Optical: systems and elements – Compound lens system – With curved reflective imaging element
Reexamination Certificate
2000-02-11
2002-07-30
Henry, Jon (Department: 2872)
Optical: systems and elements
Compound lens system
With curved reflective imaging element
C359S729000, C359S859000
Reexamination Certificate
active
06426834
ABSTRACT:
The invention relates to a method for correcting optical wavefront errors according to the preamble of claim
1
and to a telescope having the general features of claims
3
and
5
.
Optics are often concerned with point-like objects. The classical example is astronomy, where telescopes are used to register virtually point-like star images. Another, modern example is the optical linking of two satellites for the purpose of data exchange. Each satellite transmits laser light of different wavelengths to one or more remote satellites. If the intensity of the light is modulated with the data sequence to be transmitted, the term communication mode is used; if both satellites emit laser light to “find” themselves, the term acquisition mode is used. In both modes, the respective remote satellite appears virtually point-like to the other satellite, owing to the large distances (up to 70,000 km in geostationary orbit).
In both cases, the receiving optical system has to process very low light powers. The optical systems must therefore primarily have high luminous intensity. In the case of point-like objects, the irradiation intensity b at the location of the receiver is given by
b≡W/A=D
2
·sin
2
(&dgr;) (1)
where W is the incident radiant power, A is the area of the diffraction disk, D is the diameter of the entry pupil and &dgr;is the opening angle of the telescope (R.W. Pohl, Einführung in die Physik [Introduction to Physics], Volume 3, page 64). sin(&dgr;) is referred to as the numerical aperture NA of the telescope, which is derived from the F number
F
#
≡F/D
(2)
using the focal distance F according to the relationship
NA
=1/(2
*F
#
) (3)
Systems having high luminous intensity and intended for quasi-point-like objects are thus distinguished by a large diameter D or by a large numerical aperture NA or by both.
In astronomy, the approach of choosing D to be very large is adopted. The large 8 m telescopes on Mount Palomar and other astronomical centers are known. Although for reasons of technical feasibility the numerical aperture must be kept small, the gain in luminous intensity is considerable. Owing to the small numerical aperture, according to (3) the focal distance is correspondingly increased and hence also the telescope dimensions. Above a certain diameter—typically above 30 cm—it is possible only to use mirror systems since lens systems can no longer be expediently produced.
The second major advantage of reflecting mirror systems over optically refracting lens systems is their freedom from chromatic aberration. An optically corrected mirror system can thus be used both in the UV range and far into the infrared range, which is of decisive importance especially for astronomy.
On the other hand, the disadvantage of mirrors is the small field of view of only a few degrees, whereas lens systems in turn can be designed to be extremely wide-angled (cf. fisheye systems with fields of view>180°). In astronomy, this disadvantage is overcome elegantly but in a very expensive manner by enormous mechanical tracking units in order to achieve a large celestial field of view.
Historically—and also as a rule also today—lenses and mirrors having a spherical surface are produced; a production process developed to maturity over centuries. If it is necessary to position a plurality of spherical mirrors one behind the other, as in a telescope—these should have a common or optical symmetry axis (“on-axis” arrangement). If this is not the case, the result may be deteriorations in the imaging quality. In the “on-axis” mode, however, the problem of internal obscuration arises, leading on the one hand to losses of light and on the other hand—often worse—to troublesome diffraction artefacts. Diffraction artefacts adversely affect the image quality in imaging telescopes whereas, in the case of space telescopes, they lead to an undesired beam expansion and hence to critical energy losses at the remote satellite.
However, the obscuration can be avoided if the mirrors are operated “off-axis”. Consequently, a normal, rotationally symmetrical “parent” mirror is first produced but only a generally round section thereof remote from the optical axis is used. However, this results in image deterioration, the compensation of which is also the object of the present invention.
Aspherical mirrors are more difficult to produce than spherical mirrors. For very high quality requirements, however, aspherical mirrors are indispensable. In the case of aspherical shapes, a distinction is made between “conical” mirrors, such as paraboloids of revolution, ellipsoids of revolution or hyperboloids of revolution, and more complex, nonconical aspherical shapes. The degree of difficulty increases sharply in the case of the latter, particularly in the case of mirrors of large diameter and low F numbers. From F numbers of less than 2, moreover, the measurement and testing equipment has to meet very high requirements.
In satellite communication, the situation is completely different from that in astronomy. Here, primarily the telescope dimensions and the weight have to be minimized; they are essentially responsible for the high costs. A satellite telescope therefore consists predominantly of systems of smaller dimension. Mirrors are chosen since, when constructed in the highly developed lightweight construction method, they can have a significantly lower weight than optically equivalent lens systems. Lens systems are moreover very problematic for space flights since it is necessary to take expensive protective measures to prevent fogging of glass in the lenses, which is caused by the high-energy cosmic radiation.
A further advantage of mirror systems is that the volume occupied can be minimized if the mirror arrangement is “nested” in an appropriately skilled manner. However, nesting is already a necessity in the “off-axis” systems described above, but it has even further advantages:
On the one hand, a compact arrangement makes the optical system thermally more stable. Since satellites are exposed to extreme temperature differences, the following design rule is applicable: the smaller the dimension, the smaller the defocusings of the optical system which are caused by the thermal expansion of the mechanical structure. Defocusing of only a few &mgr;m can lead to unacceptable wavefront deformations.
Designers must furthermore minimize the natural vibrations of the satellite. A vibrating satellite results in a dramatic deterioration in the “pointing” stability of the optical connection: this is understood as meaning that, owing to the large distances, vibrations cause intensity variations at the location of the remote satellite. Intensity variations in turn influence the error rate of the data transmission (bit error rate) and must be compensated by an undesired reduction in the data rate. The vibrations are due to the lack of a mechanical fixed point in the orbit. Thus, each new orientation of the satellite must be mechanically opposed, i.e. it must be actively damped. This leads to control sequences which, particularly in the vicinity of natural frequencies of the structure, become very complicated. Here too, the following is therefore applicable: the smaller the size, the easier the stabilization. The manner in which vibrations are rated as critical is evident from the fact that the tolerance for the pointing stability is specified as typically 50 nanorad, i.e. a hundredth of an arc-second (!!).
There are two possibilities for transmitting sufficient optical power to the remote satellite, in spite of the small mirror diameter: powerful laser light sources are used and the numerical aperture NA of the telescope is increased according to (1).
A large NA is almost always necessary since, in addition to the pure communication and acquisition with well collimated laser light, many satellites also have to “operate” in an optically passive manner, i.e. quasi-astronomically. Thus, a satellite registers continuously selected star images according to whi
Biber Massimo
Braunecker Bernhard
Henry Jon
Leica Geosystems AG
Rothwell Figg Ernst & Manbeck
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