EP0346819A2 - Method and apparatus for the contactless measuring and optionally machining of surfaces - Google Patents

Abstract

The invention relates to methods for contactless measurement and, if necessary, machining of surfaces, in particular of large mirrors or the like, in which the difference between interferometrically recorded actual surface contour values and preselected desired surface contour values is determined in order to determine contour deviations of the surface. According to the invention, a reference contour of at least one reference element, the extent of which essentially corresponds to a measurement and, if appropriate, processing area of the surface, is measured interferometrically with respect to a standard, the geometry of which is known within the permissible contour tolerance of the surface. The surface to be measured is brought into a defined spatial position with respect to the reference element, the spacing of the reference contour from the measurement or processing area of the surface is recorded incrementally interferometrically and the respective difference between the actual and target value is determined. For machining, material may be removed from the surface, the extent of which does not exceed the permissible contour tolerance. The invention also relates to devices for carrying out these methods.

Description

The invention relates to methods for non-contact measurement and, if necessary, machining of surfaces, in particular for fine polishing of large-area mirrors or the like, in which the difference between interferometrically recorded actual surface contour values and preselected desired surface contour values is determined and given. depending on the result, a superficial material removal is carried out.

The invention also relates to devices suitable for carrying out these methods.

Known machining processes are used to produce surfaces with a high degree of shape accuracy, even with large surfaces. These methods are used, for example, in the manufacture of reflection optics, in particular for astronomy; one example are telescope mirrors for the visible and infrared spectral range.

The true-to-shape production of large workpieces in particular presents particular difficulties. This applies even if the surface is flat, spherical or rotationally symmetrical aspherical (for example parabolic). This is especially true for non-rotationally symmetrical aspherical surfaces; To date, there has been no satisfactory method for producing such surfaces.

Such workpieces are urgently needed, particularly in the field of astronomy. This applies in particular to the existing need to be able to economically produce non-rotationally symmetrical aspherical segment mirrors for large telescopes with openings of several meters.

Up to now, the classic method developed by Herschel, Ritchey, Anderson and others has been used to produce such mirrors. First, the best approximated spherical shape is created with large-area grinding and polishing bodies; with small lapping and polishing discs, the remaining deviations from the target shape are then eliminated. The polishing process has to be interrupted several times by inspections of the actual shape achieved, for which the Foucault test is still the most reliable test method.

From DE-PS 34 30 499 a method and a device for producing aspherical surfaces are already known. In this case, a flexible lapping or polishing tool is to be used, which essentially covers the entire workpiece surface to be machined at the same time and is applied to the workpiece with locally different pressures; the local pressure differences should be chosen according to the deviations of the workpiece surface from the target shape. This is implemented by a membrane that covers the entire workpiece surface and carries a large number of individual machining bodies on the workpiece side. From the other side, the membrane together with the processing bodies is pressed against the surface by individually controllable pressure shoes. The form accuracy is to be ensured by controlling the individual pressure shoes with regard to the processing pressure and by occasional measurement of the workpiece, the membrane being brought into the shape of the surface to be processed between each processing operation. For this purpose, it can be pressed, for example, on a separate tool, which has approximately the desired shape of the surface to be machined.

This method is not very suitable for the production of larger aspherical surfaces (about 1m or more) because of the uncertainty of the measuring system that then increases over the permitted tolerance band.

The object of the invention is to create a method for the contactless measurement of surfaces, in particular of large mirrors or the like, which allows an exact measurement of surfaces of any shape with little effort.

It is also an object of the invention to provide a method and a device for the machining of surfaces which allow large surfaces to be produced true to shape with any shape, including non-rotationally symmetrical aspherical ones.

In doing so, a form accuracy is sought, which excludes deviations of more than 3 x 10⁻⁸m with regard to the workpiece diameter. This means that with a mirror diameter of 1m, for example, a shape accuracy is achieved that is better than 30 nanometers. At the same time, a micro roughness of less than 10 Å rms should be achieved.

The machining of large-area workpieces should be possible, with large area being understood to mean a ratio of the diameter to the mean radius of curvature of the workpiece, which is typically less than 1 to 10. With a mirror diameter of 1m, for example, this means an average radius of curvature of more than 10m. The processing of surfaces of any shape should be possible, so that in addition to flat, spherical, rotationally symmetrical aspherical and non-rotationally symmetrical aspherical surface shapes can be formed true to shape. It should not matter whether the surface curvature is concave or convex over the entire surface or also alternating between concave and convex, such as with Schmidt plates.

It should be possible to process all polishable substrates, e.g. the processing of glass substrates, in particular quartz glass; Glass ceramic substrates such as Zerodur; Ceramic substrates and metal substrates.

The features of the independent claims serve to solve these tasks.

A basic feature of the invention lies in the concept that the actual contour of the workpiece surface can be measured in-process and, if necessary, the machining tool can be controlled in-process until the desired target contour is reached. The achievable contour accuracy is better than 25 nanometers even with very large, non-rotationally symmetrical aspheres. According to the invention, no high demands are made on the accuracy of the axes relevant to the method and the device. There is no need to control the processing tool in terms of pressure, speed, alignment or infeed. It is not necessary to interrupt the machining process for inspection purposes or even to remove the workpiece from the machining device; rather, quality control takes place during the machining process itself.

According to the invention, a reference system for measuring the surface contour is first constructed from, in particular linear, reference elements. This Be traction system can be measured interferometrically against a standard, the geometry of which is known with the necessary accuracy. This reference system is integrated in the device structure in such a way that the initial measurement of the reference elements can be carried out using the same interferometers that are also used to measure the surface. A direct connection of the measurement geometry to the accuracy of the straightness standard is obtained in a particularly simple manner, which can be implemented relatively easily even with the required high accuracy (typically less than 10 nm), for example by means of a mercury surface.

Scanning heterodyne interferometers are preferably used as interferometer measuring devices, which work with two closely adjacent wavelengths. The wavelength relationships correspond to a beat. Such heterodyne interferometers are particularly suitable because they are relatively insensitive to surface roughness.

The invention makes it possible to make no special demands on the straightness of the process and device-relevant linear axes in horizontal and vertical terms. Straight lines of 10 microns are sufficient.

If the surface is not only to be measured, but also to be machined, for example, to be ground, the removal rate does not have to be known exactly, nor is there any time control of the pressure or a normal alignment of the processing tools is necessary.

If a rotary linear axis, about which the workpiece rotates relative to the measuring devices and the processing units, and a plurality of radial linear axes, along which the measuring and machining processes take place, are preferably used, the radial deviation of the axis of rotation can be of the order of magnitude of 10 micrometers.

The preferably used heterodyne interferometers are used either for measuring or for measuring the angle between the workpiece surface and the reference element. A resolution of 1 nm is achieved in the first case, and 1/20 arcsecond in the second case.

It is particularly advantageous for several measuring devices and processing units to be suspended or supported radially alternately above the rotating workpiece during the machining of the workpiece surface. For example, three measuring devices arranged at 120 ° to each other and three processing units arranged at 120 ° to each other can be used, the angle between one measuring device and the adjacent processing unit being 60 °.

If only measurements are taken, a corresponding arrangement of three measuring devices arranged at 120 ° to one another is used; the processing units can then be completely absent or, in the case of a measuring and processing device, cannot be actuated.

The simultaneous use of several measuring systems brings a variety of advantages, for example the possibility of a mutual control of the individual measuring devices; the detection of disturbances such as vibrations, geometric changes in the supporting structure, air turbulence in the beam path, spindle stroke, etc .; the further work even in the event of a temporary failure of a measuring device and an overall much faster measurement and possibly processing, in particular if several measuring systems and possibly processing devices are used in succession in the direction of rotation.

Overall, the invention enables the measurement and shaping of large-area, also non-rotationally symmetrical aspheres with a contour accuracy better than 25 nm. The invention is of striking conceptual simplicity, since it requires a minimum of axes, does not make any extraordinary demands on the accuracy of the axes, and extensive control of the processing tools with regard to pressure, speed, alignment and infeed is not necessary. The susceptibility to faults is low due to the high redundancy in the measuring devices, which, together with the possibility of self-checking and error detection, ensures great operational reliability. Several individual parts, for example several mirror bodies of a segment mirror, can be measured simultaneously and possibly processed. The measuring or machining process does not have to be interrupted to enable the inspection and control of the surface quality; the workpiece does not have to be removed from the device will. The invention thus enables a very quick and very economical measurement and, if necessary, processing.

• Fig. 1 eine periphere Anordnung von Spiegelsegmen­ten auf einem Rundtisch einer erfindungsge­mäßen Vermessungsvorrichtung;
• Fig. 2 eine schematische Aufsicht auf die Vorrich­tung gemäß Fig. 1;
• Fig. 3 eine seitliche Schnittansicht der Vorrich­tung gemäß Fig. 1 und 2;
• Fig. 4 eine schematisierte Draufsicht auf einen Teil einer Meßeinrichtung;
• Fig. 5 eine schematisierte Seitenansicht entspre­chend Fig. 4;
• Fig. 6 eine Rückansicht der Meßeinrichtung gemäß Fig. 4 und 5;
• Fig. 7 eine schematische Aufsicht auf eine erfin­dungsgemäße Poliervorrichtung und
• Fig. 8 eine seitliche Schnittansicht der Poliervor­richtung gemäß Fig. 7.

Preferred exemplary embodiments of the invention are explained in more detail below with reference to the attached drawing. Show in it

• 1 shows a peripheral arrangement of mirror segments on a rotary table of a measuring device according to the invention;
• FIG. 2 shows a schematic plan view of the device according to FIG. 1;
• 3 shows a sectional side view of the device according to FIGS. 1 and 2;
• 4 shows a schematic plan view of part of a measuring device;
• Fig. 5 is a schematic side view corresponding to Fig. 4;
• 6 shows a rear view of the measuring device according to FIGS. 4 and 5;
• Fig. 7 is a schematic plan view of a polishing device according to the invention and
• 8 is a sectional side view of the polishing apparatus shown in FIG. 7.

The device according to the invention shown in FIGS. 1 and 2 comprises a large air-bearing rotary table on which the workpieces to be measured, in the exemplary embodiment a plurality of mirror segments 20, are assembled peripherally together with their supporting elements. The device serving as a measuring machine 10 for these mirror segments 20 comprises a base frame 12 in which a spindle 14 (FIG. 3) is mounted, which carries the mirror segments. The spindle 14 is rotatable relative to the base frame by a motor drive about a central axis of rotation 15 perpendicular to the plane of the drawing; this rotation takes place relatively slowly, for example at one revolution per minute. In addition, an encoder (not shown) for determining the angular position of the spindle 14 relative to the base frame 12 is connected to the spindle. The encoder can be designed as a glass scale and allows an accuracy of the angular position determination in the range of 10 to 20 arc seconds. The data on the position of the spindle determined by the encoder are entered into a computer.

The measuring machine 10 is preferably set up in a vibration-decoupled, air-conditioned clean room.

Measuring devices 16 are provided above the surfaces 34 of the mirror segments 20, which are firmly connected to the base frame 12 and are not rotated when the spindle 14 is rotated.

As shown in FIG. 2, three measuring devices 16 are provided. The radial angle between two measuring devices is 120 ° in each case.

The measuring devices 16 are equipped with heterodyne interferometers. In the exemplary embodiment, these correspond to the heterodyne interferometers of the Axiom 2/20 type available from Zygo, but are modified with regard to the beam path.

A laser head and receiver 22 of each interferometer is arranged near the axis of rotation of the spindle 14 in such a way that the beam path is directed radially outward from the laser, as the arrow R in FIGS. 3 to 5 indicates. The beam path back to the receiver is directed radially inwards.

A guideway 24 runs radially outward along the beam path of the laser head / receiver 22 (FIG. 3), the end of the guideway near the axis of rotation serving as a holder for the laser head / receiver 22.

A measuring head 28 of the heterodyne interferometer can be displaced in the radial direction along the guide track 24, so that it can be moved in the radial direction over the entire width of the mirror segment 20. The movement of the measuring head 28 takes place by means known in the prior art.

The straightness of the guideway 24 in horizontal and vertical terms is relatively uncritical; Graduations of 10 µm are sufficient.

A glass scale or the like serving as an encoder for the radial position of the measuring head, which is not shown in the figures, extends along the guideway 24.

4, a reference element 26 extends parallel, as shown in FIG. 4, which is formed, for example, by a polished Zerodur ruler. The reference element 26 is suspended a few millimeters above the surface 34 of the mirror segment 20, and is supported in the exemplary embodiment by the supports for the guide track 24, which rise on the one hand near the axis of rotation and on the other hand on the outer circumference of the measuring machine 10 from the base frame 12.

The measuring head 28 enables interferometric measurement both with respect to the reference element 26 and the surface 34, as indicated in FIGS. 4 to 6 on the one hand by a dotted line and on the other hand by a solid line.

The data determined by the heterodyne interferometer are also input into the computer mentioned.

The measurement process begins with the calibration of the reference elements using the assigned heterodyne interferometer. For this purpose, the measuring head 28 is moved on the guideway 24 along the assigned reference element 26, the contour of which is initially only approximately known. The measurement is carried out with reference to a straightness standard of known geometry, for example a mercury surface, with an accuracy better than 10 nm. A reference contour 26 ′ is determined on the reference element, and the setpoint values given by the data and stored in the computer can be used to determine the deviations from the setpoint geometry.

If necessary, axial runout of the spindle, vibrations and the like can be monitored and corresponding measurement data can be transmitted to the computer for compensation. For example, other independent interferometers can be used for this purpose.

A (not shown) wavelength compensator with a resolution of, for example, 5 x 10⁻⁹ detects changes in wavelength dependent on air pressure and enables a corresponding compensation of the measurement data.

During the measurement, the mentioned slow rotational movement of the spindle 14 takes place, so that, together with the radial linear movement of the measuring head 28, the surface areas to be measured are swept spirally radially inwards or outwards by the measuring devices. Because of the geometric conditions mentioned, the tolerances with respect to the main surface plane are not very critical. Of course, this does not apply to the tolerances in the direction perpendicular to the main plane, i. H. Axial direction of the spindle 14.

As already said, while the surface to be measured is moving under the measuring device 16, the actual contour is measured and stored in the computer.

To ensure that the workpiece and reference element surfaces scanned interferometrically do not result in incorrect measurements, they must remain dust-free. For removing dust and the like. a cleaning device, for example a suction device (not shown) arranged between the measuring devices, can be used.

7 and 8 show a polishing device according to the invention with which the machining method according to the invention can be carried out.

In its basic structure, the polishing device 10 'corresponds to the measuring device already described with reference to FIGS. 1 to 6. Therefore, the parts shown in Figs. 7 and 8 correspond to the parts in Figs. 1 to 6 which have the same reference numerals.

Compared to the measuring device (FIGS. 1 to 6), only processing units 18 are added to the polishing device.

The processing units 18 are also firmly connected to the base frame 12 and are not rotated when the spindle 14 is rotated. In the exemplary embodiment, three processing units 18 are each offset by 120 ° relative to the measuring devices 16 so that a processing unit 18 is located between two measuring devices 16 and the angle between adjacent measuring devices 16 and processing units 18 is just 60 °.

The processing units 18, similar to the measuring devices 16, have a guide track 32 which extends above the surface 34 of the mirror segment 20 and is supported on the non-twisted part of the polishing machine 10 '.

Along the guideway 32 is a polishing head 30 over the entire radial extent of the mirror segment 20 movable. The size and shape of the polishing pin of the polishing head 30 are matched to the geometry of the surface to be machined.

The polishing head 30 is driven and adjusted by devices known in the prior art; an encoder (not shown), which extends along the guideway 32 and which can also be formed by a glass scale, enables the respective radial position of the polishing head 30 to be determined. In operation, the polishing head 30 is always at the same distance on the basis of the data stored in the computer to the axis of rotation 15 (center of the rotary table) held like the associated interferometer measuring head 28, d. H. the measuring head of the measuring device 16 preceding in the machining direction A (FIG. 7). The polishing pins of the polishing heads 30 are placed on or lifted from the surface by computer control. The processing pressure of the polishing pins on the surface is adjusted so that the material removal between two interferometer positions is at most equal to the permissible contour tolerance (e.g. 25 nm).

The machining process begins, like the measurement process already described, with the measurement of the reference elements 26. The measurement of the actual surface contour values described above and the calculation of the deviations from the target geometry of the surface follows.

During the rotational movement of the spindle 14, machining areas are formed which are already in accordance with the written measuring areas extend radially inwards or outwards. The distance of the spiral tracks corresponds to the distance on which the arrow height of the mirror surface changes in the radial direction with respect to the reference element by a tolerance unit, for example by 25 nm. In the case of long focal length parabolic segments, this typically amounts to a few tenths of a millimeter, with a favorable design of the reference elements even only a few Millimeter. These path distances are taken as a basis when choosing the polishing pins with regard to size and shape.

While the surface is moving under the measuring device 16, its actual contour is measured and stored in terms of data in the computer. These stored actual contour data are used to control the processing unit following in processing direction A, i.e. the polishing head 30. The removal rate is set so that the removal between two successive measuring units in processing direction A is smaller than the tolerance unit (e.g. 25 nm ). This means that between two measuring processes so much material can never be removed that the contour tolerance is exceeded.

The measuring device 16 following the mentioned processing station in processing direction A determines whether the previous processing step has already brought the actual surface contour into the tolerance range of the desired surface contour. If this is the case, the subsequent processing station in this processing area is not actuated, so that no further removal takes place.

The measuring and machining processes are repeated until all mirror segments have reached the target surface contour within the tolerance.

The above-mentioned suction device advantageously serves for the removal of removal residues during processing.

The separation of the measuring process from the assigned machining process (i.e. with the same surface area) in terms of time enables local heating that occurs during machining to be reduced again before the next measurement and air vortices to subside.

If an interruption in the machining of joints and recesses between individual workpiece parts, for example mirror segments, is undesirable, fillers can be used and also polished.

It goes without saying that the entire machining process is ended as soon as the measuring units determine that the target contour has been reached for the entire surface.

Claims (58)

1. A method for the contactless measurement of surfaces, in particular of large mirrors or the like, in which the difference between interferometrically recorded actual surface contour values and preselected desired surface contour values is determined to determine contour deviations of the surface,
characterized in that a) a reference contour of at least one reference element, the extent of which essentially corresponds to a measurement area of the surface, is measured interferometrically with respect to a standard, the geometry of which is known within the permissible contour tolerance of the surface, b) that the surface to be measured is brought into a defined spatial position with respect to the reference element and c) the spacing of the reference contour from the measurement area of the surface is recorded incrementally interferometrically and the respective difference between the actual value and the target value is determined. 2. The method according to claim 1,
characterized in that the reference element and the surface to be machined are moved relative to each other. 3. The method according to claim 2,
characterized in that the reference element lies on an imaginary radial line with respect to a vertical axis of rotation through the main surface plane, about which axis of rotation the surface is rotated, so that the essentially radially extending measurement area forms part of a spiral line on the surface. 4. The method according to claim 3,
characterized in that each spiral path formed by a measurement area of the spiral line corresponds to a distance on which the arrow height of the surface changes in the radial direction with respect to the reference contour of the reference element by the contour tolerance. 5. The method according to claim 3 or 4,
characterized in that the measuring head of the interferometer is moved in incremental steps along the assigned radial line over the measurement area. 6. The method according to any one of claims 1 to 5,
characterized in that several reference elements are used successively spaced apart. 7. The method according to any one of claims 1 to 6,
characterized in that the actual and target values are determined by laser interferometers and, if necessary, the influence of the wavelength-air pressure dependency of the laser light is recorded interferometrically in real time for correction. 8. The method according to any one of claims 1 to 7,
characterized in that scanning heterodyne interferometers are used together with linear reference elements. 9. The method according to any one of claims 1 to 8,
characterized in that the main surface plane is aligned substantially horizontally with respect to the direction of gravity and the reference element is suspended or supported at a short distance from the surface above it. 10. The method according to any one of claims 1 to 9,
characterized in that the surface is prepolished in preparation for a contour accuracy in the range of a few 10⁻⁶m. 11. The method according to any one of claims 1 to 10,
characterized in that the interferometric acquisition processes, the calculations and the control and regulating processes during the measurement are carried out automatically by means of a computer. 12. The method according to claim 11,
characterized in that the geometric target data of the surface are calculated or stored by means of the computer. 13. A method for non-contact measurement and ablative machining of surfaces, in particular for the fine polishing of large-area mirrors or the like, in which the difference between interferometrically detected Surface contour actual values and preselected surface contour setpoints are determined and, depending on the result, a superficial material removal is carried out, characterized in that a) a reference contour of at least one reference element, the extent of which essentially corresponds to a processing area of the surface, is measured interferometrically with respect to a standard, the geometry of which is known within the permissible contour tolerance of the surface to be achieved, b) that the surface to be machined is brought into a defined spatial position with respect to the reference element and c) the spacing of the reference contour from the machining area of the surface is recorded incrementally interferometrically and the respective difference between the actual and target value is determined, d) that, depending on the result in the processing area, material is subsequently removed from the surface, the extent of which does not exceed the permissible contour tolerance, and e) the acquisition according to step c) and optionally the processing according to step d) are repeated until the surface contour corresponds to the target value within the tolerance. 14. The method according to claim 13,
characterized in that the material is removed by at least one drivable controlled tool of a processing unit, in particular one or more polishing pins. 15. The method according to claim 13 or 14,
characterized in that the reference element and a machining unit are arranged at a fixed distance from one another and the surface to be machined is moved relative thereto, so that the machining region is successively brought into corresponding positions with respect to the reference element and the machining unit. 16. The method according to claim 15,
characterized in that the reference element and the processing unit lie on imaginary radial lines with respect to a vertical axis of rotation through the main surface plane, about which axis of rotation the surface is rotated, so that the essentially radially extending processing area forms part of a spiral line on the surface. 17. The method according to claim 16,
characterized in that each of the spiral paths formed by the machining areas of the spiral line corresponds to a distance on which the arrow height of the surface changes in the radial direction with respect to the reference contour of the reference element by the contour tolerance. 18. The method according to claim 16 or 17,
characterized in that at least one machining tool of the machining unit and the measuring head of the interferometer are moved along the respective radial lines in incremental steps assigned to one another. 19. The method according to any one of claims 13 to 18,
characterized in that several reference elements and processing units are used alternately in succession. 20. The method according to any one of claims 13 to 19,
characterized in that the actual and target values are determined by laser interferometers and, if necessary, the influence of the wavelength-air pressure dependency of the laser light is recorded interferometrically in real time for correction. 21. The method according to any one of claims 13 to 20,
characterized in that scanning heterodyne interferometers are used together with linear reference elements. 22. The method according to any one of claims 13 to 21,
characterized in that the main surface plane is aligned substantially horizontally with respect to the direction of gravity and the reference element is suspended or supported at a short distance from the surface above it. 23. The method according to any one of claims 13 to 22,
characterized in that the surface is preparatory is polished to a contour accuracy in the range of a few 10⁻⁶m. 24. The method according to any one of claims 13 to 23, characterized in that the interferometric acquisition processes, the calculations and the control and regulation processes are carried out automatically during processing by means of a computer. 25. The method according to claim 24,
characterized in that the geometric target data of the surface are calculated or stored by means of the computer. 26. Device for the contactless measurement of surfaces, in particular of large mirrors or the like, with a support structure which receives the workpiece, the surface of which is to be measured, and measuring devices for interferometric detection of the surface contour,
characterized in that a) at least one reference element (26) is provided, which extends at a distance substantially parallel to a measurement area of the surface (34), b) an interferometer measuring device (16) is assigned to the reference element (26), by means of which the reference element (26) is measured and the distance between the reference element (26) and the measurement area is recorded, c) Devices (14) for generating a relative movement between the workpiece (20) and the measuring device (16) are provided. 27. The apparatus according to claim 26,
characterized in that a supporting structure (12, 14) is provided for receiving the workpiece with the main surface plane lying substantially horizontal with respect to the direction of gravity. 28. The device according to claim 27,
characterized in that the support structure is formed by a rotary table with a spindle (14) which is rotatable about a vertical axis of rotation. 29. The device according to claim 27 or 28,
characterized in that the support structure or the rotary table has an air-bearing spindle (14) to which an encoder is assigned for determining the angular position of the spindle with respect to the non-rotated support structure, the spindle preferably having an axial stroke of less than 0.1 arc seconds. 30. Device according to one of claims 27 to 29,
characterized in that the supporting structure comprises a base frame (12) which is not rotated during operation. 31. Device according to one of claims 26 to 30,
characterized in that the measuring devices (16) are arranged in a stationary manner above the surface (34) of the workpiece (20), in particular are suspended or supported on the base frame (12) and are not rotated when the workpiece is rotated. 32. Device according to one of claims 26 to 31,
characterized in that the measuring device has a guide track (24) which extends horizontally, in particular radially from the axis of rotation to beyond the outer boundary of the surface (34) to be machined, along which the interferometer measuring head (28) can be moved. 33. Device according to one of claims 26 to 32,
characterized in that several identical measuring devices (16) are provided, in particular three measuring devices (16) spaced at an angle of 120 ° about the axis of rotation of the rotary table. 34. Device according to one of claims 26 to 33,
characterized in that the measuring devices (16) are provided with encoders such as glass scales or the like for detecting the position, in particular the radial position, of the respective measuring head (28), the encoders in particular extending along the guideways (24). 35. Device according to one of claims 26 to 34,
characterized in that the interferometer measuring devices (16) are designed as scanning heterodyne interferometers, in particular the laser heads and receivers (22) being arranged near the axis of rotation of the rotary table or the spindle (14). 36. Device according to one of claims 26 to 35,
characterized in that an interferometric wavelength compensator is provided, the air pressure dependent wavelength changes detected for compensation. 37. Device according to one of claims 26 to 36,
characterized in that the reference elements (26) are essentially linear and in particular extend parallel to the associated guideway (24), and in particular consist of elongated mechanically dimensionally stable bodies, for example a polished Zerodur ruler. 38. Device according to one of claims 26 to 37,
characterized in that the reference elements (26) are suspended or supported at a distance of a few millimeters above the surface (34) to be processed. 39. Device according to one of claims 26 to 38,
characterized in that at least one further independent interferometer is provided for detecting the axial stroke of the spindle, for detecting vibrations of the device and the like. 40. Device according to one of claims 26 to 39,
characterized in that a computer is provided for storing the target contour data, the actual contour measurement data and for calculating the difference between the two. 41. Device for the removal of surfaces, in particular for the fine polishing of large-area mirrors or the like, with a support structure which receives the workpiece whose surface is to be processed, at least one removal tool, Devices for the relative movement of workpiece and processing tool as well as measuring devices for the interferometric detection of the surface contour,
characterized in that a) at least one reference element (26) is provided, which extends at a distance substantially parallel to a processing area of the surface (34), b) an interferometer measuring device (16) is assigned to the reference element (26), by means of which the reference element (26) is measured and the distance between the reference element (26) and the processing area is detected, c) at least one processing unit (18) spaced from the measuring device (16) is provided, by means of which the processing tool (30) can be removed and moved over the entire processing area, and d) Devices (14) for generating a relative movement between the workpiece (20) on the one hand and the measuring device (16) and the processing unit (18) on the other hand are provided to the processing area in succession with the measuring device (16) and the processing unit (18) in to bring corresponding relative positions. 42. Device according to claim 41,
characterized in that a support structure (12, 14) for receiving the workpiece with respect to the gravita tion direction is essentially horizontally lying surface main plane. 43. Device according to claim 42,
characterized in that the support structure is formed by a rotary table with a spindle (14) which is rotatable about a vertical axis of rotation. 44. Device according to claim 42 or 43,
characterized in that the support structure or the rotary table has an air-bearing spindle (14) to which an encoder is assigned for determining the angular position of the spindle with respect to the non-rotated support structure, the spindle preferably having an axial stroke of less than 0.1 arc seconds. 45. Device according to one of claims 42 to 44,
characterized in that the supporting structure comprises a base frame (12) which is not rotated during operation. 46. Device according to one of claims 41 to 45,
characterized in that the measuring devices (16) and the processing units (18) are arranged in a stationary manner above the surface (34) of the workpiece (20), in particular are suspended or supported on the base frame (12) and are not rotated when the workpiece is rotated. 47. Device according to one of claims 41 to 46,
characterized in that the measuring device and the processing unit horizontally, in particular radially from the axis of rotation to beyond the outer boundary of the surface (34) to be machined have guideways (24 or 32), along which the interferometer measuring head (28), otherwise the machining tool (30) can be moved. 48. Device according to one of claims 41 to 47,
characterized in that several identical, alternately arranged measuring devices (16) and processing units (18) are provided, in particular three measuring devices (16) spaced at an angle of 120 ° about the axis of rotation of the rotary table and three processing units (18) at an angle of 120 ° around the axis of rotation are provided so that adjacent measuring devices and processing units are at an angle of 60 ° to each other. 49. Device according to one of claims 41 to 48,
characterized in that the measuring devices (16) and the processing units (18) are provided with encoders such as, for example, glass scales or the like for detecting the position, in particular the radial position of the respective measuring head (28) or tool (30), the encoders in particular extend along the guideways (24 or 32). 50. Device according to one of claims 41 to 49,
characterized in that the interferometer measuring devices (16) are designed as scanning heterodyne interferometers, in particular the laser heads and receivers (22) being arranged near the axis of rotation of the rotary table or the spindle (14). 51. Device according to one of claims 41 to 50,
characterized in that an interferometric wavelength compensator is provided which detects changes in wavelength dependent on air pressure for compensation. 52. Device according to one of claims 41 to 51,
characterized in that the reference elements (26) are essentially linear and in particular extend parallel to the associated guideway (24), and in particular consist of elongated mechanically dimensionally stable bodies, for example a polished Zerodur ruler. 53. Device according to one of claims 41 to 52,
characterized in that the reference elements (26) are suspended or supported at a distance of a few millimeters above the surface (34) to be processed. 54. Device according to one of claims 41 to 53,
characterized in that electronic control devices for guiding the tools (30) on the radial position of the measuring head (28) of the measuring device (16) preceding the processing sequence of the surface (34) and for placing and lifting the processing tool (30) on or from the surface (34) are provided. 55. Device according to one of claims 41 to 54,
characterized in that pressure adjusting devices for the machining tool (30) are provided, which adjust the machining pressure of the tool in this way allow the material removal in a machining step to be at most equal to the permissible contour tolerance. 56. Device according to one of claims 41 to 55,
characterized in that at least one further independent interferometer is provided for detecting the axial stroke of the spindle, for detecting vibrations of the device and the like. 57. Device according to one of claims 41 to 56,
characterized in that a computer is provided for storing the target contour data, the actual contour measurement data, for calculating the difference between the two and for controlling the processing unit (18) associated with the respective measuring device (16). 58. Device according to one of claims 41 to 57,
characterized in that a cleaning device, in particular a suction device, is provided for removing removal residues from the surface.

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