So Why a Truss Dobsonian?

Dobsonian telescopes (commonly referred to as “Dobs”) are a common sight these days. Most of us know that they are conventional Newtonian reflectors on a novel, simplified altitude/azimuth mounting system. Many of us are aware that the basic design is credited to John Dobson. Some of us remember that Mr. Dobson’s concept was a “people’s telescope” - one that could be put together at practically no cost using salvaged materials and unskilled labor. He felt that everyone would want a telescope if it were affordable, and that observing the Cosmos with even a very crude instrument would encourage people to become more spiritual. He wasn’t a skilled optician or a trained engineer; his telescopes were crude but fairly effective. Mr. Dobson is still a popular figure among some groups of amateur astronomers (see http://sidewalkastronomers.com) and eagerly accepts speaking engagements (see http://www.johndobson.org). Despite an obvious lack of formal scientific training he enjoys writing and speaking about “conceptual cosmology”, wherein he replaces much of modern physics with ancient Eastern philosophy.

The simplest and least expensive Dobsonian telescopes offered today reflect both acceptance of his basic mounting design and rejection of his philosophy. Modern so-called Dobs are built to a performance standard far beyond his concept. Gone are the broomstick secondary holders, plumbing component focusers, and porthole glass mirrors. Only the mount itself is replicated, and the use of fresh materials and modern manufacturing techniques have brought about large improvements there as well. Folks today expect a Dobsonian telescope to have smooth motions and very good to excellent optics.

Because of the simplicity of the basic mounting, it’s possible to build a Dobsonian telescope in which nearly all of the cost is invested in the telescope itself. For many reasons, it is much easier to build an inexpensive, good-quality Newtonian reflector than any other telescope design. Thus, simple modern Dobs represent the most “bang for the buck” of anything on the market. Today the conventional wisdom when recommending a first telescope is to steer folks toward a Dob.

Compromises involved in using a conventional Dob involve two primary areas. First, the altitude/azimuth mounting doesn’t permit the telescope to move as the stars appear to. This apparent motion is caused by the earth’s rotation and is thus curved. This means that the instrument must be moved manually in two axes to follow an object. There are ways of getting around this, and I’ll address them later - but they involve added complexity and cost and thus move us away from some of the reasons for choosing a Dob in the first place. Second, the alt/az mounting doesn’t permit the simple addition of mechanical setting circles, making it necessary to locate objects manually by the use of charts or memorization. There are ways around this as well, but again they add cost and complexity.

I mentioned “bang for the buck” earlier. How about a brand new 6” F/8 Newtonian telescope complete with accessories for $200? A 10” for $500? A 12” for $900? A 16” for $1300? You can see why many folks are willing to accept the compromises. For these prices one can acquire a reasonably well-built telescope with surprisingly good optics. Most of these are imports from China or Taiwan - and it’s obvious that those factories have come a long way in learning to produce quality mirrors.

We can add features to these telescopes if we wish. An EQ platform may be placed under the base to provide equatorial tracking for $600 and up. Motors can be added to the mount itself to make it track in both axes or even to automatically locate and point to objects on demand for $1000 to $2000. Digital setting circles are easily installed to help locate objects for $400 or so. We can substitute premium mirrors and intricately machined focusers and mirror mounts. We can replace the solid tube of the telescope with a lighter truss structure which breaks down for transport. Each incremental investment enhances performance but moves us away from some of the economical advantages of the Dob design. In modest apertures (10” and below) most users forego the more expensive addons.

What about larger apertures? Above 12” a truss structure is nearly a requirement for portable use, as the optical tube of a 16” tube Dob weighs around 100 pounds and is the size of a water heater. A truss structure increases the cost of the instrument dramatically - but at the same time it makes other enhancements seem more worthwhile. A fairly high-quality 14” truss Dob with a hand-figured mirror and exotic hardware (mirror mounts, focuser, etc.) can be had for around $2000. That may sound like a lot - but there’s no more affordable way to acquire that much aperture with that much quality. Add motors for GoTo operation and you’re still only at $4000, and you have a setup that will travel in a passenger car and can easily be set up by one person.

All this explains why there are so many $6,000 18” Obsessions and $8,000 18” Starmasters out there. They are still called Dobsonians though they bear little resemblance to the crude “people’s telescope” of John Dobson’s dreams. What they offer is superb optics, beautifully crafted mechanical components, and amazing performance with substantial aperture in a portable package.

John Crilly
First published November, 2004 in “Night Sky”, the newsletter of the Astronomy Club of Akron, Ohio.