Refracting Telescopes

Astronomical refractors are renowned for their exquisite image quality.   Views of the Moon and planets are crisp, and stars appear pinpoint.   Refractors are ideal telescopes for terrestrial viewing too, when outfitted with an image-erecting prism.

Refractors have a long, narrow tube containing a multi-element objective (front or main) lens.  The lens focuses the incoming light and directs it out the back of the telescope.  Any quality refractor has at least a two-element, or achromatic, objective lens.  It consists of one convex lens made of crown glass and one concave lens made of flint glass.  This combination reduces the chromatic (color) aberration caused whenever white light passes through glass.  Apochromatic objective lenses have two to  four lens elements; at least one is made with fluorite or "ED" glass, which provides even better color correction.

Refractors are compact and lightweight in smaller apertures, and thus are highly portable.  The closed-tube design is rugged and practically maintenance-free.   The optics rarely, if ever, require realignment (collimation), unlike those of a Newtonian reflector.  Also unlike reflectors or Catadioptic telescopes, a refractor has a clear aperture, that is, there's no secondary mirror to obstruct any of the incoming light and degrade the image.

While they offer the best performance of any telescope type, refractors are also the most expensive per inch of aperture.  So affordable refractors tend to be small, with apertures in the 2" to 4" range.  Their lower light-gathering capability makes them best suited for viewing bright objects, such as planets, the Moon, double stars, and the more luminous deep-sky objects.

One note of caution: Don't be hood-winked by claims of excessive power, as are commonly made for telescopes you see in department stores.  That's a sure sign of an inferior product.  The practical limit for any telescope is about 45X to 60X per inch of aperture, or 144X for a 60mm (2.4") scope.  Magnification is not related to quality of performance. See Powers of a Telescope.

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