Telescope Magnification Limits: How Much Is Too Much?

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Written By: Zane Landers
Category: Learn

A common misconception I’ve noticed among newcomers is that more magnification with a telescope is better in some way. This is in spite of the fact that many celestial objects span as wide an area of sky as the Moon and are diffuse, meaning they are best viewed at lower powers to preserve brightness and detail.

Note: As the telescope’s magnification increases, the brightness of the image decreases. Pushing the magnification too high can make already faint objects nearly invisible.

Close-up of the Moon
Close-up of the Moon taken through the AstroMaster 114EQ with the Celestron NexImage 5 Solar System Imager. This magnified view requires a 6mm eyepiece

The Basics of Magnification

How Magnification Is Calculated:

Magnification of a telescope = focal length of the telescope / focal length of the eyepiece.

For instance, if a telescope has a focal length of 1500mm and is used with an eyepiece of 15mm focal length, the magnification achieved is 100x (1500/15).

So, in theory, with a small enough eyepiece (e.g., 4 mm), one can achieve extremely high magnifications (1500/4 = 375x).

Changing Eyepiece for Changing Magnification:

By using eyepieces with different focal lengths, you can attain different magnifications.

A device called a Barlow lens, inserted between the eyepiece and telescope, works by doubling or tripling the telescope’s focal length to achieve very high magnifications in lieu of a very short focal length eyepiece.

Most astronomers use at least three eyepieces and accessories like Barlow lenses, to achieve a wide variety of magnifications. The magnification they ultimately pick depends on the object observed and the viewing conditions.

How Much Magnification Is Useful for Telescopes?

While it’s tempting to zoom in closer to brighter and smaller celestial objects, such as the planets, there’s a limit to how much magnification can be usefully employed before the image starts to degrade.

A generally cited rule of thumb is that the highest useful magnification with a telescope is approximately 50x the telescope’s aperture in inches, or 2 times the aperture in millimeters. So, for a telescope with a 4-inch (i.e., 100mm) aperture, the highest useful magnification would be around 200x.

However, this is a bit of a fuzzy definition.

From my experience, pushing up to 60x or even 80x magnification per inch can sometimes be useful for splitting double stars and viewing planetary nebulae, even though I agree that theoretically it shouldn’t reveal anything more. I believe that the low stimulation of our retinas from these dimmer objects means that magnifying them may help us see more detail, even if they appear fuzzy.

The Quality of the Telescope Matters

Telescopes aren’t all made equal, even at the same aperture sizes.

A world-class Takahashi refractor or a handmade, great-quality Newtonian reflector can easily handle 50x or 60x per inch on the planets.

From what I’ve seen, a typical “good” commercial scope (a good Maksutov-Cassegrain, mid-range ED refractor, or a better sample of a commercial Dobsonian) is usually not able to provide a usefully sharp image at a magnification power as high as the stated 60x per inch. It’s generally less than 40x per inch of aperture. I’d say 35x per inch is a good rule for most telescopes for anything but planetary nebulae and some double stars, i.e., on the Moon, planets, and globular clusters.

Poor-quality telescopes (especially those coupled with bad eyepieces) don’t provide me with a sharp image at even 10x per inch (often under 60x magnification) in any circumstance.

A Factor to Consider: Atmosphere Seeing

Even if a telescope’s optics can handle a high magnification, the Earth’s atmosphere might not cooperate. On nights with poor “seeing” due to atmospheric turbulence, you may need to use much lower magnification to get clear views.

Once we get larger than about a 100-mm aperture, the atmosphere will typically be the limiting factor more than the telescope. This is the reason the scientific community put large telescopes in orbit—to get them above the atmosphere.

We are viewing the Moon, the stars, and the planets through 60 to 100 miles of air, which distorts the light coming to the telescope. The more turbulent the air, the less magnification we can apply. This is like looking through a glass bowl of boiling water. We call this atmospheric turbulence the “seeing” condition.

When the atmosphere is calm, we have a good seeing. When it is turbulent, we have bad seeing. There is nothing you can do about this.

Transparency is another factor that affects what we see. If there is a lot of air pollution, humidity, or dust in the atmosphere, it will scatter the light and reduce the magnification, which will give us the best view. Sometimes the sky looks clear, but we are actually looking through a thin layer of very high clouds. This sometimes reveals itself by showing a ring around the Moon.

Another Culprit: The Diffraction Limit

I’d warn you that we are entering physics territory here. Here is a quick and simple abstract for the most of you:

The diffraction limit is the smallest detail that a telescope can resolve due to the wave nature of light. A larger aperture will have a smaller (better) diffraction limit, allowing it to resolve finer details.

Understanding the interplay between the diffraction limit and the highest useful magnification is crucial for observing faint deep-sky objects like galaxies. While you might want to use high magnification to see finer details (approaching the diffraction limit), doing so might push the image brightness below a comfortable viewing level. Conversely, if you increase the brightness by reducing magnification, you might not be able to discern the finer details.

The diffraction limit is the smallest detail that a telescope can resolve due to the wave nature of light. When light waves pass through an aperture (like a telescope’s primary lens or mirror), they interfere and diffract, causing them to spread out of the wavefront. This phenomenon limits the telescope’s ability to distinguish between two closely spaced objects.

Different colors (or wavelengths) of light diffract differently. For instance, blue light has a shorter wavelength and will diffract less than red light. However, in practice, green light (around 550 nm) is typically used as a reference since our eyes are most sensitive to it. The differences in diffraction between red and blue light matter little; however, it is pronounced at longer wavelengths.

Infrared telescopes like the James Webb Space Telescope have slightly less resolving power than a similarly sized instrument at visual wavelengths. The problem only gets worse with radio telescopes; a giant dish the size of a football field has about the same resolving power at radio wavelengths as an ordinary 8” Dobsonian in visible light.

Regardless of the wavelength of the light involved, the diffraction limit is inversely proportional to a telescope’s aperture. A larger aperture will have a smaller (better) diffraction limit, allowing it to resolve finer details, assuming the larger instrument is under the same conditions and has the same or better quality optics as the small one.

I Recommend Staying Away from Telescopes Marketing Magnification

By now, it should go without saying: any telescope that markets itself predominantly based on magnification is likely a scam. Magnification is a variable factor determined by the combination of the telescope and eyepiece, and I’ve never noticed it to be the main selling point for any good telescopes.

Furthermore, as we’ve seen, the excessive magnifications (often 600x) advertised on the box of the small “hobby killer” telescopes advertised to beginners are literally not of any use. This is true even if the telescope is lucky enough to have good optics, eyepieces, and a sturdy enough mount—not that this is likely to be the case.

Focus on aperture and build/optical quality; our rankings and buyers’ guides have plenty of information to help you pick.

Zane Landers

An amateur astronomer and telescope maker from Connecticut who has been featured on TIME magazineNational GeographicLa Vanguardia, and Clarin, The Guardian, The Arizona Daily Star, and Astronomy Technology Today and had won the Stellafane 1st and 3rd place Junior Awards in the 2018 Convention. Zane has owned over 425 telescopes, of which around 400 he has actually gotten to take out under the stars. These range from the stuff we review on TelescopicWatch to homemade or antique telescopes; the oldest he has owned or worked on so far was an Emil Busch refractor made shortly before the outbreak of World War I. Many of these are telescopes that he repaired or built.

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