Are you in the market for a new telescope but need help deciding which one to choose? One crucial factor to consider is the focal ratio of the telescope, also known as the f-number. But what is a good focal ratio for a telescope, and how does it affect your viewing experience? In this article, I’ll cover the complexities of focal ratio and help you determine the best option for your needs.
A telescope’s focal ratio is a measure of how much light it gathers and how wide a field of view it has. The ratio of the focal length of the objective lens or primary mirror to the aperture determines it. A smaller focal ratio, also known as a faster f-number, results in a wider field of view and a brighter image, but with less detail and resolution. A larger focal ratio, or a slower f-number, has a narrower field of view but provides more detailed and higher-resolution images.
In this article, you get
A detailed look at what is the focal ratio on a telescope
A look at the different use cases for a slow vs fast telescope
To explore the best focal ratio for planets
A look at how to focal ratio affects the magnification of your scope
By the end of this article, you’ll be able to choose the best focal ratio for your next adventure under the night sky.
Let’s dive right in.
WHAT IS A GOOD FOCAL RATIO FOR A TELESCOPE?
A “good” focal ratio for a telescope depends on your specific needs and preferences.
You may prefer a lower focal ratio or faster f-number because it results in a wider apparent field of view and a brighter image. This can be especially useful for observing faint deep sky objects like nebulae and galaxies.
On the other hand, you may prefer a longer focal ratio, or slower f-number, because it provides more detailed and higher-resolution images. This can be especially useful for observing bright objects with fine detail, such as the planets and their moons.
Generally, a focal ratio of around f/8 to f/10 is considered a good balance between brightness and detail for most types of observations. However, the optimal focal ratio will depend on your local conditions and the specific characteristics of the telescope.
DOES FOCAL RATIO MATTER IN A TELESCOPE?
Yes, the focal ratio matters, as it affects the amount of light the telescope can gather and the level of detail it can resolve.
It may be helpful to try out different f-values to see which one works best for your needs.
DO YOU WANT A HIGH OR LOW F-VALUE?
Here are some general considerations to help you determine whether you want a high or low f-value:
Suppose you are interested in observing objects with fine detail, such as the planets and their moons. In that case, you may prefer a higher f-value (>f/10), or slower f-number.
A higher f-value will have a narrower field of view but provides more detailed and higher-resolution images. This can be especially useful for seeing subtle features on these objects.
Suppose you are interested in observing objects with low surface brightness, such as nebulae and galaxies. In that case, you may prefer a lower f-value (<f/8), or faster f-number.
A lower f-value will result in a wider field of view and a brighter image, making it easier to see these objects.
Suppose you are interested in a wide range of objects and want to be able to see both faint and detailed objects. In that case, choose a telescope with a moderate f-value, such as around f/8 to f/10.
This will provide a good balance between brightness and detail for most types of observations.
IS A HIGHER F RATIO BETTER?
Yes, a higher f ratio will provide you with better results for observation in the solar system, like for planets and the moon.
HOW TO CALCULATE THE FOCAL RATIO OF A TELESCOPE
Before diving any deeper into what is a good focal ratio, I’d like to go back to basics and then build up to where you can calculate the focal ratio.
FOCAL RATIO MEANING
A telescope’s focal ratio is a gauge for its light-gathering capacity and field of view. It is determined by the ratio of the telescope’s focal length to its aperture diameter.
It is commonly stated as an f-number, such as f/8 or f/10.
A lower focal ratio or faster f-number produces a larger field of vision and a brighter image. While having a smaller field of view, a greater focal ratio or slower f-number produces richer images in detail and resolution. However, the image has less clarity and resolution.
THE FOCAL LENGTH OF A TELESCOPE
The focal length of a telescope tube refers to the distance between the objective lens or mirror and the eyepiece. A longer focal length results in higher magnification and a narrower field of view. A shorter focal length results in a lower magnification and a wider field of view.
For more info on magnification, read my article on how Barlow lenses work.
WHAT IS A GOOD FOCAL LENGTH FOR A TELESCOPE?
Generally, a focal length of around 600-1000mm is considered a good balance for most types of observations. However, the optimal focal length will depend on your specific needs. I’ve listed here two examples for you to choose from:
For wide-field views of your deep sky object of choice, such as the Milky Way or large nebulae, a short focal length, such as around 300-500mm, may be preferred.
This will provide a wider field of view and a brighter image, making it easier to see faint objects.
For detailed views of objects such as the planets, a long focal length, around 800-1200mm, may be preferred.
This will provide high magnification and a narrow field of view in the eyepiece, allowing you to see subtle details on these objects.
The aperture of a telescope refers to the diameter of the main lens or mirror, which determines how much light the telescope can gather. The aperture of a telescope is typically in millimeters or inches.
It is an important consideration when choosing a telescope.
A larger aperture allows the telescope to gather more light, making it easier to see faint objects and provide more detailed and higher-resolution images.
However, a larger aperture also requires a larger and heavier telescope, which can be more expensive and difficult to transport.
THE FOCAL RATIO TELESCOPE CALCULATOR
The focal ratio telescope calculator is straightforward and can be a helpful tool for determining the optimal focal ratio for a given telescope.
To use the calculator, you need to input the focal length and aperture of the telescope, and the calculator will calculate the focal ratio based on this information.
Alternatively, you can calculate it by dividing the focal length by the aperture.
For example, suppose the focal length of a telescope is 1200mm, and the aperture is 120mm.
In that case, the focal ratio would be as follows:
Focal ratio = 1200mm / 120mm = 10
The focal ratio is an f-number, such as f/8 or f/10, rather than a decimal value. To express the focal ratio as an f-number, you can divide the focal length by the aperture and round the result to the nearest whole number.
Once you have calculated the focal ratio of a telescope, you can use this information to determine the appropriate eyepiece and accessories for the telescope and to compare the performance of different telescopes.
FOCAL RATIO FOR PLANETARY IMAGING
WHAT IS A GOOD FOCAL RATIO FOR PLANETS?
A higher focal ratio, or slower f-number, is generally suitable for observing planets because it provides more detailed and higher-resolution images.
This is because a fast focal ratio results in a narrower field of view and higher magnification, which can help to reveal subtle features on the surface of the planets.
For example, a focal ratio of around f/10 to f/15 is a good choice for observing the planets.
THE BEST FOCAL RATIO FOR PLANETS
Suppose you are interested in very high magnification and are willing to trade off some field of view and brightness. In that case, consider a higher f-number than f/15.
WHAT IS THE BEST F RATIO FOR ASTROPHOTOGRAPHY?
The best f-ratio for astrophotography depends on your preferences and the characteristics of the telescope and camera.
A lower f-ratio, or faster f-number, is better for astrophotography because it results in a wider field of view and a brighter image.
This can be especially useful for capturing large objects such as nebulae and galaxies, which may require longer exposure times to capture sufficient light.
The wider field of view and brighter image come at the cost of reduced magnification. If you are interested in capturing very detailed and high resolution images of objects such as the planets, you may want to consider a higher f-ratio by using a slow telescope.
However, a lower f-ratio can also result in less detailed and lower-resolution images.
Generally, a focal ratio of around f/5 to f/8 is a good choice for most types of astrophotography, as it provides a wide field of view and a relatively bright image while still allowing for some detail and resolution.
In addition to considering the focal ratio of a telescope, it is essential to consider other factors.
These include things like the aperture, focal length, and mount type, as these can all affect the performance and capabilities of the telescope.
It is also a good idea to consider your budget and any practical considerations, such as the telescope’s size and weight.
By considering these factors and researching, you can find the best telescope for your needs and get the most out of your observing experience.