RU2803879C1 - Method for measuring the shape of off-axis asspherical optical part - Google Patents

Abstract

FIELD: telescopes.

SUBSTANCE: controlling the shape of convex aspherical surfaces of off-axis mirrors of telescopes in the process of their shaping. The method is carried out using an interferometer and a wavefront corrector in the form of a round diffractive optical element (DOE) with a diameter larger than the size of the optical part. The DOE consists of an optical substrate, on the surface of which the main diffractive transmitting structure and additional reflective diffractive structures are applied - centering in the form of a central circle around the axis of the DOE and four elements in the form of segments of the ring along the circumference of the DOE, located diametrically opposite, as well as with four focusing elements, arranged along the circumference of the DOE diametrically opposite between the four centering elements. The DOE is installed in the coverage area of the off-axis surface, tuned with the help of additional structures relative to the interferometer and, together with the interferometer, relative to the controlled surface. Using the main diffractive structure, the interferogram of the controlled surface is recorded, which is taken into account when finishing the surface.

EFFECT: creation of an accurate and productive method of control.

1 cl, 5 dwg

Description

The invention relates to interference methods for measuring the shape of optical surfaces and is intended to control the shape of convex aspherical surfaces, for example, off-axis telescopes in the process of their shaping.

For precise shaping of optical surfaces, especially convex aspherical ones, reliable information about deviations of the surface shape from the specified one is necessary.

Numerous methods are known for monitoring axial convex aspherical elements, for example, with an auxiliary sphere of larger diameter than the controlled surface, which is called a scheme with a Hindle sphere, as well as numerous modifications of this scheme (V.A. Zverev, K.Yu. Sobolev, G.I. Tsukanova, Control of the shape of convex non-spherical surfaces of revolution, "Optical Journal", 1996, No. 12, pp. 12-19). In the classical Hindle scheme, a light beam emerging from one focus of a hyperbola falls on a controlled surface, a diverging beam centered at the second focus is reflected from it, falls on an auxiliary spherical mirror, the point of the beginning of the radius of which is aligned with the second focus, normal to the sphere, and is reflected back and returns to the first focus.

Theoretically, this method can also control an off-axis surface, but this control will be very difficult to implement when adjusting this control scheme, practically impossible.

There is also a known method for monitoring an axial convex mirror for passage through the mirror material from the rear surface and reflection from the inside from the convex surface when passing through the mirror material (T. Stewart McKechnie, “Interferometric test method for testing convex aspheric mirror surfaces,” Proc. SPIE 7739, P. 77390Y-4 and the patent “Method for controlling surface shape for large-sized convex optical surfaces” US 8,576,408 B2, published November 5, 2013). To compensate for aberrations that occur when passing through the mirror material, either a lens corrector or a wavefront corrector based on a diffractive optical element (DOE) is used. The use of this method for testing an off-axis mirror is also problematic due to the complex adjustment of the control circuit, taking into account the inhomogeneity of the material of the mirror, which, as a rule, is not designed to work in transmitted light, etc.

A control scheme is also used, again for axial convex aspherical parts with an auxiliary lens according to the Fizeau scheme (Device for monitoring convex aspherical optical surfaces of high-precision large-sized mirrors, utility model patent RU No. 169716, Publ. 03/29/2017). In this case, to control the off-axis surface, it is necessary either to manufacture an auxiliary lens of a much larger size and control the required surface with the off-axis part of the surface of this lens, or to make a concave off-axis surface of the lens inverse to the required convex off-axis surface, which is also difficult to accomplish.

There is a known method for monitoring the off-axis part of the full axial surface of a part using additional optical elements, lenses, which actually implement the Fizeau scheme (Proc. of SPIE Vol.7018, pp: 701818-1 - 701818-12). In this case, the light beam passes through an optical system, which at the output has a concave aspherical surface coinciding with the convex aspherical surface of the tested part, the Fizeau interferometer uses reflection from the control surface, which is located only a few mm from the measured surface, the light beam falls normal to convex aspherical surface and returns back. The first lens is illuminating and does not affect the measurement results.

This method also requires very precise adjustment of the relative position of the lenses and two lenses relative to the controlled part of the surface, which reduces the accuracy of control of the surface shape; in addition, it requires the manufacture of two auxiliary lenses of a rather complex configuration.

There is a known method for monitoring convex aspherical surfaces of axial parts using axial synthesized hologram optical elements (SGOE), or diffractive optical elements (DOE) (Optical Journal, Vol. 74, No. 6, 2007, p. 44, Fig. 1a). In this case, to control a convex aspherical surface, a DOE is used, in which a plane-parallel plate is used as a substrate (Fig. 1a). When a DOE is illuminated by a point light source, a converging beam of rays is formed at its output, each ray of which propagates along the corresponding calculated position of the normal to the convex aspherical surface. After reflection from the surface, the rays in the autocollimation path form an image that matches the light source. The distance between the DOE and the top of the controlled convex surface is selected at the stage of calculating the DOE as minimal as possible in order to minimize the difference in the light diameters of the convex aspherical surface and the DOE. But in this case, the method is used to control only exclusively axial convex aspherical surfaces.

The closest to the proposed invention in terms of technical essence will be a method for measuring the shape and decentering of the aspherical off-axis surface of an optical part (RU 2758928 published on November 3, 2021) using an interferometer and a wavefront corrector in the form of a combined diffractive optical element (DOE). The DOE is made in the form of a round plane-parallel plate with diffraction zonal structures deposited on its surface - the main transmitting one and two additional ones - centering reflective and focusing reflective, made around the main structure in the form of rings. The DOE is installed at the focus of the reference lens of the interferometer and, using a centering structure, is adjusted (adjusted) relative to the interferometer, and using the focusing structure, the DOE with the interferometer is adjusted relative to the controlled aspherical surface. Then standard interferometric control of the shape of the aspherical surface, and in particular, its decentration, is carried out. This DOE design is intended mainly for testing axisymmetric and concave surfaces, while testing off-axis surfaces is difficult or impossible and will not accomplish the task.

The objective of the invention is to create a more accurate and productive (accelerated) method for monitoring the convex off-axis aspherical surface of a telescope mirror with a DOE as a wavefront corrector

The technical result determined by the task is achieved by the fact that in the method of measuring the shape of an aspherical off-axis surface of an optical part using an interferometer and a wavefront corrector in the form of a round combined diffractive optical element (DOE), including the main structure and additional centering and focusing structures, with operations settings of the DOE relative to the interferometer and the optical part, in contrast to what is known for measuring convex off-axis surfaces, a DOE with a diameter larger than the overall dimensions of the part is used, with four centering reflective elements in the form of ring segments along the DOE circumference, located diametrically opposite, and a central circle around the DOE axis, as well as with four reflective focusing elements located along the circumference of the DOE, diametrically opposite between the four centering elements, while the DOE is installed in the coverage area of the off-axis surface and, after its adjustment, the interferogram of the controlled surface is recorded, which is taken into account when finishing the surface.

The technical result is ensured, first of all, by the use of a new design of DOE as a corrector. The presence of centering elements spaced around the circumference of the DOE, together with the central round zone, allows you to quickly and accurately center the DOE relative to the interferometer, and with the help of four focusing points, center the controlled surface of any shape relative to the DOE. The DOE creates a wave front emerging from it, incident normally to the surface of the test part and corresponding to the off-axis shape of this off-axis aspherical surface, reflects from it, again passes through the wave front corrector and then in the interferometer interferes with the reference wave front, creating a wave front of deviations of the surface shape from required, which is deciphered for conducting an automated shaping session.

The invention is illustrated by the drawing, where

fig. 1 - measuring circuit;

fig. 2 - DOE design with main and additional structures;

Fig.3 - location of focusing points in the area of the surface of the part;

fig. 4 - surface interferogram obtained using EBSD

fig. 5 - wavefront map corresponding to the interferogram;

The proposed method is carried out as follows:

In the diagram of Fig. 1 to control the convex aspherical surface 1 is used as a wavefront corrector of the DOE 2, which is installed between the surface 1 and the interferometer 3.

When the DOE is illuminated by a point light source of the interferometer 3, a converging beam of rays is formed at its output, each ray of which propagates along the corresponding calculated position of the normal to the convex aspherical surface 1. This calculated position is preliminarily set and measured in accordance with the calculated data (distance from the DOE to the surface mirrors).

The DOE 2 used consists of an optical round plane-parallel substrate 4 (Fig. 2), on which the main diffraction transmitting structure 5 and additional structures outside it are applied - reflective centering 6 in the form of four ring segments along the circumference of the DOE, located diametrically opposite, and a central circular zone 7 on the DOE axis, as well as four focusing elements 8 along the DOE circumference, located diametrically opposite between the centering structures 6.

First, the DOE 2 in the frame is adjusted and centered relative to the interferometer 3 (Fig. 1) using interferograms of the reference wavefront from the centering structures 6 and 7 (Fig. 2), with a minimum number of interference fringes, and relative to the measured surface 1 - using four focusing zones of the structure 8 (Fig. 2), transformed on the surface of the part in the form of luminous focused spots in the edge region of the part in given positions (Fig. 3), which ensures and guarantees the accuracy of the adjustment. As a rule, wavefront correctors (either lens or diffraction) are designed and manufactured to work with specific aspherical surfaces, taking into account the focal length and the steepness of the asphericity. An interference pattern reflected from the controlled surface in autocollimation is obtained. This picture is deciphered and a topographic map of deviations of this surface of the part from that required for the shaping session is constructed.

As an example, let us consider the control of an off-axis mirror with a diameter of 230 mm with a vertex radius of 1200 mm, a deviation of the center of the part from the optical axis of 184.8 mm with an asphericity of 128.8 μm. Control of the surface shape was carried out in a horizontal scheme (Fig. 1). The DOE is located on a substrate with a diameter of 240 mm (Fig. 2), which is slightly larger than the diameter of the mirror. The main structure 5 is located in the zone from 24 to 117 mm. The reflective centering structure 6 is located in four areas of the zone from 110 to 117 mm and in the central zone 7 with a diameter of 23.8 mm. Focusing elements 8 for installing the mirror are applied in the area of 113-119 mm. In fig. Figure 4 shows an interferogram at the final stage of shaping. In fig. Figure 5 shows a map of the wavefront reflected from the surface. Here RMS is the root mean square deviation (rms) of the wavefront from the required one, P-V is the full range of deviations, the table below shows the values of regular errors of astigmatism, triangular coma, 5th order coma with the corresponding direction angles, zonal error coefficients, as well as the magnitude of c. k.o. minus these regular errors.

As a result of the application of a method for monitoring the off-axis aspherical surface of an optical part with a DOE, the possibility of reliable control tied precisely to the coordinates of the surface has become possible, the processing time of the results of surface shape control has been reduced, and in some cases the formation of off-axis highly aspherical surfaces without this development using DOE with mathematical transformation is not possible at all seemed possible. As a result, we obtain a surface shape suitable for calculations of automated shaping sessions using a small tool.

The proposed method is industrially applicable, because it uses standard optical components. Combined DOEs are consistently produced at the Institute of Automation and Electrometry SB RAS, Novosibirsk. Compared to the prototype, the method has the advantage of higher productivity, faster and more accurate measurement of the shape of off-axis aspherical surfaces, i.e. it solves the technical problem of measuring the shape of the off-axis convex aspherical surface of an optical part.

Claims (1)

A method for measuring the shape of an off-axis aspherical surface of an optical part using an interferometer and a wavefront corrector in the form of a round combined diffraction optical element (DOE), including a main structure and additional centering and focusing structures, with the operations of adjusting the DOE relative to the interferometer and the optical part, characterized in that to measure convex off-axis surfaces, a DOE is used with a diameter larger than the dimension of the part, with four centering reflective elements in the form of ring segments along the DOE circumference, located diametrically opposite, and a central circle around the DOE axis, as well as with four reflective focusing elements located diametrically opposite along the DOE circumference between four centering elements, while the DOE is installed in the coverage area of the off-axis surface and, after its adjustment, the interferogram of the controlled surface is recorded, which is taken into account when finishing the surface.