Did Newton Invent the Newtonian?

Jack Kramer

We all know that the Newtonian telescope is named after Sir Isaac Newton. But did he really invent it? Well, yes and no. He can be credited with the Newtonian design, but not the reflector concept. In the annals of mirror-based systems, it was the Scottish astronomer and mathematician James Gregory who invented the reflecting telescope. His design, published in a book called Optica Promota in 1663, used a parabolic primary mirror and concave ellipsoidal secondary that redirected the light back through a hole in the primary. Though many telescopes of Gregorian design eventually were built, James Gregory himself never actually made one. At the same time, Isaac Newton was working on his own reflecting telescope and in 1668 achieved the notoriety of building the first working reflector telescope. Its design was essentially the same as what we today think of as a "Newtonian". He did it entirely on his own, even making some of the tools used. With an aperture of 1-inch and a tube length of 6-inches, it magnified 30 times. Newton later built an improved reflector with an aperture of nearly 2-inches. In 1672 he made the 6-inch telescope that is commonly illustrated as his. (A reproduction of it is shown here.) Newton's study of light convinced him that refracting telescopes of that day were flawed, since they were subject to color interference. But his spherical primary mirrors produced their own aberrations, so the Newtonian design was not embraced by the astronomical community for about another hundred years. The most serious drawback to reflector telescopes of the time was the type of material available with which to make the mirrors. James Gregory's concept of using a parabolic primary mirror was sound. But Newton's design provided a larger field of view and ultimately supplanted the Gregorian in popularity. The rest is history.

Published in the January 2007 issue of the NightTimes