Lens sag is a problem that sometimes afflicts very large refracting telescopes. It is the equivalent of mirror sag in reflecting telescope

Reflecting telescope

A reflecting telescope is an optical telescope which uses a single or combination of curved mirrors that reflect light and form an image. The reflecting telescope was invented in the 17th century as an alternative to the refracting telescope which, at that time, was a design that suffered from...

s. It occurs when the physical weight of the glass causes a distortion in the shape of the lens

Lens (optics)

A lens is an optical device with perfect or approximate axial symmetry which transmits and refracts light, converging or diverging the beam. A simple lens consists of a single optical element...

 because the lens can only be supported by the edges. A mirror on the other hand can be effectively supported by the entire opposite face, making mirror sag much less of a problem. One expensive solution to lens sag is to place the telescope in orbit

Orbit

In physics, an orbit is the gravitationally curved path of an object around a point in space, for example the orbit of a planet around the center of a star system, such as the Solar System...

 around the Earth

Earth

Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...

.

The technical limit concerning lens sag was reached at the Yerkes refractor (1897) with an aperture of 40" (102 cm), where the sagging already causes small optical distortions. The 1890ies therefore mark the end of the great refractor

Great refractor

Great refractor refers to a large telescope with a lens, usually the largest refractor at an observatory with an equatorial mount. The preeminence and success of this style in observational astronomy was an era in telescope use in the 19th and early 20th century. Great refractors were large...

s era.