What is Magnification in Telescopes?

Magnification describes the factor in which an image is magnified. As magnification increases, images will appear larger. As magnification decreases, the field of view will broaden and the image will become clearer.

How to Calculate Magnification in Telescopes?

The magnification of your telescope is determined by the focal and eyepiece lengths. To calculate this, divide your telescope’s focal length by the length of the eyepiece. For example, a telescope with a 1000mm focal length and 25mm eyepiece would have 40x magnification. Additional optical devices, such as Barlow Lenses can further increase this magnification. 

Magnification = Focal Length / Eyepiece Length

You can also use telescope magnification calculators to do the math.

What Magnification Should You Use in Telescopes?

The ideal magnification depends on your observational location and goal. A 50x magnification is sufficient for viewing objects like the moon or star clusters. This provides a clear, wide view, but isn’t very helpful when viewing extremely distant objects. In order to view more distant celestial bodies, you’ll need higher magnification. For example, you’ll need at least 100x magnification to view Jupiter or Saturn.

What is Maximum Useful Magnification in Telescopes?

Depending on the size of your aperture, your telescope will lose clarity beyond a certain level of magnification. This value is called the maximum useful magnification and can be found by multiplying your aperture size (mm) by 2x. For example, a 50mm aperture would yield a maximum useful magnification of 100x. However, it’s important to consider effects, such as atmospheric turbulence, that reduce the maximum useful magnification.

How to Increase Magnification in Telescopes?

Magnification is the product of your telescope’s focal length and eyepiece length. Therefore, to increase magnification, you must increase one of these factors. To increase the focal length of your telescope, you can use a Barlow lens. Barlow lenses are concave lenses used to increase the focal length, most commonly by a factor of 2x. To increase the length of your eyepiece, you can either purchase a larger eyepiece or combine or “nest” eyepieces. You can also increase your maximum useful magnification by using adaptive optics or lucky imaging to improve the image quality.

How does Field of View Relate to Magnification in Telescopes?

As your telescope’s magnification increases, the field of view decreases. This is because the image enlarges while the frame remains the same size. In astronomy, the true field defines the angle of the sky seen. To find the true field, you can divide the apparent field (listed with the eyepiece) by the magnification. By doing this, you can determine how much of the sky will be visible at a given magnification.

True Field = Apparent Field x Magnification

How does Atmospheric Turbulence Affect Magnification in Telescopes?

Atmospheric turbulence is caused by the presence of air currents with different temperatures and densities in the Earth’s atmosphere. These air currents cause light from distant objects to bend and become distorted as it passes through the atmosphere, leading to the phenomenon of atmospheric turbulence. It’s important to consider atmospheric turbulence when determining magnification as it reduces the maximum useful magnification. To overcome the atmosphere’s effect on observation, telescopes like the Hubble and James Webb Space Telescopes observe from outside the atmosphere. You can purchase adaptive optics to mitigate the effects of atmospheric turbulence on magnification.

When to Use High Magnification in Telescopes?

High magnification in telescopes is ideal when viewing details of small or distant objects. This includes moons, planets, star clusters, planetary nebulae, small galaxies and double stars. Advanced telescopes, like the James Webb Space Telescope, use magnifications of over 1000x to peer back billions of years in time.

When to Use Low Magnification in Telescopes?

Low magnification in telescopes is ideal when surveying large portions of the sky or capturing wide-field images of the sky. Wide-field images, such as open clusters and galaxies may appear too magnified to distinguish at high magnifications. For example, viewing Alcyon, a popular star system, at a magnification of 200x would only show you a portion of the image. Other low-magnification observations include The Beehive Cluster, the Pleiades and Hyades.