Dobsonians vs Newtonian Telescopes: What’s the Difference?

A History of Newtonians and Dobsonians

After the telescope was invented in approximately 1607 by a Dutch optician and then refined by Galileo and Kepler, there was a major issue with these early instruments. They used simple, crudely manufactured lenses and suffered from major optical defects, chiefly chromatic aberration.

When a simple lens is used to focus light, it behaves as a weak prism, splitting up the white light reflected off of the moon and planets and generated by stars into their constituent colors, resulting in a blurry orange or blue fringe on objects. Early refractor telescopes suffered immensely from this, and the only cure was to make extremely long telescopes with very slight curvature on the lenses and to stop down the aperture by blocking the majority of its area.

In order to build telescopes longer than a couple inches in aperture, enormous aerial telescopes were built, suspending the objective lenses on poles and observing with an eyepiece held taught at the end of a tether. To prevent false color fringing from ruining the images, the focal length had to be exponentially longer when the aperture was increased.

As early as the 1630s and certainly by the 1660s, opticians and mathematicians had also become interested in whether a mirror could be used to focus light instead of a lens. James Gregory, a mathematician, came up with a plan for a telescope that used two curved mirrors—a large concave primary mirror in the back with a precise parabolic curvature and a smaller concave secondary mirror at the front with a precise elliptical curvature—and a glass lens at the back to examine the image. This Gregorian telescope wouldn’t be built until 1673, by Robert Hooke.

Before this first design for a reflecting telescope could be built, Isaac Newton was working on a problem of optics. He was trying to understand how light could be made up of different colors and how it could be split apart by glass lenses and prisms. He devised an experimental apparatus to test whether mirrors could get around the chromatic aberration problem, and so he built his design for a reflector telescope.

His first reflector telescope, built in 1668, was a ball-mounted telescope with about a 1.3-inch aperture, a spherical primary mirror made of speculum metal, a flat diagonal secondary mirror in front, and a 5mm eyepiece in the front side. This would be the first reflecting telescope ever built, and as crude as it was, it was used by Newton to examine Sirius at ‘high’ power and confirm that his reflector didn’t show any false color. This was used as part of his evidence that white light was composed of multiple colors—as a sort of counterexample.

Reflectors (mostly Gregorians) were gradually adopted by some astronomers because they could be much more compact than refractors and had no false color. However, they would be much dimmer, since two reflections on 60% reflective speculum metal resulted in a 30% reduction in image brightness; hence, they were only useful for looking at bright objects.

Newton’s design, however, was seldom used. It had a spherical mirror, so it still suffered from spherical aberration. Newton was also quite secretive with his manufacturing methods. It wasn’t until John Hadley reinvented the design in 1721 that it began to be taken seriously. Hadley actually documented how he designed and figured his telescope, so other telescope makers could replicate the design. His six-inch reflector was probably the sharpest reflector telescope in the world when it was built, since he used a star test method to precisely figure the mirror into a paraboloid.

Achromatic refractors, which controlled the false color fringing, had begun to be built by the 1750s, changing the game and giving refractors an edge again. The two-curved-mirror Gregorian design would eventually be superseded by the Cassegrain, where the concave ellipsoid secondary mirror was replaced by a convex hyperboloid, which improved the field of view and collimation tolerances.

Over the 18th and 19th centuries, Newtonians using Hadley’s construction methods would become more and more popular. Even with their severe light absorption problems, large-aperture reflectors could be made much larger than lenses, which would droop when made about a meter or larger, as they could only be suspended by their edges and not their entire backs. When a technique to coat a glass mirror in silver was developed in 1857, Newtonians became much more powerful, as their light transmission compared favorably to refractors. In addition, the mirror would not have to be refigured into a precise optical shape when it tarnished; it would just have to be stripped and recoated. (Nowadays glass mirrors are coated in non-tarnishing aluminum using industrial vacuum chambers)

Newtonians remained the easiest telescopes to build and could easily reach much larger sizes. By the 1950s, amateur astronomers seemed to settle on either a very nice refractor or a 6 to 8 inch newtonian, invariably attached to a complex equatorial mount to allow it to track the sky (or even do film photography with a guide scope and slow motion cables), as the classic option.

From the 1940s to the 1960s, a Vedantic monk in California, John Dobson, became interested in reconciling astronomy and his faith and showing people the universe. He built telescopes out of essentially garbage—salvaged ship portholes for the mirror blanks, wood and cardboard concrete form tubes for the structure, and thrift-store binoculars were dismantled for their eyepieces. When figuring the mirror, Dobson essentially used the exact same method used by John Hadley two centuries prior and used star tests to precisely parabolize the mirror.

After leaving the monastery, John Dobson founded the San Francisco Sidewalk Astronomers and popularized what he called a “sidewalk telescope.” It was, to paraphrase, “a tube on a box, held together by gravity, and powered by yogurt and eggs.” Dobson used the simplest method possible to aim the telescope above the horizon: a wooden rockerbox that goes up, down, and all around. (He claimed he wasn’t smart enough to build a mount that could track.) The result was a telescope that was quick and easy to construct, yet still extremely powerful.

Apertura DT6
A Dobsonian on field

Dobson never really liked calling it the “Dobsonian” telescope—it was a term applied by others. He insisted that he didn’t make anything new. “Dobsonian revolution this and that,” he would say during his speeches—he had lots of ‘schticks’ that he would often repeat in his lectures—”all the world’s revolutions were fought with cannons on ‘Dobsonian’ mounts.”

Indeed, Dobson’s contribution wasn’t so much that he’d invented anything—though most telescope mounts have historically been complicated assemblies, whether they were altitude-azimuth or equatorial, there had been amateur telescopes made with similar simple mounts, and certainly the concept of a rocker mount was nothing new. And the telescope was merely a junky version of a concept from over two centuries prior. Dobson’s contribution was that he popularized these ideas and showed that you didn’t need a complex, expensive equatorial mount—you could focus on the optical quality and let that be that.

The first commercial Dobsonians appeared in the 1980s with Coulter optics. They had various sizes, from 8 inches and up, and were essentially the platonic ideal of a Dobsonian. Wooden rocker, belt sling, cardboard tube, and good optics. They could have been made by anyone, but they were made by Coulter, so you didn’t have to.

As the Dobsonian concept spread through amateur telescope-making circles, some telescope makers made alterations to make more complicated designs and build in ‘premium’ features. The truss tube Dobsonian allowed for a more compact type of telescope that could be taken apart and put back together for easier transportation (Dobson’s 24 incher lived in a dedicated trailer, but large-aperture truss dobs could fit in a pickup truck or perhaps even a car trunk). In 1989, Obsession Telescopes introduced their premium Dobsonians—large aperture, high-quality telescopes—which are still around today.

Nowadays, the modern commercial Dobsonian telescope imported from China or Taiwan is something of a balance between those two extremes. The telescopes are manufactured with steel or aluminum tubes, mirror cells that properly hold the mirror and allow for collimation, decent to excellent optical quality, particle board mounts that the user assembles themselves from a flat-packed box, and machined (or sometimes injection-molded plastic) crayford or rack-and-pinion focusers. The modern Dobsonian is inexpensive but of high mechanical and optical quality, with a simple, inexpensive, and sturdy rockerbox mount. It is these features that allow for Dobsonians to be the best “bang for your buck” in the majority of situations. Even as telescope prices rise, the Dobsonians we have today are only about the same price as those you could get from Coulter in the 1980s, NOT accounting for inflation.

What’s the difference between a Dobsonian and a Newtonian?

This isn’t exactly the right question, but it’s a frequently asked one. There isn’t a difference exactly; it’s more like a Dobsonian is a subset of Newtonian telescopes, or a Newtonian telescope is a part of a Dobsonian.

A Newtonian telescope is a reflector telescope with a curved primary mirror in the back, a flat diagonal mirror in the front, and a focuser to use an eyepiece or camera in the side of the front. It is the cheapest type of telescope to manufacture, mostly because it requires only one precisely figured surface—the parabolic primary mirror.

A Dobsonian telescope is a Newtonian telescope on a simple, sturdy, easy-to-use rockerbox mount that goes up, down, and all around. A Dobsonian mount is the cheapest way to mount an optical tube, but because it sits on the ground, it’s unsuitable for refractors and cassegrains, which have their eyepieces in the back, so aside from a few rare tabletop models, Dobsonians are almost always Newtonians. A Dobsonian mount is an especially cheap and simple form of an altitude-azimuth mount.

Should I get a Dobsonian or another Newtonian?

The difference between a Dobsonian and a Newtonian is purely in the mount, so what types of mounts are there? There are German equatorial mounts, which allow a single axis of motion to be used to track the sky against the rotation of the Earth, and fork-arm altitude-azimuth mounts. Most non-Dobsonian mounts have their own tripod. 

Altitude-Azimuth mounts are as simple to use as Dobsonians (Dobsonians are a subset of Altaz telescopes). Typically, you’ll see Altaz mounts on smaller telescopes. A short-focal-length 4.5”, 5”, or 6” Newtonian can work well on certain kinds of altaz mounts. I often use my Zhumell Z130, originally a tabletop dobsonian, on a Vixen Porta mount, which is sturdy, not too heavy (though heavier than the original), and has a tripod built in, meaning I don’t have to worry about finding a sturdy table at just the right height.

Some small Newtonians can work fine on a sturdy photo tripod-style altaz mount, though there are balance concerns when pointing such a mount far above the horizon. 

However, the flimsy fork-of-doom Altaz mounts are a no-go. These can be identified by having a “slow motion bar” attached from the mount to the telescope next to the eyepiece to slightly sturdy it up. These are signs of a hobbykiller-type mount, which is so shaky as to be virtually impossible to use except at low magnification. You will usually only see this type of mount on 3” Newtonians or maybe a 4.5” short tube, often with a spherical mirror.

Equatorial mounts are occasionally billed as an ergonomic/ease-of-use improvement—with just one axis of rotation, the telescope will track the rotation of the Earth. Unfortunately, GEM-mounted Newtonians are ergonomically frustrating. As the telescope is moved from one part of the sky to another, the eyepiece can end up in some weird orientations, necessitating a procedure where you lock down the mount, loosen the tube rings, rotate the telescope until the eyepiece and finderscope are in a usable orientation, make sure you haven’t lost your forward/backward balancing, tighten the tube rings, and readjust the mount. It is doable, but not fun to have to do it all night with a lot of objects to look for.

At one time, equatorial mounts were the only way astronomers could reliably find dim deep-sky objects. They would use setting circles to line up the telescope, rather than using a go-to computer, or using star maps, constellations, and asterisms to “star-hop” to the object as you would in a Dobsonian.

Nowadays, beginner-level GEM-mounted Newtonians are a joke. The telescopes are usually too heavy for the mount, so they’ll shake and wobble at high power or even lose their aim. Despite how fancy these mounts can seem, they often lead to disappointment and frustration, as a beginner will have problems understanding how even a high-quality equatorial mount works and balances, let alone one that doesn’t even work right. If they have setting circles, they’re usually decorative.

Expensive, high-quality equatorial-mounted Newtonians can be found, particularly using an EQ3/CG-4 mount. Equatorial mounts are also necessary when doing astrophotography (after all, the focuser orientation doesn’t matter with a camera.) 

Although there are some computerized push-to or go-to Dobsonians, most computerized mounted Newtonians are either on go-to German equatorial mounts, or on go-to single-arm altitude-azimuth fork mounts. Computerized GEM Newtonians can do astrophotography, but they still require tube rotation when doing visual work, and that might risk upsetting the computer alignment. Altaz go-to mounts are a mixed bag. They can’t be used for serious astrophotography, but they’re often sufficient for visual use. Some of them can’t be used manually at all and rely entirely on motors to move, which in my view is a dealbreaker—what if it breaks or the power goes out? Then the telescope will be bricked. Some altaz mounts are just as cheap and finicky as cheap beginner hobbykiller mounts, but with the added issue of the goto feature being imprecise and annoying. Choosing a computerized mount should be done on a case-by-case basis; read reviews.

Fundamentally, the problem with non-Dobsonian Newtonians is that tripods and good-quality mounts that are sturdy enough to hold the telescope without it shaking are expensive. Often, they’re about as expensive as the telescope itself, meaning that you’ll be paying double for a non-Dob Newtonian than you would for an equivalent Dobsonian.

Non-Dob Newtonians can certainly be fine instruments, and they have their niches, but Dobsonians remain the best bang for your buck in most cases, and they’re usually the most beginner-friendly and ergonomically friendly.

Leave a Comment