The Gregorian Telescope

   The form of reflecting telescope suggested by James Gregory (1638-1675) predates the familiar form of reflector which Isaac Newton (1642-1727) first designed and made in 1668. The problem with the reflecting telescope is how to view the image formed by the primary mirror. With a single concave mirror, this image is formed on the optical axis, and the head of the observer then gets in the way. Newton solved the problem by using a small diagonal mirror to deflect the image to the side, where it may be viewed through the eyepiece.

   Gregory's solution was to place a small, short focal length, concave mirror along the optical axis; this can be seen in the right-hand picture. The rays reflect back toward the primary mirror and pass through a small hole in its center. The resulting image is then viewed with the aid of the eyepiece.

   This example is at the Museum of the United States Military Academy at West Point, New York. It is unmarked, and the primary mirror is about three inches in diameter. Other examples of refracting telescopes may be found in the Optics section.
 

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