Lens sag

The deflection of lenses ( lens sag English ) is an effect, which limits the maximum size of a lens telescope. Theoretically, the problem was already known in the 18th century, but was for the former lens maximum diameter of 30 cm to neglect yet. As in Europe, the first giant telescopes were built, the 1875/80 the University Observatories of Vienna and Pulkovo limited (mostly of course for reasons of cost ) to 28 or 30 inches aperture.

Clearly noticeable bend the lens was in the 1890s, when the trend in the United States for the construction of telescopes led with about 35 inches. In 1897 built the Yerkes refractor (102 cm) made ​​it difficult to deformation of the planned double-star projects, but did not affect the very successful astrophotography. Therefore no major lens telescopes more were built in the sequence, but instead constructed novel mirror telescopes.

The lens deflection is a result of the high weight of such telescope lenses and changed the calculated ideal form of composite lenses ( achromats ) so that the diffraction disk of stars not turn out exactly symmetrical. Is the telescope directed to the zenith, the deformation is symmetric, but with inclined telescope the objective lens at the lower edge are thicker than the upper edge, which produces a similar aberrations astigmatism.

Although a large lens is more thermally stable than a mirror, but their deflection is much stronger than that of an equally large concave mirror. Because a lens can be supported by the version only at the edge, while a mirror is supported along its entire surface.