What is chromatic aberration? For what kinds of telescopes does it occur? How can it be corrected?

Refracting telescopes are a type of optical telescope that utilize lenses to gather and focus light from distant objects. The primary lens used is called an objective lens, which is responsible for bending incoming light rays to converge at a focal point where an image is formed. One of the inherent traits of this design is the use of transparent materials to direct light to the eyepiece.

However, due to the phenomenon known as lens dispersion, different wavelengths of light are refracted at slightly different angles. Consequently, this results in what is known as chromatic aberration, where colors don't converge at the same point, causing fringes of color around the image and reducing its clarity. This aberration is prominent in instruments that solely rely on lenses for image formation, such as refracting telescopes.

To address the issue of chromatic aberration in refracting telescopes, achromatic lenses are employed. These lenses are meticulously engineered composite lenses, typically made up of at least two different types of glass - crown and flint glass. Each of these glasses has distinct refractive properties.

An achromatic lens usually consists of a convex lens made from crown glass and a concave lens made of flint glass. The convex lens refracts a spectrum of colors towards a single point, while the concave lens is designed to correct the dispersion caused by the convex lens. With their powers combined, they act to bring two primary colors (usually red and blue) to focus at the same point, significantly reducing chromatic aberration and enhancing the quality of the image formed.

Lens dispersion refers to the splitting of white light into its constituent colors (spectrum) when passing through a transparent material, such as a lens. This occurs because different colors of light travel at varying speeds when they move through materials like glass. Due to physical properties, each color bends (refracts) by a different amount, with shorter wavelengths (blue) refracting more than longer wavelengths (red).

The result of lens dispersion can be aesthetically pleasing, as seen in a rainbow, but it is a nuisance in optics, specifically when precision is required, like in telescopes. Dispersion is why a simple lens cannot focus all colors to a single sharp point, resulting in images that have colored edges or halos, degrading overall sharpness and clarity. To produce high-quality, crisp images without the rainbow-like artifacts caused by chromatic aberration, carefully designed achromatic lenses are implemented in optical systems.