Optical: systems and elements – Diffraction – From zone plate
Reexamination Certificate
1998-04-20
2001-04-17
Chang, Audrey (Department: 2872)
Optical: systems and elements
Diffraction
From zone plate
C359S566000, C359S399000, C359S418000, C244S003160, C244S003170
Reexamination Certificate
active
06219185
ABSTRACT:
FIELD OF THE INVENTION
This invention relates in general to earth observation and astronomical observation from satellites and, in particular, to a large aperture diffractive telescope for performing such observations from intermediate or geosynchronous earth orbit. The invention pertains especially to a space-based diffractive telescope having a relatively large aperture objective lens and a separate spaced-apart eyepiece.
BACKGROUND OF THE INVENTION
Present space-based earth observation is generally from satellites in low earth orbit. This low earth orbit observation is usually accomplished with reflective telescopes. Since the position of such a low earth orbit satellite is continually changing with respect to any location on the earth's surface, any area of the earth's surface can only be viewed by the satellite for a brief time as the satellite passes over the particular area of interest on its orbit. Furthermore, if the area of interest on the earth's surface does not come within the field of regard of the satellite within an acceptable period of time, the satellite must have the capability of substantially modifying its orbit to pass over the area of interest if the desired observation is to be obtained.
A telescope in geosynchronous earth orbit, in contrast, can observe any position within its field of regard whenever desired and for as long as necessary. However, geosynchronous earth orbit is
100
times higher than low earth orbit so that to get the same resolution from geosynchronous earth orbit as from low earth orbit, the aperture of the telescope needs to be 100 times greater. Sub-meter earth observation from geosynchronous earth orbit and high resolution astronomy require space telescopes having apertures in the 10's of meters. It is apparent that a space telescope having such a large aperture would be very advantageous.
In the past, considerable effort has been spent attempting to design reflective telescopes of such size, but two basic difficulties have arisen: achieving and maintaining sub-wavelength tolerances over the large apertures, and designing telescopes which are light and compactly packaged for launch and eventual deployment into orbit. The telescope must be launchable (i.e., light weight and folded-up at launch) yet deploy to optical precision tolerances (fractional wavelengths). This has not yet been accomplished.
One prior concept concentrated on optical precision by using rigid one-to-three meter mirror segments in an aperture array wherein launchability concerns lead to the design of unfoldable segmented sparse aperture arrays. Another concept concentrated on reducing mass and improving deployability by employing a thin membrane mirror. However, optical precision concerns demand the presence of high frequency (space and time) adaptive optics.
SUMMARY OF THE INVENTION
The present invention is a lightweight, easy-to-deploy, full-aperture space telescope which solves both the launch and precision problems associated with conventional concepts. The present invention overcomes the difficulties inherent in the reflective telescope design for geosynchronous earth orbit (or any planetary orbits higher than the equivalent of low-earth orbit) by providing a diffractive telescope including two separate spacecraft located far apart but acting together as an eyeglass. One spacecraft is an eyepiece which is similar to a conventional, one-meter-class, space telescope and which would be too small to be useful from geosynchronous earth orbit by itself. The second spacecraft is comprised of an objective lens which functions as a magnifying glass and provides the necessary large aperture for observation by the eyepiece from geosynchronous earth orbit.
The present invention solves the problem of obtaining a high tolerance over a large aperture by the use of a transmissive optic for the large-aperture primary optic instead of a reflective optic. When a reflective surface is used to bend light through an angle x, the optical path error induced by any small surface ripple is (1+cos x) times the size of the ripple. If, instead, a transmissive surface is used, the optical error multiplier is (1−cos x). For small angles, the optical errors induced by ripples in transmissive surfaces are thus smaller than those for reflective surfaces by a factor of x
2
/4. Expressed in terms of the tolerable ripple size, transmissive optics have a 16 times F# squared advantage over reflective optics. This advantage can be exploited by adopting a high F Number (hereinafter F#) design. For example, at an F# of 100, a transmissive primary optic tolerates ripples 160,000 times larger than can be tolerated by a reflective primary optic. In physical terms, a typical 300 angstrom tolerance for a visible-light reflector grows to 0.5 cm for a transmissive design. This huge advantage greatly eases the practical implementation of large space telescopes.
Space implementation difficulties and launch considerations are solved by using a very thin, flexible membrane for the transmissive primary optic of the present inventions. Membranes (only 10's of microns thick) are extremely light, easily packaged for launch, and potentially simple to deploy. The present invention gains these advantages by using diffraction rather than refraction as the basis of the transmissive magnifying optic. Diffractive lenses can be implemented with very thin membranes, while refractive lenses are much thicker, leading to systems which are more rigid, and harder to package and deploy.
The combination of the large F# required for the primary optic and its large aperture would require that a telescope have a focal length measured in kilometers. Placing a rigid, Earth-like, telescope of this length in space would entail severe weight, packaging and deployment problems. These difficulties are eliminated by separating the telescope into two spacecraft, the objective lens and the eyeglass and the eyepiece. Each are readily emplaced in space; the objective lens by virtue of its membrane implementation, and the eyepiece because of its more conventional (Hubble-Telescope-like) dimensions and rigid construction.
The eyeglass telescope allows 24-hour a day continuous high-resolution earth observations to be performed because it would be in geosynchronous earth orbit (GEO) (40,000 kilometer (km)). Given present-day optics, such resolutions have been only possible from low-earth orbit (LEO) (100-1000 km), and such orbits do not allow the observer to park on the object for longer than a few minutes. The observation times of such LEO-based imagery follow a schedule dictated by the orbit that is easy for other parties to predict. The eyeglass telescope of the present invention is also applicable to earth observation from mid-range earth orbit (5000-10,000 km) or for orbiting observation of other bodies of the solar system.
It is therefore an object of the present invention to provide earth and/or astronomical observation.
Another object of the present invention is to provide a high-image-quality space telescope having a relatively large aperture.
These and other objects and advantages and features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings wherein like or similar elements are identified by the same reference characters in the several views.
REFERENCES:
patent: 4131791 (1978-12-01), Lego
patent: 4453224 (1984-06-01), Crooks
patent: 5548439 (1996-08-01), Smith
Chang Audrey
Daubenspeck William C.
Gottlieb Paul A.
O'Dwyer Thomas
The United States of America as represented by the United States
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