A Newtonian telescope is a type of telescope that was invented by the Englishscientist Sir Isaac Newton. The telescope sends an image to the back of the tube and then reflects it off an angled mirror back up the tube. The image then goes into an eyepiece and into the person's eye. The eyepiece uses one or more lenses that are used to make the image larger.
Early Newtonian telescopes were made for real science work. Now, "newtonians" are mostly used by amateur astronomers. The most popular type is the Dobsonian telescope, which is on a simple rockerbox mount. People like them because they are cheap to make and easy to use.
References
Smith, Warren J., Modern Optical Engineering, McGraw-Hill Inc., 1966, p. 400.
