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28.7 x 34.3 cm ⏐ 11.3 x 13.5 in (300dpi)
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The Gregorian telescope is reflecting telescope designed by Scottish mathematician and astronomer James Gregory in the 17th century, and first built in 1673 by Robert Hooke. The design pre-dates the first practical reflecting telescope, the Newtonian telescope, built by Isaac Newton in 1668, but was not successfully built until 5 years after Newton's first reflecting telescope. The Gregorian telescope consists of two concave mirrors; the primary mirror collects the light and brings it to a focus before the secondary mirror where it is reflected back through a hole in the centre of the primary, and thence out the bottom end of the instrument where it can be viewed with the aid of the eyepiece. This design solved the problem of viewing the image in a reflector by allowing the observer to stand behind the primary mirror. It renders an upright image, making it useful for terrestrial observations. It also works as a telephoto in that the tube is much.
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