Lens deflection

The deflection of lenses (English lens sag ) is an optical deformation of lenses. The lens deflection is a result of the heavy weight of large telescope lenses and changes the calculated ideal shape of the assembled lenses ( achromatic lenses ).

The effect limits the maximum size of a lens telescope . Due to the changed ideal shape, the star's diffraction disks are no longer exactly symmetrical. If the telescope is directed towards the zenith , the deformation is symmetrical, but when the telescope is tilted the objective lenses are thicker at the lower edge than at the upper edge, which creates an aberration similar to astigmatism .

A large lens is thermally more stable than a mirror, but its deflection is much greater than that of a concave mirror of the same size . Because a lens can only be supported by its frame at the edge, while a mirror rests on its entire surface.

history

Theoretically, the problem was already known in the 18th century, but was still negligible for the lens diameters of 30 cm at that time. When the first giant telescopes were built in Europe , the university observatories of Vienna and Pulkowo in 1875/80 were limited to 28 and 30 inch apertures (mainly for reasons of cost, of course) .

The bending of the lens became clearly noticeable in the 1890s, when the trend in the USA led to the construction of telescopes over 35 inches. In the case of the Yerkes refractor (102 cm) built in 1897 , the deformation made the planned double star projects more difficult , but did not impair the very successful astrophotography . As a result, larger lens telescopes were no longer built, but instead new types of mirror telescopes were constructed.

literature

• J. Flügge: Guide to geometrical optics and optical computing (205 p., Chapter 4.2), Göttingen 1956
• J. Bennett, M. Donahue, N. Schneider, M. Voith: Astronomie (1170 p., Chapter 6.2), Ed .: Harald Lesch, 5th edition (Pearson-Studienverlag, Munich-Boston-Harlow-Sydney-Madrid 2010