Refractors
There are two basic ways to bring light rays to a focal point. The earliest method used by telescope makers, was to
bend the rays by passing them through one or more pieces of glass which had curved, polished surfaces. This method
produces a type of telescope called a refractor.
Lenses refract light at different angles depending upon the wavelength of the light, so a single-lens refractor introduces so much chromatic aberration that a celestial object cannot be focused well enough to see very clearly. Opticians discovered that a second, convex lens cemented to the concave primary lens can offset much of this chromatic aberration and this type of doublet primary lens could yield a useable telescope; a scope with a doublet primary lens assembly is called an achromatic refractor. However, a doublet lens system still cannot bring the wavelengths of light at the far ends of the visible spectrum to a common focus - to accomplish this a third lens element is required. Triplet lens systems can eliminate chromatic aberration especially if the lens elements are manufactured from specialized glass compounds; scopes with triplet lens systems are called apochromatic refractors.
Lenses refract light at different angles depending upon the wavelength of the light, so a single-lens refractor introduces so much chromatic aberration that a celestial object cannot be focused well enough to see very clearly. Opticians discovered that a second, convex lens cemented to the concave primary lens can offset much of this chromatic aberration and this type of doublet primary lens could yield a useable telescope; a scope with a doublet primary lens assembly is called an achromatic refractor. However, a doublet lens system still cannot bring the wavelengths of light at the far ends of the visible spectrum to a common focus - to accomplish this a third lens element is required. Triplet lens systems can eliminate chromatic aberration especially if the lens elements are manufactured from specialized glass compounds; scopes with triplet lens systems are called apochromatic refractors.