History of the Telescope: Refractor vs Reflector

In 1609 Galileo pointed his rudimentary telescope at the heavens, finding three and then four moons orbiting Jupiter. He combined two polished glass lenses, slightly convex at different angles, and was able to magnify the image. This wasn’t a new technology; the basic monocular had been generally used for terrestrial purposes, allowing humans to peer across valleys and mountaintops, or at their military foes, from great distances.

This type of magnifying device is known as a refractor scope, for the way it bends (refracts) the light as it passes through a lens. The total light gathered across the surface is then bent to converge on a single point; the distance from the lens to the point of convergence is known as the focal length. By combining two lenses of different focal lengths at either end of a tube, it was discovered that the image could be greatly magnified.

The technology of polishing lenses exploded, with royalty hiring glass makers to design bigger and better lenses for government-sanctioned observatories and royal astronomers. But as lenses get bigger, the process of refraction gets more complicated. Like a child’s toy prism — or the cover of a Pink Floyd album — the light we see enter the glass divides into different colors as the various wavelengths slow to different speeds in passing through the dense medium of the glass. When this happens in a telescope the effect is called chromatic aberration, where the colors at the eyepiece don’t match up quite right and the image looks fuzzy.

Another problem is that as lenses get bigger, the glass gets very heavy, and it can only be supported from the thin edge of the lens, so as not to obstruct the light. And as lenses got bigger, so did the tubes.

To address this, a little-known polymath named Isaac Newton came up with a different type of magnification process in 1668, using a concave mirror to reflect the light back to the observer’s eye. A secondary mirror above the main mirror diverts the light to the side of the telescope. This solves all the main problems of the refractor, because the light does not divide into different wavelengths through the lens, the mirror can be fully supported at the end and therefore much more firmly, and reflecting the light back up through the tube and out essentially uses the tube twice, greatly reducing the length of the tube relative to refractors to achieve the same focal length.

Some of the largest refracting telescopes have been used for great discoveries, such as the 24-inch telescope at Lowell Observatory, used in discovering the red shift of galaxies and mapping the moon for the Apollo missions. Reflectors, on the other hand, have grown much, much larger, including many across Arizona, like the Discovery Telescope in Happy Jack, with its four-meter primary mirror. This is also the format commonly used for the great space telescopes, with the James Webb Space Telescope’s 18 gold hexagonal mirrors unfolding last month to a completed primary mirror diameter of 6.5 meters.

So which is best for you? Well, it depends on what you want to do. Many of the images we have featured in this column over the years have been from Joel Cohen, who has a seven-inch refractor he uses for astrophotography. My personal telescope is a type of reflector called a Dobsonian, great for viewing but less so for photos. Whichever route you choose, I wish you clear skies!

Images by Joel Cohen, Adam England and Lowell Observatory.

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