Lens sag

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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]

References

  1. ↑ "Refracting Telescopes". Las Cumbres Observatory . Retrieved 2 June 2025.
  2. 1 2 3 Simmons, Mike (2008). "Building the 60-inch Telescope". Mount Wilson Observatory . Retrieved 2 June 2025.
  3. ↑ Gibilisco, Stan (2002). Physics Demystified. McGraw-Hill Education. p. 515. ISBN   978-0071382014 . Retrieved 2 June 2025.
  4. ↑ Cottrell, Geoff (2023). Observational Astronomy: A Very Short Introduction. OUP Oxford. p. 13. ISBN   978-0192665515 . Retrieved 2 June 2025.


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