Newtonians & Dobsonians
Newtonians and their variant, the Dobsonian, are types of reflecting telescopes. Using a simple concave primary mirror design with a flat secondary, Newtonian designs have the advantage of being inexpensive to produce, but they also provide a crisp view. These simple designs are highly popular among amateur astronomers worldwide. If you want a lot of bang for your buck, a Newtonian or a Dobsonian may be just what you are looking for!
What is the difference between a Newtonian and a Dobsonian? A Newtonian is an optical design, and a Dobsonian is a mount style. Normally, a Dobsonian is a Newtonian, but a Newtonian is not always a Dobsonian! You can buy a Newtonian style telescope on any mount: equatorial, alt-azimuth, and even Dobsonian. Feel free to check them out because you will always get a lot of aperture for the money with the versatile and budget-friendly Newtonian.
The History of the Newtonian
The seventeenth century was a golden time for astronomy and telescopes. The very first refractors had begun to appear on the scene and so had the very first reflector telescopes. Like the predecessors of both models, the major flaw lay in shaping the optics correctly, and – like Galileo – Sir Isaac Newton was a far more advanced optician than the average. While Galileo chose to work with lenses, Newton chose to work with concave mirrors to gather and focus light. His improved method came to be known as the Newtonian Reflector Telescope.
The first Newtonian Telescope appeared in about the year 1668, and introduced the one design manifestation that no one could figure out how to get around – once the light was gathered by the parabola and refocused back to a point on the focal plane, how did you view it without obstructing the light gathering source? Newton’s plan was simple and elegant. He introduced a very small, secondary mirror at the focal point and aligned it with the center of the parabola where most incoming parallel light rays are less effective. Held in place by thin vanes called the “spider” assembly; this secondary mirror captured the refocused light path in a non-magnified way and was aimed at a porthole on the top side of the optical tube. From there a series of lenses, called the eyepiece, is used to focus on the secondary mirror and study the image. The first practical working model of the Newtonian Telescope went into production in roughly the year 1689. The simple and sensible design has endured through the years!
Why Choose A Newtonian Telescope
There are many, many reasons to choose a Newtonian Telescope, but first and foremost is the fact you are able to get a far larger light gathering surface at a far more economical price. Because light does not pass through the mirror, exotic glasses aren’t necessary – only a glass capable of holding the configuration. Cost is concentrated on the reflective coatings used. In refractor telescopes, large lenses sag under the weight of gravity, large mirrors do not – therefore very, very large telescopes must be the Newtonian design. Since only one optical surface has to be machined to perfection, again, the expense is less and the chances of getting an optically perfect scope for a smaller price tag becomes greater. And last, but not least, because the distance to focus like or parabola can be changed, the Newtonian telescope can vary greatly in focal lengths. This allows the user to choose a long focal length telescope for more magnification factors and photographic abilities – or a short focal length for portability and wider fields of view with short exposure times
To be fair and like any other optical design, the Newtonian does have some drawbacks. Incorrectly ground mirrors can suffer from coma – the distortion of image at the edges of the mirror. By virtue of design, the spider assembly which holds the secondary mirror also obstructs some incoming light and can produce diffraction spikes which can reduce contrast. Because of the suspended secondary design, collimation will be a regular part of the Newtonian owner's routine.
For anyone looking for a large aperture but with a limited budget, the Newtonian is the smartest choice you can make.
Evolution of the Newtonian Reflector Telescope
Over the years the Newtonian evolved to incorporate other designs as well. By adding a lens assembly, the Schimdt-Newtonian was born – a hybrid of both telescope types. The Cassegrain, the Ritchey-Chretien, the Dall-Kirkham, the Gregorian, the Herschelian, and even the Schiefspiegler are all variants of the Newtonian design.
Some of the most famous telescopes in the world have borrowed from the Newtonian telescope. Just take a look at the 5-meter (200-inch) Hale Telescope at Palomar Observatory. Its primary mirror is almost 17 feet across! How about the 10-meter Keck Telescope at the W.M. Keck Observatory? The primary mirror is composed of 36 hexagonal mirrors put together to act as one large mirror 10 meters in diameter. The 100-Inch Hooker Telescope at Mt. Wilson and the Hubble Space Telescope are all are variants of the Newtonian Telescope.
The Dobsonian reflector is based on the idea that a telescope can be inexpensive and very easy to use. Looking at the Newtonian design, John Dobson realized that the major concern was the primary mirror. The body of the tube and the telescope's base could be made of almost any inexpensive material so long as they were stable and could support the weight of the mirror. The Dobsonian reflector is nothing more than a well-spaced set of mirrors in an optical tube assembly and mounted on a rocker box with side bearings – which, in turn, is set on a spinning base like a Lazy Susan. When the weight is balanced correctly on the side bearings, the dob telescope becomes incredibly easy to use with nothing more than up-and-down and side-to-side movements. In the late 1980′s the Dobsonian reflector design soon hit the telescope manufacturing business and became instantly popular. With less moving parts, the simple alt-az cabinet was very cost effective to manufacture and the major companies offered ever larger mirrors and prices backyard astronomers could afford. With time, the design even evolved to include truss-tubes and off-axis models. Because of simple freedom, the Dobsonian reflector “light bucket” is one of the most popular telescopes today!