The focal ratio is a metric used to determine a telescope’s brightness, magnification and field of view. This number, often referred to as the ‘f-number’, describes the relationship between focal length and aperture size.

## How to Calculate Focal Ratios in Telescopes?

The focal ratio can be found by dividing the focal length by the diameter of the aperture. For example, a telescope may have a 100 mm aperture and a focal length of 1200 mm. By dividing 1200 by 100, we find a focal ratio of 12, commonly expressed as ‘f/12’.

The formula for focal ratio is: Focal Ratio = (Focal Length) / (Aperture)

You can also use a focal ratio calculator to do the math.

## Which Focal Ratio Should You Use for Astronomy?

Different focal ratios are beneficial for different viewing purposes. For example, a long focal ratio gives a higher magnification, but it also provides a narrower field of view. Focal ratios greater than f/6 are ideal for observing celestial bodies like moons and planets.

On the other hand, a lower focal ratio is best to see full views of star clusters and galaxies. This is because they have less magnification, but they give you a broader and brighter view of the sky. In astrophotography, focal ratios under f/6 are typically used to maximize the amount of light in an image.

## How does Focal Ratio in Telescopes Affect Light-Gathering?

Low focal ratios allow for telescopes to gather more light and produce brighter images. For example, a telescope with focal ratio of f/5 will show an image of four times the brightness as a telescope with focal ratio of f/10, all other things being equal.

## How does Focal Ratio in Telescopes Affect Magnification?

A high focal ratio implies higher magnification and a narrower field of view with a given eyepiece, which is great for observing the moon and planets and double stars. For such objects, a focal ratio of f/10 or more is ideal. Low focal ratios have less magnification and wider fields of view.

## Advantages of Low Focal Ratios in Telescopes

A low focal ratio means that the light from a distant object enters the telescope faster and with a wider angle, which can be useful for observing fast-moving objects like comets, planets, or the moon. They also allow a telescope to gather more light in a shorter amount of time, making it easier to observe distant celestial bodies in detail.

## Disadvantages of Low Focal Ratios in Telescopes

Telescopes with lower focal ratios don’t provide as strong of magnification, making it difficult to view distant objects in detail. They are also more susceptible to aberrations such as coma and field curvature because light enters at a wider angle.

## Advantages of High Focal Ratios in Telescopes

A high focal ratio yields a higher magnification and smaller field of view, making it great for observing celestial objects like the moon and planets. Telescopes with higher focal ratios are also easier to focus as it allows for a narrower beam of light.

## Disadvantages of High Focal Ratios in Telescopes

Telescopes with higher focal ratios have longer exposure times and a narrower field of view, causing object enters the telescope more slowly. This can be problematic when following fast-moving objects or making wide-view observations. Because light travels a longer distance to the eyepiece, chromatic aberration is typically more pronounced than in telescopes with low focal ratios.

## Which Telescopes Have a Low Focal Ratio?

There are many telescopes designed with low focal ratios, allowing for a wide view of the night sky. While it’s also important to consider your budget, experience and goals, knowing a telescope’s focal ratio is a great way to understand its observational ability.

- Orion SkyQuest XT8 has a focal ratio of f/5.9
- Sky-Watcher Collapsible Dobsonian has a focal ratio of f/4.7
- William Optics ZenithStar 73 has a focal ratio of f/5.9

## Which Telescopes Have a High Focal Ratio?

There are many telescopes with high focal ratios, allowing for a detailed view of the cosmos. Below are three powerful telescopes, from reputable manufacturers, that will provide a deep view of space.

- Orion 10022 StarMax 90mm TableTop has a focal ratio of 13.9
- Meade LS-8 ACF has a focal ratio of f/10
- Celestron EdgeHD 9.25 has a focal ratio of f/10