Telescopes can have many shapes and designs, that were developed over the centuries. The variety of designs also means different capabilities, dimensions and image quality.
For an introduction on telescopes, check this article out!
The very first telescopes appeared around 1609. It is a simple design that uses a set of lenses to refract the light towards an eyepiece. Thus the name: refracting telescope, or refractor.
Although the original design was quite simple, it evolved quite a lot since the 17th century, giving birth to 2 variants, known as achromatic and apochromatic refractors. The reason is that, by design, a basic refractor suffer from optical aberrations.
The main problem is chromatic aberration, caused by the diffraction of the light. Each wavelength composing the visible light behaves differently when it goes through glass. Therefore, the focal point is not the same for all wavelengths (i.e. colors), which leads to chromatic aberrations.
Another problem are the spherical aberrations: when 2 rays of light enter in 2 different points of the lens, their focal point will be different, which causes a blur.
Achromatic (AC) and Apochromatic (APO) refractors are similar in design, but the latter tends to correct the aberrations found in the former, and is therefore optically better.
The standard way to correct optical aberrations is to create a compound lens, made of several lens elements grouped together, instead of only one lens element. This compound lens does a much better job than a single lens when it comes to avoiding optical aberrations.
Additionally, the terms doublet, triplet and even quadruplet are often used to indicate how much lenses the compound lens has (2, 3, 4…). Adding a lens can solve a given issue. Consequently, adding several lens elements, usually means less aberrations and a better image quality… but also more weight and a higher price tag.
Note: Almost all camera lenses actually have a refractor design, though it can be much more complex than a telescope, in order to add the ability to zoom, autofocus and change the aperture. A refracting telescope can be seen as a prime lens with a constant aperture and a manual (and limited) focus.
Another way to reduce optical flaws is to use low-dispersion glass. The term ED is often used for telescopes and camera lenses, and means Extra-low Dispersion. These glasses have a low refractive index, which means that they are less prone to splitting the light into its different colours.
Rainbows and prisms are common examples of what dispersion is. When the sunlight goes through a prism (or through drops of water), the glass (or the water) has a higher refractive index than vacuum. Consequently, the light is split into its different colors.
- Achromatic and Apochromatic Refractor Designs on Starizona