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From Galileo to Newton

5.3 - The Evolution of the Telescope

J. Hoewel's 150-foot long telescope (Machina coelestis, 1673) Diagram of Newton's telescope (I. Newton, Optice, 1740) Diagram of a Gregorian reflecting telescope Diagram of a Newtonian reflecting telescope Diagram of a Cassegrain reflecting telescope

Already the first telescope makers had become empirically aware that the effects of chromatic aberration (which was not yet theoretically recognised) substantially diminish when the ratio between focal length and diameter of the objective is increased. This increase is not proportional to the diameter of the objective, but to its square, a fact that led to an extraordinary increment in the length of telescopes, some of them becoming tens of meters long within a few decades. The search for alternative solutions was based on the utilisation of mirrors, which are not affected by chromatic aberration. In 1663 James Gregory (1638-1675) invented an instrument consisting of a primary parabolic mirror and a secondary elliptical one. In 1668 Isaac Newton built an instrument, it too with a parabolic objective, in which a small flat mirror, inclined 45°, deviated the optical beam to the side. Lastly, in 1672, Laurent Cassegrain (c. 1629-1693) proposed an instrument that also had a parabolic objective, but a secondary mirror that was convex and hyperbolic. But what prevented the success of the reflector telescope was the low reflecting power of the mirrors of the time, made of a copper and tin alloy called speculum.

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