Optical: systems and elements – Compound lens system – Telescope
Reexamination Certificate
1998-09-16
2001-07-10
Spyrou, Cassandra (Department: 2872)
Optical: systems and elements
Compound lens system
Telescope
C359S429000
Reexamination Certificate
active
06259558
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a device for the directional transmission and the directional reception of modulated light waves between geostationary satellites, or respectively geostationary satellites still close to earth by means of a telescope.
BACKGROUND OF THE INVENTION
Fiber-optical communications systems have revolutionized the wire-dependent data transmission over large distances within a few years. In connection with directional radio installations, which had been dominant up to that time, systems already in service today can be considered to be superior in every respect in view of the available bandwidth. Only mobile communications are able to profit indirectly from this advance by means of efficient fixed networks, since cellular networks also need to utilize narrow-band and trouble-prone radio on a portion of the transmission path. In connection with the transmission via or between satellites, large distances still need to be overcome, which absolutely requires large transmission outputs and antennas, which in turn runs counter to the desire for systems as compact and as light as possible for space travel. For this reason efforts were undertaken fairly soon after the triumphal march of the fiber-optical communication technology to also use its advantages for optical communications in free space by means of suitable systems.
New factors in the fiber-optical communications technology, inherent in the system, have shown themselves to be limiting, in particular in connection with bridging great distances, especially the dispersion in the dielectric wave guide used for transmission, and various non-linear effects of its material. Optical communication in free space again meant the return of old limiting effects of radio technology and wire-dependent communications. Here, the loss of signal output on the transmission path and the effects of foreign signals dominated again. However, in fiber-optical communications the extreme limits of the energy of a symbol used for transmission are not expressed by the terms describing the phenomenon of thermal noise, but by means of photons per bit.
For example, at an error quotient of 1/1,000,000,000, 10.5 photons per bit are inherently required for the assured transmission of data by means of intensity modulation (J. S. Senior, “Optical Fiber Communications, Principles and Practice”, second edition, Prentice Hall, pp. 469 to 471).
Better results can be achieved with pulse-position modulation, as well as various coherent techniques, in particular methods with homodyne transmission. The best realized results were obtained by means of homodyne superimposition (less than 30 photons per bit). Since there is a clear requirement for low energy consumption for space-based systems, an optical system for data transmission between distant geostationary satellites should transmit and receive light waves by the largest possible and very accurately aligned aperture. This, in turn, can only be realized, starting at a defined size and while maintaining a low weight, in the form of a reflecting telescope. Reflecting telescopes in the so-called coaxial form are known in numerous designs, the systems in accordance with Gregory, Cassegrain and Schmidt should be mentioned (Eugene Hecht, “Optics”, second edition, Addison-Wesley Publishing Company, Reading, MA, USA, pp. 197,198).
Common to all of them is the system-related disadvantage of the partial central covering of the aperture by the collecting mirrors and their suspension devices. In this case a compromise between mechanical sturdiness and losses because of covering of the aperture must be found.
Generally, an additional screen is required, which prevents the reflection of scattered light, which is encouraged by the collecting mirror and its suspension, in the direction of the light to be received. The simultaneous use of such a telescope for radiating a light wave as well as for receiving an oppositely entering light wave generally results in significant disadvantages, since the said collecting mirror and its suspension reflects a portion of the high output transmitted light in the direction of the simultaneously entering light wave and results in interferences because of superimposition. Accepting great losses regarding the imaging quality, this problem can be bypassed by the use of an oblique reflecting telescope proposed by Kutter. However, the mentioned imaging errors result in the waste of valuable transmission output.
OBJECT AND SUMMARY OF THE INVENTION
It is therefore the object of the invention, which will be described in what follows, to overcome the mentioned disadvantages of the prior art by employing an oblique reflecting telescope, which is free of imaging errors but permits a simultaneous bidirectional use for transmitting and receiving a light wave.
The said oblique reflecting telescope is constructed from a plurality of mirrors with refractive power and has a surface which provides images in a refraction-limited manner which, in contrast to the system developed by Kutter, have a common optical axis. The characteristic feature of the optical systems lies in that the surfaces of the individual mirrors can be imagined as partial surfaces cut out of the axis-symmetrical mirrors arranged on a common optical axis. Cutting out partial surfaces creates the actual structure of an oblique reflecting telescope, but is also necessary for preventing mutual obscuration and because of the impossibility of the intersection of mirror surfaces.
A further characteristic of the invention lies in the structure of the housing containing the mirrors of the telescope. To the extent necessary for maintaining the imaging quality, the relative position of the mirrors in respect to each other is stabilized by a separate support frame made of Invar or a comparable material. This means can be omitted when using a mechanically and thermally sufficient stable housing.
In accordance with the invention, the housing for the oblique reflecting telescope is made of a particularly light and thermally stable structure. This consists of a honeycomb structure known from aircraft manufacturing, which is closed off at both sides by a material with poor thermal expansion properties. In addition to the proven Invar, the considerably lighter glass-ceramic material Zerodur® has been used for the first time in the present invention for these layers which, compared with Invar, in addition shows considerably less thermal expansion. Furthermore, a plastic material was used for the first time for the same purpose. It is a very temperature-resistant thermoplastic material (Peak), which is mixed with a large proportion of stabilizing fiberglass sections, which see to sufficient mechanical and thermal stability.
In addition, the housing for the telescope is designed in such a way that a screen, which is not as urgently necessary because of the lack of a collecting mirror, has already been integrated over a relatively short length.
An essential advantage of the invention lies in the low weight of the telescope, which is seated so it is rotatable around one or several axes, in particular when it is possible to omit a supporting frame inside of the housing because of the advantageous static properties of the housing, which has been assembled for reasons of weight saving from the plates manufactured in the sandwich structure with the aluminum honeycomb mentioned, which is possible in particular if, in accordance with a further characteristic of the invention, the mass of the mirror body used has been reduced to a minimum, determined by the strength requirements, by recesses made by means of bores on its side facing away from the light.
Because of the employment of an oblique reflecting telescope, which provides error-free images, a further advantage lies in that it is possible, corresponding to telescopes which mainly operate with lenses, to use more than two mirrors with refractive power, by means of which it is possible to transfer the advantages, which can be achieved with refractive systems completely equipped
Fischer Edgar
Herren Andreas
Sanvido Saverio
Chambers Guy W.
Contraves Space AG
Robinson Mark A.
Spyrou Cassandra
Townsend and Townsend / and Crew LLP
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