Why choosing a mirror telescope without central obstruction ?
The performance of a telescope is defined by ist resolution, the amount of light gathered, and its ability to image faint intensity differences of the objects observed. The latter can be described in terms of the contrast tranfer from object contrast to image contrast. A central secondary mirror within the lightpath results in a distortion of the passing wavefront of light. This effect has strong influence to the contrast transfer which is significantly reduced. That means that small intensity differences in objects of different size can not be longer distinguished in the image. The resolution is not affected seriously by the central obstruction. You can choose a refractor telescope to avoid the central obstruction of mirror telescopes and the optical performance lost. But a lens system suffers from chromatic abberations and a high effort in the production is necessary to correct this deviations of the focal length at different wavelength. For a triplett lens 6 surfaces have to be manufactured and to be centered precisely. The glass has to be of high transparency and homogenity and two or three special types of glass with different refracive index are required. This explains the high costs of large refractor lenses of high quality. The Schiefspiegler avoids this dificulties by using only 2 or more mirror surfaces which are tilted with respect to each other . The secondary can be located outside of the lightpath to avoid the central obstruction. Only this surfaces have to be produced very precisely. This reduces the costs of these mirror systems compared to that of refractors. The mirrors have to be adjusted precisely to compensate for image erors resulting from the tilted arrangement. This can be realised by a stable mechanical construction of the telscope frame and the mirror cells. The folded lightpath reduces the length of the tube.
The new Multi-Schiefspiegler
The newly and patented Multi-Schiefspiegler uses the concept of the Classical Schiefspiegler of A. Kutter and developed it further. A compact tube and focal ratios ranging from f/10 to f/15 can be realised. The image shows the optical design using three tilted mirrors. The second mirror is used twice so this results in a total of 4 reflections , the sequence of reflections is 1-2-3-4. Finally an image is formed near the primary mirror. This design results in a refractor like arrangement, the eyepiece is located at the backside of the telescope and not very far away from the center of gravity, where the tube is attached to a mount. Because you look in a direction which is inclined with respect to the incoming light a finder is always be necessary to locate the object within the image plane. This is a little bit unusual compared to a refractor but can easily be learned.
The new Multi-Schiefspiegler design allows to corect image errors also for apertures largwer than 300 mm and for more than 0.5° field of view. Because the telescope is without central obstruction and chromatic errors a high optical performanc can be dreived which is comparable to that of high quality refractors of equal aperture. The light path is folded 4 times. Therefore the tube length is reduced to about 40% of the corresponding focal length.
The new Multi-Schiefspiegler uses a medium focal ratio and can be used to observe and photograph all observable types of astronomical objects. The design results in a compact and transportable telescope of highest optical quality.
