Newtonian Telescope
Written by Tammy Plotner
Are you curious about the term Newtonian telescope? What is a Newtonian telescope, what different applications does it apply to, and most of all what does Sir Isaac Newton have to do with it? If you have questions like these and more, then follow along as we take a closer look at the Newtonian telescope.
Sir Isaac Newton and the Newtonian Telescope
The seventeenth century was a golden time for astronomy and telescopes. The very first refractors had began to appear on the scene and were improved by Galileo 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. While Galileo chose to work with lenses, Newton chose to work with concave mirrors to gather and focus light and 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 simply 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 Newtonian Telescope is a simple and elegant design which has endured through the years!
Why Choose A Newtonian Telescope
There are many, many reasons to choose a Newtonian Telescope, but first a 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 and coatings count here! In refractor telescopes, large lenses sag under the weight of gravity, while 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 become greater. And last, but not least, because the parabola can be changed, the Newtonian telescope can vary greatly in focal lengths – the distance the light must travel to reach a focal point. 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.
To be fair, one must point out that the Newtonian telescope does also have a few flaws. 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. Last, but not least, Newtonian telescope must periodically have their optics realigned in a process called collimation – a simple adjustment much like tuning a guitar.
The Evolving Newtonian Telescope
Over the years the Newtonian Telescope 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 Gregorain, 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 across. How about the 100-Inch Hooker Telescope at Mt. Wilson? Or the Hubble Space Telescope? All are variants of the Newtonian Telescope.
Filed under: Astronomy
