Dobsonian Telescope | Universe Today

Written by Tammy Plotner

Are you curious about what a dobsonian telescope is? What does the design entail? Who created it and what's its history? If you'd like to know more about how the dobsonian telescope originated, how it works and what modern dobsonian telescopes look like, follow along!

John Dobson and the San Franscisco SideWalk Astronomers

No one really takes credit for the simply rocker-box design that ended up being incorporated into a telescope, but no one disputes that John Dobson and the San Franscisco SideWalk Astronomers revolutionized the way the design impacted amateur astronomy. When this simple altazimuth mount appeared in the 1960s, it was a concept to allow the average individual to realize that a large aperture telescope could be made from just about anything. Up until that time, large reflector telescopes meant heavy mirrors and complicated mounts, but Mr. Dobson changed all that. Using thin mirrors made for salvaged portholes and telescope tubes made from concrete forms, these two simple items were easily combined with simple mirror cells and rocker box style mounts. The design soon became known as the "dobsonian".

Behind the Dobsonian Telescope Design

The principle is easy enough – a dobsonian telescope is nothing more than a Newtonian reflector on a self-styled altazimuth mount. The telescope itself consists of a primary mirror at the base which reflects light to a secondary mirror towards the top of the assembly. The telescope "tube" can be constructed of anything – from cardboard sonotube to aluminum struts – its only purpose is to hold the secondary mirror at the proper focal length and direct the light back towards the eyepiece located along the side. The altazimuth mount is equally simple. Alt-Az is just a formal term for a mount that moves either up or down – left or right. In the case of the dobsonian telescope, it became very easy to devise an mount created on the principle of a rocker-box.

The rocker-box, or base, of a dobsonian telescope is generally made of wood an performs identically to a lazy susan. At the base is a bearing or pivot point which allows the box to turn fully 360 degrees. On top this base is a box assembly to hold the telescope tube. On either side of the telescope tube, a large circular "bearing" is attached and placed in a fitted semi-circular cut-out in the box base. One side of the box is left open so the telescope tube end will clear, while the opposing side is cut down to about half the size to keep the telescope to no more than a horizontal position.

While the design must also incorporate balance and smooth movements, this incredibly simple, durable and easy to use telescope soon became a choice among amateurs. With no difficult to learn setting circles or complicated mounts, the dobsonian taught observers that portability and size could become a reality. While the dobsonian telescope design isn't perfect for every astronomy application, even the most hardened astronomer enjoys the simplicity and ease with which these rugged and durable telescopes perform.

Modern Dobsonian Telescopes

Since the dobsonian revolution, many amateurs prefer to make their own telescopes. And why not? With such a simple design, hand figuring your own mirror and customizing the tube and mount becomes a loving hobby – not a necessity for those who simply want a big telescope. At every star party you attend, there are hundreds of different styles, ranging from extravagant wooden creations to simple plumbing supplies. No matter what they are made of, they all have on thing in common – they are a Newtonian reflector.

Thanks to John Dobson and the dobsonian telescope revolution, large aperture telescopes have become affordable and commonplace. Small dobsonian design telescopes with an aperture of 4.5" begin about $200 and range upwards to 18" and $5000. Almost all telescope manufacturers carry a dobsonian telescope design. Among them you will find collapsible tubes, truss tubes, metal tubes and even some of the original cardboard sonotubes. The base has also evolved, with spring assemblies to aid in balance and even computer guidance aids to help you locate objects!

The revolution has begun…

Filed under: Astronomy