What inherent trade-off makes manufacturing a large, flawless objective lens for a refractor difficult and expensive?

Chromatic aberration (color fringing)

A significant challenge associated with large refracting telescopes stems from the material limitations of glass lenses, specifically the phenomenon known as chromatic aberration. Chromatic aberration occurs because different wavelengths (colors) of light refract at slightly different angles when passing through glass, causing the colors to focus at slightly different points. This results in visible color fringing around bright objects, degrading image quality. Manufacturing a large objective lens that is entirely free from this defect is exceptionally difficult and cost-prohibitive, which is why, despite their image clarity benefits, very large telescopes often opt for mirror designs that circumvent this specific optical hurdle.