Telescope Magnification Limits: How Much Is Too Much?

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Written By: Zane Landers
Category: Learn

A common misconception in astronomy, especially 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. 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 will require a 6mm eyepiece

The Basics of Magnification

How It’s Calculated: The magnification of a telescope is determined by dividing the focal length of the telescope by the focal length of the eyepiece. For instance, if a telescope has a focal length of 1000mm and is used with an eyepiece of 10mm focal length, the magnification achieved is 100x.

Changing Magnification: By swapping out eyepieces with different focal lengths, you can adjust the magnification. So, in theory, with a small enough eyepiece, one can achieve extremely high 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, depending on the object observed and the viewing conditions.

What Limits the Telescope’s Magnification?

And 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. 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, and 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.

How Much Magnification Is Useful for Telescopes?

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 aperture, the highest useful magnification would be around 200x. However, this is a bit of a fuzzy definition. Pushing up to 60x or even 80x magnification per inch can be useful for splitting double stars and viewing planetary nebulae, even though it should theoretically not reveal anything more. The low stimulation of our retinas from these dimmer objects means that making them appear oversized may, in fact, help us see more detail, even if they appear fuzzy.

Telescopes aren’t all made equal, even at the same aperture sizes. While a world-class Takahashi refractor or a handmade Newtonian reflector can handle 50x or 60x per inch on the planets, a typical “good” commercial scope like 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 lower than this. It’s generally less than 40x per inch of aperture; 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) may not be able to provide a sharp image at even 10x per inch (often under 60x) in any circumstance, however.

Likewise, 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.

Understanding the interplay between the diffraction limit and the highest useful magnification is crucial for observing faint deep-sky objects like galaxies. For instance, 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.

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 it is not usually the prime selling point. Furthermore, as we’ve seen, the excessive magnifications (often 600x) advertised on the box of the small “hobby killer” telescopes advertised to beginners may literally not be of any use, 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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