--2--
The Schiefspiegler is a reflecting telescope of high definition for visual and photographic work on the moon, the planets and double stars. By avoiding the additional diffraction-effects on the spiders and secondaries or flats of all centric reflectors the original high defi- nition of a long focus mirror is preserved in it.
The fundamental system of the Schiefspiegler is the Cassegrainian, but not in the classical form with a short focus (1:3 - 1:6) paraboloidal main mirror and the usual 3 or 4 times amplifying ratio by a convex- hyperboloidal secondary, but a device which gets its final ratio of aperture 1 : 20 by the low primary's focal ratio 1:12 and the unusual low amplifying ratio A = 1,667. This little "optical round about way" preserves the favourable features of the long focus primary and allows the application of spherical curves for primary and secondary until a telescope's aperture of 9″ ( 230 mm). For telescopes with more than 9" the spherical aberration is eliminated by elliptical deformation of the primary ( 0,56 of a full parabola, i.e. 0,56 y12/r1, while the secondary keeps its spherical curve in any case.
Besides the ratio of aperture for the primary and the amplifying ratio A there is another parameter, which determines the dimensions of the fun- damental system: The distance between the center of the primary and the second focus b (v. fig.1). Expressed in fractions of the primary's focal length f1 we find a convenient value for b = 0,165f1. All other dimensions will result from the well known cassegrainian formulae in fig.1.
The numerical values of the radii of curvature of primary and secondary of above mentioned fundamental system are approximately equal, i.e., the condition Petzval is practically accomplished and a flat field of view guaranteed. Of course it is possible to make the r.o.c. of pri- mary and secondary absolutely equal, but the overall length of the te- lescope will then become a little longer. The ratio of aperture of the primary should not exceed in any case the value 1:12, but a lower value ( f.i. 1:13 - 1:14) simplifies the later correction of the off-axis faults yet results in a considerable lengthening of the system. Above mentioned design has proved optimal. For special,effects, where a very long focal length is desired ( photographic lunar and planetary work) the amplifying ratio A can be enlarged till 2 or 2,5 , but the really flat field of view is then limited to 25 - 28 minutes of arc.
Above mentioned design is impossible for the classical axial type of the Cassegrain, because the diameter of the secondary and with it the damages of definition become too large.