Refracting Telescopes

A refracting telescope is the type of telescope most people recognize. It was the first type of telescope invented, and Galileo was the first to turn such a telescope to the night sky back in 1609.  Refractors collect light with a large glass objective lens at the top of the tube.  The lens focuses the light at the bottom of the tube where the image is magnified by an eyepiece.

A refracting telescope for astronomy

A refracting telescope for astronomy

Early refractors had a single objective lens with spherical curvature. This caused much image distortion near the edge of the field of view.  Also red, green, and blue light rays focused at different points, a problem called “chromatic aberration”.  This caused the images of stars and planets to have colored “halos”.  To reduce these problems, early refractors needed large focal ratios, which meant even telescopes with a small objective lens were enormously long… ten to twenty feet or more. But in the mid-1750s, a lawyer named Chester Moore invented a refractor with two objective lenses, each of which was made from a different kind of glass to partially correct for chromatic aberration. Today, all refractors have this type of “achromatic” objective.

Refractors produce the highest contrast of any telescope, which makes them superb for visual observation of fine detail on the Moon and planets, especially refractors with achromatic lenses and a focal ratio of f/10 or longer.  Still, even achromatic refractors, especially those with a focal ratio of f/8 or less, tend to show false color, especially on brighter objects. While generally low maintenance, refractors are expensive compared to their aperture and light-gathering capacity.  But refractors are light, simple to use, mechanically robust, and show sharp images of brighter celestial objects. If you’re mostly interested in the Moon and planets, then a achromatic refractor may be the right scope for you.  You can get an 80 mm or 90 mm refractor and a reasonably solid mount for $300-$400. 

In the past twenty years or so, new fluorite glasses and innovations in telescope design have led to “apochromatic” refractor lenses that show virtually no false color, even at focal ratios of f/6 or f/7. “Apos”, as they’re called, are fantastic telescopes, with razor sharp images and superb contrast. They are favored by astrophotographers and visual observers of the Moon and planets, as well as larger nebulae and star clusters along the Milky Way.

refractor

A 90 mm achromatic refractor on a simple alt-azimuth mount (credit: Celestron)

The down side? Top-notch apochromats are shockingly expensive, up to $1000 per inch of aperture. In the past few years, however, many well-reviewed refractors that use simpler “ED” lens designs and new materials have hit the market. For example, you can get a decent 4-inch ED optical tube assembly (mount not included) that gives you tack-sharp images of planets and wonderful contrast on binary stars and stunning wide-field views of the Milky Way for only $1,000 or so. If you have good dark sky and your main interest is in objects that benefit from ED or apochromatic lenses, you should try such a telescope at a star party and see if it’s right for you.

Refractor Pros

  • Excellent image contrast and sharpness
  • Mechanically robust; little alignment required
  • Easy to use

Refractor Cons

  • Smallest aperture per dollar
  • Some false color (chromatic aberration) with achromatic lenses

Refractors are best for

  • Observing Moon, planets, and bright deep-sky objects
  • With an extra optical element to make the image rightside-up, small refractors can be used for terrestrial observing
  • Low f-ratio ED and apochromatic refractors give stunning wide-field views of night sky