Refracting Telescopes

Abstract

The earliest of telescopes was the refracting or lens type. Initial versions used a single objective lens to collect and focus the image and a simple eyepiece to magnify it. Single lenses by their very nature have a number of optical shortcomings, chief among these being chromatic (or color) aberration. This causes light of different colors to come to focus at different points, producing image-degrading prismatic haloes to appear around the Moon, planets and the brighter stars. Another serious problem is that of spherical aberration—the inability of a single lens to bring all light rays to the same focus. But, as focal length increases, these aberrations tend to decrease. Attempting to take advantage of this, telescopes became ever-longer, reaching unwieldy lengths of as much as 150 ft! (It was using primitive single-lens refractors that Galileo made his historic discoveries. And while he did not invent the telescope itself, he’s credited with being the first person to apply it to celestial observation—and to publish what he saw.)