Yolo-Schiefspiegler

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The Yolo Schiefspiegler is used mainly by Astro amateurs astronomical telescope ( reflector ) with two mutually tilted, long focal length concave mirror n whose light path crossed, so as X-shaped, extends. This design allows an image without shadowing from the secondary mirror and holder (unobstructed beam path), which improves image definition and contrast performance compared to conventional mirror telescope systems according to Newton or Cassegrain .

As a two-mirror reflector, it can, due to its principle, exceed the image quality of expensive, apochromatic refractors with comparable openings with only low production costs, but with high demands on an adequate quality of the two optical surfaces . Yolos have so far been manufactured with an aperture of up to 12 inches (~ 30 cm) , a German star friend has a 20-inch Yolo under construction.

The Yolo was designed in the early 1960s by the US mechanical engineering professor Arthur S. Leonard and named by him after his favorite California vacation area, Yolo County (near Sacramento).

Beam path in the Yolo-Schiefspiegler

Both mirrors are designed as very long focal length , concave spherical spheres (concave mirror with N ~ 20-40), which are quite easy to produce and can be checked individually during grinding and polishing, but not that easy because of the long focal lengths and the large measuring length required are to be checked. The Yolo can be an attractive alternative to the Newtonian telescope for more experienced amateur mirror grinders . However, the low convergence of the beam path requires a large secondary mirror with about 60 to 80% of the main mirror diameter, depending on the design of the overall system, the resulting "top-heaviness" compared to conventional mirror systems with smaller secondary mirrors should be taken into account when the tube construction is stable.

The ingenious simplicity of the optical design, the free configurability of the Yolo and the possibility of self-assembly of this high-performance mirror system make, in addition to the high image quality, the great attraction of this type of telescope. The angle and distance conditions between the mirrors according to Leonard must be observed.

Tilting the mirrors causes serious imaging errors, namely coma and astigmatism , which must be corrected. Coma is already minimized by Leonard's design and the choice of the correct tilt angle for both mirrors.

In the case of larger, more powerful systems, the opening error may also have to be corrected.

There are three ways to correct the remaining astigmatism:

  • One of the two concave mirrors, usually the secondary mirror, can be toroidally braced by a spring . This adjustable tension, when correctly adjusted at high magnification on the star, results in an opposite astigmatism that compensates for the error.
  • Both mirrors can be installed in clamping brackets so that the toroidal correction can be distributed over both optical elements and thus more favorable optical and mechanical conditions can be achieved.
  • Instead of a spherical mirror, a toroid , i.e. a toroidally ground secondary mirror, is used. With this mirror, the vertical and horizontal focal lengths are different. Such a mirror can be made from a spherical mirror by longitudinal polishing that extends the focal length and transverse polishing that shortens the focal length. The toroidal correction can also be achieved with the help of a clamp mount, in which the pre-ground secondary mirror is installed, through further processing up to polishing. Finally, the result can be checked in the finished Yolo system in autocollimation .

The opening error is compensated for by corrective elliptical polishing of the spherical mirror shape of one or both mirrors.

The box-shaped design of the Yolo is similarly unusual and bulky in appearance as the Kutter-Schiefspiegler , the length is similar, but the resulting effective focal length is significantly shorter. Focal ratios of 1:10 to 1:15 are common . H. the Yolo can be made with a smaller aperture than the Kutter-Schiefspiegler (usually 1:20 or more) and thus has a greater light intensity , which makes it more universally usable for sky observation. In addition, in contrast to the larger, three-part, original Kutter Schiefspieglern, the Yolo neither requires a convex secondary mirror that is difficult to test, nor a difficult-to-produce, extremely long-focal-length, wedge-shaped correction lens that can itself bring chroma into the system. Occasionally "faster" Yolos with up to f / 8 were built, the then increasing optical errors of higher order and their correction make such Yolo aplanas into demanding building projects.

The high contrast performance, the inherent color purity and the long focal length system predestine the Yolo for observation with high magnification with a small field of view, preferred observation objects are therefore planets, moon, double stars, globular clusters and other high-resolution celestial objects. In practice, a third mirror is required for observation close to the zenith, the flat zenith mirror to be mounted in the beam path in front of the eyepiece .

The Solano telescope according to Arthur S. Leonard is the tri-mirror version of the Yolo Schiefspiegler, with a third, extremely long focal length, convex auxiliary mirror near the eyepiece, which also takes on the function of the zenith mirror. In the Solano all surfaces are spherical, no toric correction is necessary.

Because of their half-open tube construction, the Yolo and Kutter, like all classic, centric mirror systems, such as Newton and Cassegrain, often suffer from tube seeing in practical use . Small temperature differences of the telescope components near the optical path, especially the mirrors themselves, cause air streaks, which often limit the maximum magnification that can be sensibly used. This disruptive effect can be minimized by ventilating the telescope for a long enough time before observation, by forced ventilation using fan (s) and an open lattice tube construction.

Yolo Schiefspiegler are better known among astronomical amateurs in the USA than in Europe, where the Swiss SAG led by Herwin G. Ziegler in particular promotes Yolo self-construction and promotes it through self-assembly groups.

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