Lens sag

Not to be confused with Sagitta (optics).

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.^[1] Making the lens thick enough to prevent deformation would cause it to absorb too much light to be useful.^[2] A mirror on the other hand can be effectively supported by the entire opposite face, making mirror sag much less of a problem.^[2] One expensive solution to lens sag is to place the telescope in orbit around the Earth.^[3]

The technical limit concerning lens sag was reached at the Yerkes refractor (1897) with an aperture of 40 in (100 cm).^[2] Hence the 1890s marked the high point of the great refractors era.^[4]