Ramsden eyepiece
The Ramsden eyepiece is one of the two-lens eyepieces; it was conceived in the 18th century by the English optician Jesse Ramsden (1735–1800) for telescopes and microscopes. It is a further development of the Huygens eyepiece from 1670, one of the oldest almost color-pure lens systems. However, this only had a very small field of view and was not suitable for the installation of a crosshair .
That was exactly what Ramsden wanted to do with developing an eyepiece for measuring telescopes . As with Huygens, it has two plano-convex lenses, but the first lens is inverted; its flat side points towards the lens . In addition, there is an intermediate image area on the flat side of the first lens, so that a crosshair eyepiece with line marks for measuring telescopes can be used there (outside the eyepiece mount, i.e. easily accessible mechanically) .
A small disadvantage is that the exit pupil is on the flat side of the eye lens, which is why the field of view cannot be completely surveyed (the Mittenzwey eyepiece, developed around the same time, is cheaper in this regard ). This restriction can be reduced by moving the lenses closer together, but the achromatic condition is no longer met. The later Kellner eyepiece , in which the simple eye lens was replaced by a doublet ( achromat ), offers a remedy .
use
Although the Ramsden design is now considered obsolete, it is still of certain importance for inexpensive telescopes and also for simple reading microscopes for measuring devices due to its comparatively simple production . The fact that you have to bring your eye close to the eyepiece (as with the Huygens eyepiece) is particularly disadvantageous for those who wear glasses. As a simple optical system, its components are also included in some DIY kits.
Very successfully used z. B. Friedrich Wilhelm Bessel the Ramsden eyepiece for the precise measurement of comet orbits . For the 1807 comet, he added a screw micrometer to the eyepiece to optimize measurement accuracy.
literature
- Rudolf Brandt : The telescope of the star friend. Kosmos-Verlag, Stuttgart 1958, p. 22 f.
- Albert König, Horst Köhler: The telescopes and range finders . Springer-Verlag, ~ 1970/2014.
- Helmut Naumann, G. Schröder, M. Löffler-Mang: Manual components of optics: Fundamentals, materials, devices, measurement technology , chapter 10.4 (lenses and eyepieces for telescopes).