• The Battle of Lenses and Mirrors


    The first mirror telescope was built in 1668 by Isaac Newton. It was a very small instrument - its concave mirror was smaller than a matchbox. Nevertheless, Jupiter’s very distant satellites were clearly visible in this telescope. Mirror telescopes were called reflectors.

    The larger the “light funnel” of the telescope, the more light enters it, and the stronger the telescope. In a reflecting telescope, a concave mirror serves as a “light funnel”. It is clear that the builders of the telescopes tried to make a mirror as large as possible.
     

    Newton's reflector
    Newton's reflector
     The astronomer Herschel built two hundred years ago a gigantic telescope for those times: the diameter of its mirror was more than a meter. The mirror was not made of glass, but of an alloy of copper and tin. Copper, as you know, expands greatly from heat. It was worth touching a mirror with a finger - and it, under the influence of heat, already changed its shape. Of course, such an insignificant change in shape was not noticeable. But it was enough for the light rays to deviate to the side, and the image in the telescope became unclear. So Herschel himself preferred to observe the sky not through this large telescope, but through others, smaller.


    A glass mirror would be incomparably better than a metal one. But then they did not know how to make perfectly clean, uniform glass. After they learned to cook optical glass, they began to build telescopes with glass mirrors.

    Modern Newton Reflector
    Modern Newton Reflector

    Mirror telescopes have some advantages over lens telescopes. In lens telescopes, rainbow bands appear at the edges of the image as a result of refraction of the rays. In reflectors, color bands do not appear, because the light in them does not refract, but only reflects. The quality requirements for glass for a mirror are much weaker than for glass for a lens, since only the surface of the glass, on which a reflective layer of silver or aluminum is applied, works in the mirror.

    The battle between lenses and mirrors has been going on ever since the invention of mirror telescopes. The dispute between astronomers about which telescopes are better would continue now if they could not find a way to reconcile them. One of these ways is the creation of meniscus telescopes.

  • You might also like