Building a Newtonian Reflecting Telescope — Mirrors Instead of Lenses

In 1668, Isaac Newton built the first practical reflecting telescope — using a curved mirror instead of a lens to gather and focus light. Newton's design solved the fundamental problem of Galileo's refractor: chromatic aberration, where a simple lens bends different colours of light by different amounts, producing coloured fringes around every bright object. A mirror reflects all colours equally, producing sharp, colour-free images. Newton's telescope uses a concave primary mirror at the bottom of a tube to gather light and reflect it to a focus. A small flat secondary mirror (the diagonal) intercepts the converging light near the top of the tube and deflects it 90 degrees out the side of the tube, where the eyepiece magnifies the image. This elegant design — now called the Newtonian reflector — remains the most popular telescope design for amateur astronomers because it delivers large apertures at low cost. This blueprint builds a functional Newtonian telescope using a concave mirror and a flat diagonal, mounted in a simple tube on a basic altitude-azimuth mount.