lens sag

{{distinguish|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.{{cite web |url=https://lco.global/spacebook/telescopes/refracting-telescopes/ |title=Refracting Telescopes |work=Las Cumbres Observatory |access-date=2 June 2025}} Making the lens thick enough to prevent deformation would cause it to absorb too much light to be useful.{{cite web |last=Simmons |first=Mike |url=https://www.mtwilson.edu/building-the-60-inch-telescope/ |title=Building the 60-inch Telescope |work=Mount Wilson Observatory |date=2008 |access-date=2 June 2025}} 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 around the Earth.{{cite book |last=Gibilisco |first=Stan |url=https://archive.org/details/physicsdemystifi0000gibi/page/515 |title=Physics Demystified |publisher=McGraw-Hill Education |page=515 |date=2002 |access-date=2 June 2025 |isbn=978-0071382014}}

The technical limit concerning lens sag was reached at the Yerkes refractor (1897) with an aperture of {{convert|40|in|cm|abbr=on}}. Hence the 1890s marked the high point of the great refractors era.{{cite book |last=Cottrell |first=Geoff |url=https://www.google.com/books/edition/Observational_Astronomy/OW28EAAAQBAJ?hl=en&gbpv=1&pg=PA13 |title=Observational Astronomy: A Very Short Introduction |page=13 |publisher=OUP Oxford |date=2023 |access-date=2 June 2025 |isbn=978-0192665515}}