The Double Gauss objective optimizations
The mathematician Gauss once suggested that a telescope objective could be made with two meniscus — shaped elements. The idea was that such a system would be free from spherochromatism. However, this arrangement has other serious disadvantages and it has not been used in any large telescope. After lot of experiments it was recognised that two such objectives mounted symmetrically about the central stop might make a good photographic objective. One of the most popular Double Gauss objectives is designed in the famous German optical company Zeiss and it was called Biotar. The Biotar objective in its basic form consists of a single positive lens, followed by a negative meniscus doublet that is concave to the rear, followed by another negative meniscus doublet that is concave to the front, and a final positive single lens. The thicknesses of meniscus components are approximately equal and the arrangement of refractive indices is often symmetric with n1≈ n6, n 2 ≈ n5 and n3 ≈ n4. This is an exceedingly powerful design form, and many high performance objectives are modifications or elaborations of this type. If the back focal length is made short and the elements are strongly curved about the central stop, fairly wide fields may be covered. Conversely, a long system with flatter curves will cover a narrow field at high aperture.
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