Lens sag
Lens sag is a problem that sometimes afflicts very large refracting telescopes. It is the equivalent of mirror sag in reflecting telescopes. It occurs when the physical weight of the glass causes a distortion in the shape of the lens because the lens can only be supported by the edges.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> Making the lens thick enough to prevent deformation would cause it to absorb too much light to be useful.<ref name="mtwilson">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> A mirror on the other hand can be effectively supported by the entire opposite face, making mirror sag much less of a problem.<ref name="mtwilson" /> One expensive solution to lens sag is to place the telescope in orbit around the Earth.<ref>Template:Cite book</ref>
The technical limit concerning lens sag was reached at the Yerkes refractor (1897) with an aperture of Template:Convert.<ref name="mtwilson" /> Hence the 1890s marked the high point of the great refractors era.<ref>Template:Cite book</ref>