Cutter-Schiefspiegler
The Kutter-Schiefspiegler is a reflecting telescope primarily for sky observation, which was developed by Anton Kutter at the suggestion of Anton Staus in the 1930s for self-construction by astronomers. It represents an obstruction-free modification of the Cassegrain reflector , the mirrors of which are tilted against each other ( arranged obliquely ). This design allows imaging without shadowing from the secondary mirror and its holder. Overall, the result is a Z-shaped beam path (in contrast to the Yolo-Schiefspiegler with its x-shaped beam path) and thus a comparatively short overall length for these long-focal length instruments, but also an unusual and somewhat bulky design.
The concave main mirror throws the incident light onto a convex secondary mirror shifted from the entrance beam path, the system focus is then laterally offset behind the main mirror to make the focal plane easily accessible.
Tilting the mirrors causes the aberrations coma and astigmatism , which must be corrected. There are several options for correction:
- The convex secondary mirror can be tilted to a precise degree (anastigmatic design) and the imaging errors can be kept small thanks to a particularly long aperture ratio (ratio of focal length to lens diameter) of 1:28. Kutter recommends this two-mirror construction up to an opening of about 5 inches.
- The secondary mirror can also be corrected aspherically, according to Kutter up to a maximum of 6 inches aperture .
- A long-focal length correction lens (wedge lens) is also inserted in the beam path after the secondary mirror, this is Kutter's three-part, catadioptric design, with an aperture ratio of 1:20, for openings from 6 inches.
- A correction lens system is installed near the focus . This construction was realized by American amateurs.
- The opening error of the long focal length main mirror (f / 12) of the three-part cutter systems is corrected by elliptical correction ("parabolicising") of the spherical mirror contour during polishing.
The self-grinding of a three-part cutter system is a demanding project even for experienced mirror grinders. While the main mirror is still quite easy to manufacture, this does not apply to the hyperboleconvex secondary mirror and certainly not to the wedge-shaped, extremely long focal length correction lens.
Finished cutter mirror systems are supplied by the Belgian company Lichtenknecker, and finished cutter mirror systems by the Swiss company AOK.
The image definition and the contrast performance of a Schiefspiegler is superior to that of a Newtonian telescope or Cassegrain telescope, since there are no disruptive, shading or diffraction-causing components in the beam path, an apochromatic refractor with the same aperture is at least on par with an apochromatic refractor with the same aperture, a disadvantage can be the greater susceptibility generally present in the open Schiefspiegler construction to local, thermally induced air currents ("tube seeing "), which has a disruptive effect due to streaks and flickering in the field of view when observing at high magnifications and thus limits the ability to enlarge. Ventilating the instrument for a long time before observing it for temperature compensation and installing a fan can reduce this.
Kutter recommended its construction up to a maximum opening of 16 inches (400 mm), 12 inch cutters were built several times, Anton Kutter himself operated one in his amateur observatory.
The Messelberg observatory of Sternfreunde Donzdorf eV, (Württ.), Operates a 30 cm Schiefspiegler cutter.
literature
- Anton Kutter: The Schiefspiegler - A mirror telescope for high image definition . Bookstore Fritz Weichhardt [in Komm.], Biberach an der Riss 1953 ( [1] - 91 p.: With 31 illustrations, 1 plate, size 8 [out of print]).