|
 |
|
|
|
|
|
 |
|
|
|
|
|
A new mirror telescope design of the Schiefspiegler type was developed and has been realised by several different instruments.
These telescope designs have been developed to fulfil the demands of amateur astronomers for compact unobstructed mirror telescopes.
Mirror telescopes of the Schiefspiegler type are free from central obstructions and allow observations with an almost perfect image quality, like that of high quality refractors.
Schiefspiegler telescopes are free from color abberations. They have to be carefully adjusted to compensate for image errors and to obtain maximum performance. Adjustment can be simplified by using special mirror cells and by locating adjustment screws within the telescopes plane of symmetry.
|
|
|
|
|
|
|
|
|
|
|
|
The optics of a Schiefspiegler consists of two or more mirrors with a rotational-symmetric figure. In the simplest case the mirrors are spherical and they can be produced easily by experienced opticians or even by amateurs. Therefore the costs of the optics is far less than that of a refractor lens of comparable size.
The simplest form of a Schiefspiegler was developed by A. Kutter [1-3].
It uses a tilted concave primary mirror (1) and a tilted convex secondary mirror (2).
By tilting of the primary mirror, the secondary mirror can be located outside of the incoming light and therefore avoids the central obstruction which is typical for Newtonian or Cassegrain telescopes.
This obstruction results in a light loss and which is more important – it causes diffraction of light. The latter reduces image contrast of small details, which is especially important for observations of moon and planets.
|
|
|
|
|
|
|
|
|
|
|
|
The image above displays a perspective view of the optical path (it is not exactly drawn in the plane of symmetry). The tilt of the primary mirror (No. 1) induces significant image errors (astigmatism and coma) in the central part of the image (and even more off-axis).
A defined tilt of the secondary mirror (No. 2) can produce the same image errors with opposite sign and allows for its compensation.
Unfortunately, these errors can not be compensated completely, because the optical design owns not enough degrees of freedom. The image errors can be minimised by realising Schiefspieglers with small apertures and low f-ratios (smaller than about f/18).
In its simplest form both mirrors use spherical surfaces of equal radii.
The small light power (focal ratio) of the Schiefspiegler restricts the observed targets practically to the moon and planets. In principle this telescope can also be used for deep-sky objects, but eyepieces with extremly long focal lengths are required.
Furthermore the optical layout results a long tube and therefore requires a stable mounting.
Most of the Schiefspieglers use a fixed observing place.
The new type of a Schiefspiegler- telescope avoids these disadvantages almost completely.
|
|
|
|
|
|
The optics of the Multi-Schiefspiegler consists of three mirrors. One of them is used twice which results in a total of 4 reflections. The additional degrees of freedom in the optical design allow to compensate the image errors on the optical axis almost perfectly. Even for 0.5° field of view a diffraction limited image quality is obtained.
It is possible to construct compact telescopes with medium light power (up to f/10) and of rather large apertures. These telescopes can be used for observing almost all types of astronomical objects. The compact design allows to transport it to favourable observation sites.
This optical system uses the secondary mirror twice, the latter is used for the 2. and the 4. reflection. This mirror has to be about 5-15% larger in diameter than the primary mirror (1), the exact value depends on the optical layout.
The concave primary (1) and the convex secondary mirror (2+4) have spherical surfaces and the third mirror (3) a parabolic surface. The image plane (F) is located in the vicinity of the primary mirror.
The resulting tilt of the image plane amounts to 1.2°- 2° and can be neglected visually and in photography. Image distortion is less than 1.2%, depending on focal ratio and aperture.
|
|
|
|
|
|
|
|
|
|
|
|
The numbers describe the sequence of the reflections (its a 3-dimensional view). Only the marginal light-rays within the telescope´s plane of symmetry (meridional plane) are shown.
The centre of the fourth reflection (4) is shifted with respect to that of reflection (2) along the meridional plane. This is necessary to make the image accessible to the observer (at the image plane) .
The f-ratio of the optical system is mainly determined by the focal length of the parabolic mirror. The conic constant k = 0 defines a spherical curve and k = - 1 a parabolic curve (see table below).
Three optical layouts (Design 1-3) for a Multi-Schiefspiegler with apertures of 140/150/200 mm and f-ratio f/11 were calculated by using the TCT-software of J. Sasian [4].
The corresponding diagrams and a data table for design 1-3 are given at the follwing side:
spot-diagrams
All spot-diagrams have been produced by using the winspot-software of D. Stevick [4].
A spot - diagram describes the collimation of a parallel bundle of light within the image plane. In this case it is calculated for a bundle of light rays parallel to the optical axis and for 4 tilted bundles (tilt angle 0.25° each) entering the aperture from 4 different directions.
The circle around the central spot represents the diameter of the Airy-disc.
All image errors, which result in a collimation of the rays within this circle are not detectable.
Reflection No. 4 denotes the additional reflection on the secondary mirror. The mirror diameters in brackets are necessary to ensure an unvignetted field of view of 0.5°. Due to this illuminated areas and the separation of the centres of the 2. and 4. reflection an oversized secondary mirror is needed (about 9% larger than the aperture).
By using a standard size of 152 mm for the diameter of the secondary mirror the aperture has to be restricted to 145 mm. For an aperture of 150 mm the secondary mirror should have a diameter of about 160 mm.
Since the optical system of the Multi-Schiefspiegler uses only spherical and parabolic mirrors they could be commercially produced with high quality.
Amateur telescope makers can produce the optics by their own hands and are encouraged in doing so. It has to be considered that the accuracy of each mirrors surface should be doubled compared to that of a single mirror optic to yield the same wavefront accuracy (Rayleigh criteria [3]).
Otherwise a higher amount of wavefront distortion might appear, which limits the optical performance of the telescope.
|
|
|
|
|
|
|
|
|
