Kutter 150F24

The Schiefspiegler telescope has high focal ratio, large focal length and no obstruction. Nevertheless, in contrast with comparable refractors, the dimensions can be kept within bounds because it is a folded system. Also due to the lack of chromatism, teh Kutter is an ideal system for observation and photography of solar system objects.

The Kutter system has a concave primary and a convex secondary of similar focal length. The tilted secondary corrects for the coma introduced by the off-axis use of the main mirror. Because the secondary is convex it is also possible to correct the induced astigmatism, but at a different angle. By adding a cylindrical correction to primary or secondary, both abberrations can be almost cancelled in one system. This is an advantage over the Yolo design since the compensation of residual astigmatism can be achieved with only a relatively mild additional correction. Normally this additional correction is achieved by one or two lenses, but in this telescope design uses a toroid primary mirror.

The general design considerations and mathematics for this type of telescope are discussed in the "Schiefspiegler toolkit" article on this website.

The approximate design goal of the target telescope:

  • Primary: D=150mm, R=4520mm
  • Secondary: D=72mm, R=-4520mm
  • System: F=3600mm, Image: FoV=0.4° (26mm)
  • Dimensions: p'=e=1.4m

Version 1

The first version of this 150F24 Kutter uses a corrected (toroid or cylindrical) primary mirror, because it seemed to be easier to control than correcting the secondary. However, obtaining a smooth sphere by knife-edge testing is already a challenge at F/15, let alone to produce a smooth cylinder correction on it.

Although it was already a pleasure to use on e.g. the moon, the results left some to desire. Especially the surface smoothness of the primary has suffered considerably from the toroid correction attempts.

Version 2

The second version is based on the mechanics of the first version, but designed to use a cylindrically corrected secondary and a smooth spherical primary. Complete cancellation of astigmatism at the zero-coma secondary tilt angle would require a heavy 7 fringe correction, not so easy to achieve while preserving smoothness. So, reducing the secondary tilt results in a still acceptable coma but significantly less astigmatism. This would require only 1 or 2 fringes of toroid corresction in the secondary. The astigmatism is then fully compensated and the residual coma is comparable to the (much larger) all-spherical anastigmatic design. However, as a first-order approach the all-spherical anastigmat is tried to see how it performs in practise.

Below, myself using the version 1.