS MEANT BY FAST VS SLOW SCOPES?

A fast telescope has a small focal ratio (low f-number, short focal ratio), typically f/4 or lower. A slow telescope has a large focal ratio (high f-number, long focal ratio), typically f/8 or higher.

An easy way to remember this is to think of a fast sports car. A sports car is fast because it is small and short.

On the other hand, a truck is large, long and slow. The truck is similar to a long focal length telescope with a small aperture (diameter). In this analogy, a truck’s f-number would be large (long and slow).

The term “fast” in this context refers to the ability of the telescope to gather light. A fast scope with a small focal ratio can gather more light in a shorter time than a slow telescope with a large focal ratio.

A fast telescope can produce a brighter image with less exposure time, which helps you observe faint objects such as galaxies and nebulae.

However, a fast telescope also has a narrower field of view and higher magnification, which may only be ideal for some types of observation.

On the other hand, a slow telescope has a wider field of view and lower magnification, making it better suited for wide-field observing and imaging.

The choice between a fast and slow telescope depends on your observing goals and the type of objects you want to observe.

A fast telescope may be more suitable for deep-sky observing and astrophotography.

In contrast, a slow telescope may be better for wide-field and planetary observations. To learn more about seeing planets, read my article Can You See Planets With A Telescope?

ARE FAST OR SLOW TELESCOPES BETTER FOR VISUAL ASTRONOMY?

The choice between a fast and slow telescope for visual astronomy depends on the type of objects you want to observe and your personal preferences.

A fast telescope with a small focal ratio and wide aperture can gather more light and produce brighter, higher-contrast images. This makes it well-suited for observing faint objects such as galaxies, nebulae, and star clusters, which can be challenging to see in a slow telescope. A fast telescope can also provide higher magnification, which can help study small, detailed features on planets and the Moon.

However, a fast telescope has a narrow field of view. It is less convenient for observing large celestial objects such as the Milky Way or the Andromeda Galaxy.

Some people prefer the wide field of view and low magnification of a slow telescope. In contrast, others may enjoy a fast telescope’s high magnification and bright images. Try out different telescopes and eyepieces to see which combination works best.

HOW DO I CHOOSE THE CORRECT FOCAL RATIO FOR MY NEEDS?

There are a few factors to consider when choosing the proper focal ratio for your needs:

  • Observing goals: Think about what type of objects you want to observe and what performance you need from your telescope.
  • Aperture: The aperture of the telescope is essential when choosing the proper focal ratio. A larger aperture can gather more light and produce brighter, higher-contrast images. Still, it may also require a longer focal length to achieve the desired magnification.
  • Magnification: Consider the range of magnifications you want to use for your observation. Short focal length telescopes with small focal ratios will provide lower magnification and a wider field of view. A long focal length and large focal ratio will provide higher magnification and a narrower field of view.
  • Portability: If you plan to transport your telescope to different observing locations, consider the size and weight of the instrument. A telescope with a shorter focal length and lower focal ratio may be more portable and easier to handle than a telescope with a longer focal length and higher f-number.

IS A FAST TELESCOPE BETTER FOR ASTROPHOTOGRAPHY?

Generally, a fast telescope with a small focal ratio is better suited for astrophotography than a slow telescope with a large focal ratio.

This is because a fast telescope can gather more light in a shorter time, producing brighter, higher-contrast images with less exposure time.

A small focal ratio also allows for a wider field of view, which can help capture galaxies and nebulae.

However, it’s important to note that there are other factors to consider when choosing a telescope for astrophotography, such as the size and quality of the optics, the mount stability, and the availability of accessories such as coma correctors and field flatteners.

A fast telescope may not necessarily be the best choice for all types of astrophotography. It may be necessary to use a slower telescope or a combination of telescopes to achieve the desired performance and field of view.

WHEN WOULD I NEED A LARGER FIELD OF VIEW?

A larger field of view can be helpful in several situations, including:

  • Observing or imaging large objects: A larger field of view allows you to see more of an object at once, which can help you observe larger objects like the Andromeda Galaxy.
  • Scanning the sky for deep space objects: A wider field of view can make it easier to locate objects in the sky, particularly if you are searching for faint objects.
  • Observing at low magnifications: A wider field of view can help you observe at low magnifications, which can be more comfortable and easier on the eyes.
  • Public outreach and education: A larger field of view can help demonstrate the scale and beauty of the night sky to a group of people, such as during a public outreach event or educational program.

HOW DOES THE F-NUMBER DETERMINE YOUR MAGNIFICATION?

The f-number does not directly determine the magnification of a telescope.

The telescope’s focal length determines the magnification of a telescope and the given eyepiece used.

The magnification is calculated by dividing the mm focal length of the telescope by the mm eyepiece focal length.

For example, use a telescope with a focal length of 1000mm and an eyepiece with a focal length of 25mm. The magnification will be 40x (1000/25 = 40).

MAGNIFICATION FORMULA

Magnification = Telescope focal length / Eyepiece focal length

The f-number of a telescope can affect the magnification indirectly by impacting the amount of light that the telescope can gather.

A telescope with a smaller focal ratio and wider aperture can gather more light, producing brighter, higher-contrast images with less exposure time. This can allow for higher magnifications to be used without losing image quality.

On the other hand, a telescope with a larger f-number and narrower aperture will gather less light and requires longer exposure times or less magnification to produce acceptable images.

In general, the f-number of a telescope is one of several factors to consider when choosing the right telescope for your needs.

WHEN WOULD I USE A FOCAL REDUCER?

A focal reducer is an optical device used to reduce a telescope’s focal length and focal ratio, which widens its field of view.

ASTROPHOTOGRAPHY

For deep space photography, a focal reducer is typically used to increase the field of view and reduce the exposure time required to capture an image. This can be particularly useful for imaging large objects such as galaxies and nebulae or for capturing wide-field images of the Milky Way or the Andromeda Galaxy.

VISUAL OBSERVATION

For visual observation, an increased field of view reduces the magnification of a telescope, making it helpful for observing wide-field objects, scanning the sky for faint or elusive objects, and increasing the comfort and ease of use of a telescope.

A focal reducer is a useful accessory in your observing kit, as it can allow you to customize the performance of your telescope to suit your specific needs and goals.

WHAT’S THE IMPORTANCE OF A TELESCOPE’S EYEPIECE?

The eyepiece of a telescope is a critical component used to magnify the image of an object being viewed through the telescope.

It consists of a lens or combination of lenses designed to focus the light collected by the telescope’s objective lens or mirror.

This focusing action increases the apparent size of the image and allows the observer to see more detail.

There are many different types of eyepieces. Some eyepieces provide a wide field of view. In contrast, others provide a higher level of magnification.

In general, the eyepiece is an integral part of the telescope system. It is crucial in determining the final image observed through the telescope.

HOW DO DIFFERENT EYEPIECES AFFECT THE FOCAL RATIO OF A TELESCOPE?

The eyepiece of a telescope does not directly affect the focal ratio of the telescope itself. Still, it can have an indirect effect on the overall performance of the telescope. This is because the eyepiece is used to magnify the image formed by the objective lens or mirror, and the eyepiece’s choice can affect the system’s overall magnification.

For example, using an eyepiece with a shorter focal length will typically result in a higher image magnification, while using an eyepiece with a longer focal length will typically result in a lower magnification.

The overall effect on the system’s magnification will depend on the specific eyepiece being used and the telescope’s focal length.

It is also worth noting that the choice of the eyepiece can affect other aspects of the telescope’s performance, such as the field of view and the brightness of the image, which can, in turn, affect the overall quality of the observation.

WHAT IS A GOOD FOCAL RATIO FOR A TELESCOPE?

The ideal focal ratio for a telescope will depend on your specific needs and preferences.

Smaller focal ratios (f/4 to f/8) are generally considered good for wide-field observations, such as observing the Milky Way or large nebulae.

In comparison, larger focal ratios (f/10 and above) are typically better for higher magnification observations, such as planetary and lunar observations.

ARE THERE BENEFITS OF TELESCOPES HAVING A SMALL APERTURE AND LONG FOCAL LENGTH?

Telescopes with small apertures and long focal lengths can have some benefits in certain observing situations. However, there may be better choices in some cases. Some of the potential benefits include the following:

  • Cost: They tend to be less expensive than a larger telescope aperture since they require less material and are easier to manufacture.
  • Portability: They are generally smaller and lighter than telescopes with larger apertures, making them more portable and easier to transport.
  • Ease of use: They may be easier for some observers, especially those new to astronomy. They may be more forgiving of misalignment or other setup errors and require less precise focusing.
  • High magnification: They can be used to achieve high magnifications, which can help observe small, faint objects or study larger objects’ fine details.

However, it is essential to remember that telescopes with small apertures and long focal lengths can also have some limitations.

They may not perform as well as telescopes with larger apertures in low-light situations. Additionally, they may have a narrower field of view and a lower overall image quality level than telescopes with larger apertures.

CONCLUSION

The focal ratio of a given telescope refers to the ratio of the focal length to its aperture.

It is an essential factor to consider when choosing a telescope because it determines the telescope’s field of view and the size of the image it produces.

The focal ratio is the simple formula F/D, where F is the focal length and D is the aperture of the telescope.

Understanding the focal ratio can help you select a telescope that meets your observing needs and preferences.

For more information on some introductory telescope topics, read my articles

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About the Author

Chris Klein, Amateur Astronomy Advisor

Chris Klein is an amateur astronomy advisor, astrophotographer, and entrepreneur. Go here to read his incredible story "From $50,000 in Debt to Award-Winning Photographer Living in Switzerland". If you want to send Chris a quick message, then visit his contact page here.

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