GB2128733A

GB2128733A - An adaptive optical system

Info

Publication number: GB2128733A
Application number: GB08326088A
Authority: GB
Inventors: Dr Rundolf Protz; Reinhard Czichy
Original assignee: Messerschmitt Bolkow Blohm AG
Current assignee: Airbus Defence and Space GmbH
Priority date: 1982-10-14
Filing date: 1983-09-29
Publication date: 1984-05-02
Also published as: ES8405959A1; GB8326088D0; ES526502A0; DE3238108A1; FR2534701A1

Abstract

The adaptive optical system serves to correct dynamic image defects, particularly those caused by atmospheric disturbances, and includes a correcting mirror 6 having a controllably deformable surface, an opto-electrical sensor 1 within an image plane of the system and, connected thereto, a control unit 5 for the correcting mirror 6. The sensor 1 is in the form of an image definition sensor having high local resolution with which spatial and temporal variations of the image are picked up in the form of definition signals. Control signals for actuating the correcting mirror 6 are then formed from the definition signals. <IMAGE>

Description

SPECIFICATION An adaptive optical system This invention relates to an adaptive optical system for correcting dynamic image defects particularly those caused by atmospheric disturbances, said system including at least one correcting mirror having a controllably deformable surface, an opto-electrical sensor within an image plane of the system, and a control unit for the correcting mirror, which unit is connected to the sensor.

Of the previously-known adaptive optical systems (see, for example, John W. Hardy, "ACTIVE OPTICS : A NEW TECHNOLOGY FOR THE CONTROL OF LIGHT"; Proceedings of the IEEE, Vol. 66, No. 6, June 1978) only those with a so-called wave front correction have gained special prominence. In this respect, mirrors having a deformable surface which are equipped, for example, with piezoelectrical adjusting members have proved themselves as a wave front correction element. In this respect, serving as a sensor element is a so-called wave front detector, from which the adjusting members of the mirror are supplied with adjusting signals by way of a computer so that wave front deformations are corrected.Since, with such systems which detect and correct the wave fronts, it is not the quality of an attained image but the deformation of a wave front arriving in the optical system which is measured, a punctiform reference light source, for example a star, is necessary. The efficacy of wave front sensors in conjunction with an incoherent large-area source of light, such as the surface of the sun, is at least doubtful.

The above-mentioned document also discloses an adaptive optical system in which, by means of an intensity measurement, conclusions are drawn regarding the image definition and the disturbance of the wave front.

With the aid of a mirror having a controllably deformable surface and a prior-connected controllable diaphragm a correction of wave front deformations is undertaken in a similar manner. This method, too, is not suitable for incoherent large-area sources of light, as represented for example by the surface of the sun, since no clear connection exists between the intensity increase and the image quality.

An object of the invention, is therefore, to provide an adaptive optical system for dynamic image defects, particularly those caused by atmospheric disturbances, with which above all large-area sources of light can be observed.

Pursuant hereto, the present invention provides an adaptive optical system for correcting dynamic image defects, particularly those caused by atmosperic disturbances, said system including at least one correcting mirror having a controllably deformable surface, an opto-electrical sensor within an image plane of the system, and a control unit for the correcting mirror, which unit is connected to the sensor, characterised in that the sensor is in the form of an image definition sensor having high local resolution, with which spatial and temporal variations of the image are picked up in the form of definition signals and control signals for actuating the correcting mirror are formed from the definition signals.

In the system in accordance with the invention, the properties of a large-area source of light or a corresponding image are specifically utilised for image optimisation. By means of a high-resolution image definition sensor, for example a diode array, it is not, as in the case of the previously mentioned method, the total intensity but the intensity distribution in the image plane which is ascertained. Then electronically in accordance with known methods a measure of the image definition is obtained therefrom. The control of the adjusting members of the correcting mirror can likewise be effected in accordance with known methods, for example in accordance with the "trial and error" principle.

The adaptive optical system of the invention will be described in more detail with reference to the accompanying drawings, in which: Figure 1 is a diagram of a first embodiment of an adaptive optical system of the invention having a diode array with parallel outputs of the signal lines; Figure 2 is a diagram of a second embodiment of an adaptive optical system of the invention having a self-scanning diode array; Figure 3 is a diagram of a third embodiment of an adaptive optical system of the invention for use in observing the sun.

In the first embodiment shown in Fig. 1, the light coming from a telescope (not shown) is reflected by way of an adaptive mirror 6 and by way of a beam divider 7, a part thereof is deflected onto a planar or twodimensional diode array 1. The intensity distribution within this image is converted by the diode array 1 into electrical signals. These intensity signals are squared in electronic multipliers 2 and added up in a summing amplifier 3, whereby the definition signal S =7 12(x,y)dxdy is formed. The adaptive mirror 6 may, for example, be of the kind described at the beginning of the article of J.W Hardy and may, for example, have piezo-electrical adjusting members. In other words, the mirror 6 may be subdivided into a plurality of mirror elements which are movable perpendicular to the mirror plane by way of adjusting members.

The individual adjusting members of the mirror 6 are simultaneously shifted sinusoidally with a small amplitude, for example 0.1 ym, by means of an electronics control unit 5. In a manner which is phase-dependent upon this shift, a change in the image defini tion is established at the detector 1 and a corresponding control signal is fed to the adjusting members by way of the electronics control unit 5. The individual adjusting members of the adaptive mirror 6 are distinguished by different wobble frequencies with which the adjusting members are shifted sinusoidally in the above-mentioned manner. These frequencies are discriminated into definition signals by phase-coupled band-selective amplifiers 4, for example so-called lock-in amplifiers, and subsequently supplied as a characteristic adjusting signal to the corresponding adjusting member.

In the second embodiment shown in Fig. 2, control of the individual adjusting members of an adaptive mirror 1 6 is effected in a timesequential manner with the aid of a selfscanning diode array 11. In this respect, the adjusting members are shifted individually temporally one after the other, out of their neutral position firstly in the positive and subsequently in the negative direction and in each case the image is subsequently read out by the diode array 11. In a subsequent computer 12, for both positions of the respective adjusting member the definition function is numerically calculated and the respective definition signals stored. The difference between the two definition signals then yields the adjusting signal for the respective adjusting member of the adaptive mirror.This computing and adjusting procedure is performed continuously for all the adjusting members.

The two methods described for controlling the adaptive signals are independent of the coherence properties of the light source. Also, in this respect the spatial extent plays no part.

It is evident from the fundamental construction of the above-described adaptive optical systems that, in contrast to the wave front correction procedures mentioned at the beginning hereof, adjusting problems are of subordinate importance.

Fig. 3 shows an adaptive optical system which is suitable for observing the sun and which is arranged in a parallel beam path between the exit window of a telescope 30 with an adaptation lens 31 and the image plane 32. To correct the image, this is passed by way of a collecting lens 33 and a beam divider 27 and, with an intensity of about 5 percent, reaches a detector 21 having high local resolution. In a computer 22 which is connected subsequent to the detector 21, in accordance with one of the previously described methods, disturbances of the image definition and, by way of a correlation calculation, image shifts are ascertained. By way of an electronics control unit 25, image shifts are then compensated by the mirror 28, which has a mirror surface which is tiltable about two axes and which is moved by means of piezo-electrical adjusting members. Any lacks of image definition caused by disturbances are corrected by means of the adaptive mirror 26 in accordance with the exemplified embodiments shown in Figs. 1 and 2.

Claims

1. An adaptive optical system for correcting dynamic image defects, particularly those caused by atmospheric disturbances, said system including at least one correcting mirror having a controllably deformable surface, an opto-electrical sensor within an image plane of the system, and a control unit for the correcting mirror, which unit is connected to the sensor, characterised in that the sensor is in the form of an image definition sensor having high local resolution, with which spatial and temporal variations of the image are picked up in the form of definition signals and control signals for actuating the correcting mirror are formed from the definition signals.

2. An adaptive optical system as claimed in claim 1, characterised by having respectively, arranged one after the other in the parallel beam path of the system, a first mirror having a mirror surface which is tiltable about two axes and a second mirror which is subdivided into a plurality of mirror elements which are movable perpendicularly to the mirror plane by way of adjusting members.

3. An adaptive optical system as claimed in claim 2, characterised in that the individual mirror elements of the second mirror are simultaneously periodically moved with different frequencies, in that the different frequencies are discriminated from the definition signals and in that in each case control signals for the associated adjusting member are formed from the signals discriminated in this way.

4. An adaptive optical system as claimed in claim 2, characterised in that the individual mirror elements are shifted by means of the adjusting members sequentially out of their respective rest positions by the same amount firstly in the positive direction and subsequently in the negative direction, and in each case a definition signal of the total image is calculated and a control signal for the respective adjusting member is formed from the difference of the two definition signals.

5. An adaptive optical system as claimed in any preceding claim, characterised in that continuously by means of a correlator the image shift between each two points in time is ascertained and an adjusting signal for the first mirror is formed therefrom.

6. An adaptive optical system substantially as hereinbefore described with reference to and as illustrated in Fig. 1, Fig. 2 or Fig. 3 of the accompanying drawings.