Why are large refracting telescopes difficult to build?

Large refracting telescopes are difficult to build primarily due to the physical and optical limitations inherent in using large glass lenses.

A major challenge is the management of chromatic aberration, where different wavelengths of light fail to focus at the same point, causing color distortion. While this can be minimized by adding components to the objective lens, it increases the complexity and weight of the system.1

Furthermore, as the size of the lens increases, the glass becomes difficult to manufacture and support. Because a lens must be supported only by its edges, a very large lens may sag under its own weight, leading to optical distortions. Additionally, the coefficients of expansion for the glass must be carefully matched to prevent aberrations caused by temperature fluctuations during nighttime observations.1

Historically, the construction of very long refractors—sometimes reaching 61 metres—proved to be too awkward and ineffective for practical use. Consequently, modern astronomy has shifted toward reflecting telescopes, which avoid these issues by using mirrors that can be supported from the back and are not subject to chromatic aberration.23