Newtonian Reflectors and Dobsonian Telescopes

A Newtonian reflector telescope on an equatorial mount (credit: Skywatcher).

Reflecting telescopes (or reflectors) collect light using a curved mirror at the rear of the main tube rather than a lens at the front end. Isaac Newton gets credit for inventing the first reflecting telescope in the late 17^th century. He used a second small diagonal mirror to direct light out the side of the telescope to an eyepiece. His immensely practical design, now called the Newtonian reflector, is the main type of purely reflecting telescope in use today by amateur astronomers.

For a given aperture, Newtonian reflectors are generally the least expensive telescope because, unlike the lenses of a refractor, only one surface of a mirror needs careful figuring and polishing. And since no light passes through a mirror, less expensive glass is used. Some homemade reflectors even use glass from the discarded portholes of a ship!

The layout of a Newtonian reflector. The front of the tube is open; the mirror sits at the back of the tube. A smaller secondary mirror reflects light out the side to an eyepiece.

Newtonians are a great value. A top-of-the line 4-inch refractor costs as much, or more than, a 12-inch Newtonian reflector, yet the reflector has 9 times the light gathering capability. Newtonian reflectors occasionally require adjustment of their optical alignment– especially if the scope gets bumped. The adjustment is not complicated, but it takes a little practice. Newtonians also tend to be bulky, with tube lengths of 4-5 feet or more. Newtonians also have a type of optical aberration called “coma”. This makes stars appear wedge-shaped at the edge of the field of view. But it’s not a big deal if the focal ratio is bigger than f/6 or so. Another advantage of Newtonians: because mirrors reflect all colors of visible light exactly the same, reflecting telescopes have no chromatic aberration.

Dobsonian Reflectors

Newtonians came back into style in a big way in the 1980’s when telescope makers commercialized a design by the former monk and astronomy popularizer John Dobson.

The “Dobsonian” telescope is just an inexpensive large-aperture Newtonian on a simple alt-azimuth mount that points left-right (in azimuth) and up-down (in altitude). Dobsonians can’t be beat for purely visual deep-space observing. With huge mirrors (up to 36” in diameter) these new-age Newtonians are called “light buckets” because of their immense light-collecting ability.

If you’re interested in strictly visual observing, and you want the most aperture (and light-gathering capability) for your money, a Dobsonian telescope is well worth your consideration. An 8-inch (200 mm) f/6 Dobsonian with a focal length of 1200 mm is an excellent workhorse telescope, even for beginners. For most observers, it is not too long or heavy to move, yet it still has enough aperture to show thousands of deep-sky objects. Yet a quality Dobsonian scope of this size can cost less than $500.

In the past 10-15 years, telescope makers have developed more innovative Dobsonian designs to make it easier to transport large telescopes to observing sites. The ‘truss tube’ design, in particular, eliminates the need for a telescope tube the size of a hot-water heater. The telescope comes in three main sections: the mirror box which contains the primary mirror, the secondary cage which houses the secondary mirror and focuser, and a set of truss tubes which connect the two. With this assembly, a stargazer can single-handedly transport a 12″ or 14″ telescope to a dark sky site and set it up in about 20 minutes. And simple versions of 12″ truss-tube Dobs, for example, are available for less than $1,000 in the U.S. This is a good time to be a stargazer.