GB2182783A - Improvements to thin film mirrors - Google Patents

Abstract

Thin film mirrors formed by stretching a diaphragm of reflecting material over a usually circular frame and adjusting the curvature by lowering the air pressure on one side of the diaphragm suffer from defects caused by changes in barometric pressure, wind forces and lack of uniformity of the membrane material. This invention enables correction of these defects by subjecting a diaphragm to localised electrostatic forces which in the limit can replace air pressure and/or provide a dynamically changeable surface curvature. The electrostatic forces may be applied by a series of concentric ring electrodes. The segments and/or the diaphragm may be divided into segments such that independent potential feeds may be applied to different areas of the diaphragm. <IMAGE>

Description

SPECIFICATION Improvements to thin film mirrors It is known art to form a reflecting mirror by stretching a thin film of (aluminised) material over a circular hoop or drum and adjusting the curvature by lowering the air pressure on one side of the diaphragm. Such mirrors have the advantage of low cost construction but are affected by external influences such as changes in barometric pressure, wind forces, etc., and by internal defects such as anisotropy of the diaphragm material, which may cause the curvature to vary with angle, producing astigmatism. Also the curvature which a perfectly circular, perfectly isotropic, diaphragm takes up when subjected to a constant uniform pressure is closer to a catenoid than a paraboloid which is usually required.

The object of this invention is to correct some or all of these defects by subjecting the diaphragm to additional localised forces which may be preset or servo-controlled and in the limit to replace the differential air pressure by electrostatic attraction should conditions arise, e.g. in space, where air pressure is not naturally present or is otherwise undesirable.

In its simplest embodiment the invention takes the form of a series of conducting concentric stationary rings adjacent to one side of the diaphragm by means of which electrostatic forces may be exerted on the diaphragm to modify the curvature by creating electric fields between the rings and conducting diaphragm.

By way of example, the conducting rings may be formed on a flat or curved rigid circular non-conducting sheet with a raised lip by spraying graphite or metal conductor through a mask or by etching a copper laminate in the manner of printed circuits. The reflecting diaphragm, possibly made of aluminised polyethelene terephthalate, is then stretched over the raised lip against a sealing ring and held rigidly or by radial tension springs. Air is exhausted from the space between the diaphragm and the backing until the basic curvature is formed. The field strengths are chosen so that the combination of diaphragm elasticity and tension, differential air pressure and electrostatic forces produces the required curvature by means of a stable balance of forces using fixed graded voltages on the concentric conducting rings.

Should it not be possible to achieve stability in this simple way because of the combination of material properties, size and curvature, the electric fields may be modified by servo control of the voltages on the rings to linearise the normally inverse-square-law forces, sensing the position of the diaphragm with respect to the rings by capacitive, optical or other well-known methods and adjusting it, by variation of the applied voltages, to predetermined positions.By way of example, a small area of each ring may be isolated and the capacitance between it and the diaphragm used as one element of a bridge fed with high frequency alternating current, the output of the bridge when amplified and phase-sensitively rectified is used to augment, or reduce, the polarising voltage and hence the electric field between the diaphragm and the ring until the diaphragm is positioned correctly and the bridge balanced, as shown in Fig. 1. By these means it is possible to dispense entirely with differential air pressure as a means of control of curvature should this be desirable, as well as enabling a clbse approximation to parabolic or any other desired curvature to be obtained with an isotropic diaphragm or one with uniform edge tension.

Further correction is possible if the concentric rings are each divided into a number of segments each separately sensed and controlled as in the previous paragraph.

Provided the diaphragm position sensing takes place within a known frame of reference, e.g. a fixed backing plate, then correction can be made for defects which arise from lack of circular symmetry, e.g. diaphragm anisotropy or gravity. A microprocessor arranged to read each position sensor in sequence and adjust the appropriate control voltage on a capacitor supplying the appropriate segment would keep the diaphragm in any desired curvature as determined by the software program.

A further sophistication can be introduced whereby, to counteract optical defects introduced dynamically by vibration or movement of the atmosphere, each segment can have a fast-reacting closed loop which will stabilise its position. Depending on the accuracy and speed of response required this can be achieved by a fast scanning microprocessor as above or by a separate hardware closed loop for each segment.

If the individual position sensors are replaced by a very fast optical scanning mechanism taking as its reference a source of light outside the atmosphere, e.g. a bright star, as reflected sequentially by the segments of the mirror diaphragm opposite the control segments it should be practicable to effect some improvement to the degree of deviation of the image of the star over at least part of the spectrum of atmospheric turbulence frequencies.In such an application of high sophistication, there could be advantage in making the control double-sided or "push-pull" and hence linear in the manner of the "constant charge" electrostatic loudspeaker (B.P. 815978) where the front electrode in the optical path is transparent, by way of example being made from a sheet of glass with conducting electrodes, corresponding to the rear segments, formed by evaporating stannous chloride on the inner surface through a mask. To achieve a linear relationship between electric field and force on the diaphragm the optical reflecting coating must be of high electrical resistance to prevent the redistribution of charge or alternatively split into conducting segments corresponding to the fixed electrodes, each isolated from the adjacent segments by high resistance elements.Such an arrangement would have negligible effect on the coherence of the reflected optical image, except as determined deliberately by the control elements in causing relative movement of sections of the diaphragm. Each section has its own control system in the form of an amplifier with a balanced output connected to produce a positive or negative directional electrostatic field between the stationary electrodes and with a source of high voltage producing a constant static charge on the mirror diaphragm with respect to the mean stationary electrode potential as shown in B.P. 815978.Overall control is achieved by scanning the amplifier inputs, e.g. by CMOS analogue switches, to correspond with the position of the aforementioned optical sensor, applying feedback to reduce the sensed error of position of the reference and holding the resulting control voltage on memory between successive scans. The basic curvature of the diaphragm is obtained by suitably biasing the electrostatic field between each electrode pair.

Concerning the previous paragraph, the compensation and enhancement of images distorted by atmospheric turbulence, etc., is a very complex subject and an ambitious goal, which will involve the acquisition of knowledge and understanding of the problem. The solution is likely to involve very high speed data acquisition and processing in order to produce control signals which will tilt the reflector diaphragm or sections of it in the desired directions. The relevant section of this patent application does not purport to be a solution to this problem, but only to provide the basic tool in the form of a rapidly controllable mirror whose detailed surface profiles can be instantly changed under suitable control.

Claims (11)

1. A mirror formed by a thin sheet of material (diaphragm, film) in which the curvature for optical purposes is formed wholly or partially by electrostatic forces distributed in a suitable manner over its surface.

2. A mirror as in Claim 1 in which the electrostatic forces are applied to modify the curvature produced by a differential gas pressure between the film and the backing plate.

3. A mirror as in Claims 1 and 2 in which the electrostatic forces are applied from a backing plate by means of concentric ring electrodes each maintained at a suitable fixed potential with respect to the film which is rendered conducting if necessary to provide a force over the area of film adjacent to it which produces the desired curvature in conjunction with the restoring force of the tension in the film.

4. A mirror as in Claims 1 and 2 and 3 in which the concentric rings are divided into segments each maintained at a suitable fixed potential with respect to the film.

5. A mirror as previously claimed in which the fixed electrodes are disposed in any suitable positions in order to produce desired optical characteristics or compensate for defects in material characteristics.

6. A mirror as previously claimed in which the coating on the film is divided into sections corresponding to those of the fixed electrodes with provision for independent potential feeds to the sections.

7. A mirror as previously claimed in which the electrostatic fields are servo controlled by sensing the position of the diaphragm by any well-known means and adjusting the voltage between the controlling electrodes to maintain or vary this position as required.

8. A mirror as in Claim 7 in which the sensing and voltage adjustments are made sequentially and held for most of the cycle on storage means to minimise the use of equipment and power.

9. A mirror as in Claims 1 and 2 in which there is a set of fixed front electrodes as well as rear electrodes to give push-pull control the front electrodes being optically transparent.

10. A mirror as in Claim 9 in which the reflecting diaphragm is of high resistivity, e.g.

10'0 Q/square to give constant charge operation.

11. A mirror as in Claim 9 in which the coating of the reflecting diaphragm is divided into sections each isolated from the adjacent sections by high resistance strips to discourage redistribution of the charge between sections as the capacitance changes due tb diaphragm movement.