Editing Lens sag

{{distinguish|Sagitta (optics)}}
{{Unreferenced|date=December 2009}}

'''Lens sag''' is a problem that sometimes afflicts very large [[refracting telescope#Technical considerations|refracting telescope]]s. It is the equivalent of mirror sag in [[reflecting telescope]]s. It occurs when the physical weight of the glass causes a distortion in the shape of the [[lens (optics)|lens]] because the lens can only be supported by the edges.[https://escooptics.com/blogs/news/concepts-in-light-and-optics-lenses-part-2] 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]].

The technical limit concerning lens sag was reached at the [[Yerkes refractor]] (1897) with an aperture of {{convert|40|in|cm|abbr=on}}, where the sagging already causes small optical distortions. Hence the 1890s mark the end of the [[great refractor]]s era.

==References==
{{Reflist}}

{{DEFAULTSORT:Lens Sag}}
[[Category:Refracting telescopes]]


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