Dobsonian vs Newtonian – whats the difference?

When researching different telescope options, you may come across some seemingly odd terminology such as “Newt” or “Dob.” 

This is understandably confusing, but the Dobsonian vs. Newtonian conundrum is something that can be easily explained.

Newtonian and Dobsonian telescopes do not necessarily refer to two different types of telescopes. Instead, Dobsonian telescopes are often Newtonian telescopes sitting on a specific base designed by John Dobson in 1965.

The Origin of Newtonian Telescopes

The Newtonian telescope, invented by Sir Isaac Newton in 1668, is a reflecting telescope that typically uses a concave mirror and a flat diagonal mirror.

Newton’s desire to prove that white light is composed of a spectrum of colors is what pushed him to invent this telescope.

Before his invention, refracting telescopes that existed couldn’t be used in his experiments as they suffered from color distortion.

The lens of the refracting telescopes acted as prisms, breaking light into seven colors of the rainbow. Newton knew that this problem could be solved by using a telescope that did not utilize a lens, the reflecting telescope.

Dobson’s Spin on Traditional Newtonians

Using a bit of ingenuity, John Dobson took the Newtonian telescope and made it affordable and accessible to everyday consumers. There were four main ways that he achieved this goal

The Altazimuth Mount

Traditional Newtonian telescopes used more complex mounts that were huge and expensive to construct.

In addition to being cheaper, the Dobsonian altazimuth base is also more portable than equatorial mounts.

Furthermore, the simplicity of the Dobsonian base resulted in an easy manufacturing process from readily available components.

Thin Mirrors

John Dobson used mirrors made out of the glass from surplus ship portholes with a thickness ratio of 1:16 (1 cm thick for every 16 cm in diameter).

He used these rather than the Pyrex mirror blanks with a ratio of 1:6, which were more expensive. In addition, the altazimuth mount on the Dobsonian telescope provided even support to the thin mirrors, preventing them from sagging and flexing.

The potential for this was the main reason for using thick glasses in traditional Newtonian telescopes.

Construction Tubes

Instead of using aluminum or fiberglass telescope tubes, Dobson opted to use thick compressed paper tubes (sonotubes) typically used on construction sites.

He then painted the compressed paper tubes or coated them with plastic to prevent them from being affected by moisture.

Furthermore, the sonotubes were more durable during transportation, as fiberglass and aluminum were prone to dents and shattering from impacts during transport.

A square mirror box

A plywood box was used for the tube base and mirror housing, into which the compressed paper tube was inserted. This gave a rigid flat surface to attach the mirror supports and made it easy to attach the trunnions.

Dobsonian telescopes have since evolved with different designs; however, many still follow the four main concepts above, making them portable and affordable to amateur astronomers.

Differences in Use

Traditional Newtonian telescopes do not have any built-in mechanisms for attaching them to a mount. Instead, you would have to put tube rings on the telescope itself and then mount a dovetail bar onto the tube rings.

Once you’ve done this, you must choose what type of mount you want to use for viewing. 

Most telescopes you see on Amazon or random pages around the web sit on a tripod. Unfortunately, those tripods are often horribly shaky and provide a terrible viewing experience for the user.

That shakiness would be amplified 10 fold if you tried to put a sizeable Newtonian telescope on a cheap consumer tripod.

Dobsonian telescopes are a little different than traditional Newtonians since the tubes of the telescopes have trunnions on each side. These allow it to sit securely in the altazimuth Dobsonian base.

Once the telescope is placed into the base, it’s secured by two long screws on either side.

These screws have handles attached, allowing the user to do all of this by hand, with no special tools. They can also be tightened down to keep the telescope from moving once you have acquired your visual target.

The altazimuth base supports the telescope as it rotates on two perpendicular axes, vertical and horizontal.

This vertical and horizontal movement gives the Dobsonian telescope the ability to point to any astronomical object in the sky.

Newtonian Telescopes for astrophotography

If you are considering using a Dobsonian telescope for astrophotography, I would advise only using it for planetary imaging.

Because Dobsonians are designed for visual purposes, their focal points are different than imaging Newtonians.

This means that you will likely have difficulty focusing a DSLR camera with a Dobsonian telescope because the camera’s sensor will be too far away from the secondary mirror.

One way around this is to mod the Dobsonian into an imaging Newtonian, where the modifications will compensate for the shallower focal point.

This can be done by either moving the primary forward or replacing the focuser with the shallowest one that you can find.

The other way to change the focal point is by using a barlow lens. Using a barlow will work well for planetary imaging, but again is not something that you want to rely on for deep space astrophotography.

The main reason for this is the focal length of the telescope. I recommend a focal length of around 450mm or less for anyone just starting DSO astrophotography.

Reduce this to 300mm maximum if you start with a star tracker rather than a proper equatorial mount.

You can expect to take some reasonably long sub-exposures at these focal lengths on an equatorial mount and maintain round stars, even if you are not guiding.

We are not tracking the sky on an equatorial mount with a Dobsonian, and the focal length is much longer.

An 8-inch Dobsonian is a fairly common entry-level telescope for most stargazers. The focal length of these telescopes is already 1200mm. If you add a 2x barlow, you are doubling that focal length to 2400 mm.

The 500 rule is the most basic exposure rule in astrophotography. It gives you a rough idea of how long you can expose at a given focal length before star trails become noticeable.

The calculation is simple, it’s 500 / The focal length. If you are photographing the Milky way at 14mm with an APS-C DSLR, you can take images of at least 20 seconds without star trails.

Try to do the same thing with the 8-inch dob and a 2x barlow, and that exposure time drops to .13 seconds.

As you can imagine, sub-exposures this short will not work well for long-exposure astrophotography, which is what is required to image DSOs.

If you would like to use a Newtonian telescope for deep space astrophotography, look for ones specifically designed for imaging.

These are the “ds” models in the Skywatcher lineup of telescopes. An example of this would be the 130p-ds.

With a 650 mm focal length and a price of $260, it’s one of the best values out there for an imaging telescope.

Conclusion

In a nutshell, all Dobsonian telescopes are Newtonians, but not all Newtonians are Dobsonians.

Dobsonian telescopes are great for the amateur stargazer because the included altazimuth base makes them exceptionally user-friendly. 

Dobsonian telescopes are the best bang for your buck when it comes to visual astronomy. Newtonians can also be used for astrophotography, but Dobsonians are only recommended for planetary astrophotography.

The position of the primary mirror can make them difficult to focus without a barlow, which increases the focal length past the point of being usable for deep space astrophotography.

If deep space imaging is your goal, make sure to get an equatorial mount and a dedicated imaging Newtonian.

These often have shorter tubes, and the distance between the primary and secondary mirrors allows for prime focus astrophotography without the need for a barlow.