WO2012169935A2 - Device for aligning a two-mirror centered optical system - Google Patents

Device for aligning a two-mirror centered optical system Download PDF

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Publication number
WO2012169935A2
WO2012169935A2 PCT/RU2012/000437 RU2012000437W WO2012169935A2 WO 2012169935 A2 WO2012169935 A2 WO 2012169935A2 RU 2012000437 W RU2012000437 W RU 2012000437W WO 2012169935 A2 WO2012169935 A2 WO 2012169935A2
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WO
WIPO (PCT)
Prior art keywords
mirror
light source
reflecting surface
aligning
axis
Prior art date
Application number
PCT/RU2012/000437
Other languages
French (fr)
Other versions
WO2012169935A3 (en
Inventor
Villen Arnoldovich BALOEV
Vladimir Petrovich IVANOV
Nikolay Petrovich LARIONOV
Anatoliy Vasilevich LUKIN
Andrey Nikolaevich MELNIKOV
Alexandr Fridrikhovich SCOCHILOV
Andrey Mikhailovich URASKIN
Yury Petrovich CHUGUNOV
Original Assignee
Open Joint Stock Company "Scientific And Production Association" State Institute Of Applied Optics" (Ojsc "Npo "Gipo")
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Application filed by Open Joint Stock Company "Scientific And Production Association" State Institute Of Applied Optics" (Ojsc "Npo "Gipo") filed Critical Open Joint Stock Company "Scientific And Production Association" State Institute Of Applied Optics" (Ojsc "Npo "Gipo")
Publication of WO2012169935A2 publication Critical patent/WO2012169935A2/en
Publication of WO2012169935A3 publication Critical patent/WO2012169935A3/en

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Classifications

    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B7/00—Mountings, adjusting means, or light-tight connections, for optical elements
    • G02B7/18—Mountings, adjusting means, or light-tight connections, for optical elements for prisms; for mirrors
    • G02B7/182—Mountings, adjusting means, or light-tight connections, for optical elements for prisms; for mirrors for mirrors
    • G02B7/1822—Mountings, adjusting means, or light-tight connections, for optical elements for prisms; for mirrors for mirrors comprising means for aligning the optical axis
    • G02B7/1827—Motorised alignment
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01B—MEASURING LENGTH, THICKNESS OR SIMILAR LINEAR DIMENSIONS; MEASURING ANGLES; MEASURING AREAS; MEASURING IRREGULARITIES OF SURFACES OR CONTOURS
    • G01B11/00—Measuring arrangements characterised by the use of optical techniques
    • G01B11/26—Measuring arrangements characterised by the use of optical techniques for measuring angles or tapers; for testing the alignment of axes
    • G01B11/27—Measuring arrangements characterised by the use of optical techniques for measuring angles or tapers; for testing the alignment of axes for testing the alignment of axes
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01B—MEASURING LENGTH, THICKNESS OR SIMILAR LINEAR DIMENSIONS; MEASURING ANGLES; MEASURING AREAS; MEASURING IRREGULARITIES OF SURFACES OR CONTOURS
    • G01B9/00—Measuring instruments characterised by the use of optical techniques
    • G01B9/02—Interferometers
    • G01B9/02015—Interferometers characterised by the beam path configuration
    • G01B9/02016—Interferometers characterised by the beam path configuration contacting two or more objects
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01B—MEASURING LENGTH, THICKNESS OR SIMILAR LINEAR DIMENSIONS; MEASURING ANGLES; MEASURING AREAS; MEASURING IRREGULARITIES OF SURFACES OR CONTOURS
    • G01B9/00—Measuring instruments characterised by the use of optical techniques
    • G01B9/02—Interferometers
    • G01B9/02055—Reduction or prevention of errors; Testing; Calibration
    • G01B9/02056—Passive reduction of errors
    • G01B9/02058—Passive reduction of errors by particular optical compensation or alignment elements, e.g. dispersion compensation
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01B—MEASURING LENGTH, THICKNESS OR SIMILAR LINEAR DIMENSIONS; MEASURING ANGLES; MEASURING AREAS; MEASURING IRREGULARITIES OF SURFACES OR CONTOURS
    • G01B9/00—Measuring instruments characterised by the use of optical techniques
    • G01B9/02—Interferometers
    • G01B9/02055—Reduction or prevention of errors; Testing; Calibration
    • G01B9/02056—Passive reduction of errors
    • G01B9/02061—Reduction or prevention of effects of tilts or misalignment
    • G—PHYSICS
    • G03—PHOTOGRAPHY; CINEMATOGRAPHY; ANALOGOUS TECHNIQUES USING WAVES OTHER THAN OPTICAL WAVES; ELECTROGRAPHY; HOLOGRAPHY
    • G03H—HOLOGRAPHIC PROCESSES OR APPARATUS
    • G03H1/00—Holographic processes or apparatus using light, infrared or ultraviolet waves for obtaining holograms or for obtaining an image from them; Details peculiar thereto
    • G03H1/04—Processes or apparatus for producing holograms
    • G03H1/08—Synthesising holograms, i.e. holograms synthesized from objects or objects from holograms
    • G—PHYSICS
    • G03—PHOTOGRAPHY; CINEMATOGRAPHY; ANALOGOUS TECHNIQUES USING WAVES OTHER THAN OPTICAL WAVES; ELECTROGRAPHY; HOLOGRAPHY
    • G03H—HOLOGRAPHIC PROCESSES OR APPARATUS
    • G03H1/00—Holographic processes or apparatus using light, infrared or ultraviolet waves for obtaining holograms or for obtaining an image from them; Details peculiar thereto
    • G03H1/22—Processes or apparatus for obtaining an optical image from holograms
    • G03H1/2202—Reconstruction geometries or arrangements
    • G03H2001/2223—Particular relationship between light source, hologram and observer
    • G03H2001/2231—Reflection reconstruction
    • G—PHYSICS
    • G03—PHOTOGRAPHY; CINEMATOGRAPHY; ANALOGOUS TECHNIQUES USING WAVES OTHER THAN OPTICAL WAVES; ELECTROGRAPHY; HOLOGRAPHY
    • G03H—HOLOGRAPHIC PROCESSES OR APPARATUS
    • G03H2270/00—Substrate bearing the hologram
    • G03H2270/55—Substrate bearing the hologram being an optical element, e.g. spectacles

Definitions

  • the present invention relates to inspection technology and may be used for aligning two-mirror centered optical systems, including those of Cassegrain and Ritchey-Chretien telescopes, when assembling and aligning them in a workshop and in process of their routine operation at ground- and space-based observatories.
  • the device contains auxiliary optical elements which are a spherical hole, made in the central zone of the secondary mirror reflecting surface, the curvature centre being in the equivalent focus e q ' of the telescope, and a flat mirror with an annular aperture ("a collar"), surrounding the secondary mirror on its outer diameter side and rigidly fixecf to said mirror; with the flat mirror reflecting surface facing the telescope primary mirror reflecting surface.
  • auxiliary optical elements which are a spherical hole, made in the central zone of the secondary mirror reflecting surface, the curvature centre being in the equivalent focus e q ' of the telescope, and a flat mirror with an annular aperture (“a collar”), surrounding the secondary mirror on its outer diameter side and rigidly fixecf to said mirror; with the flat mirror reflecting surface facing the telescope primary mirror reflecting surface.
  • said flat annular mirror should be perpendicular to the secondary mirror optical axis and the hole curvature center should be located on said axis.
  • said spherical hole forms its autocollimating image.
  • a second autocollimating image of the point light source is formed exactly the same way, its beams being reflected subsequently from the secondary mirror reflecting surface, a respective zone of the primary mirror reflecting surface, the flat annular mirror reflecting surface (from "the collar") and then in the back path of rays from the reflecting surfaces of the primary and secondary mirrors of the system.
  • the telescope optical system is considered to be aligned.
  • a disadvantage of this device is its low alignment reliability due to inevitable errors in the course of manufacturing the secondary mirror having a spherical hole and a flat annular mirror ("a collar"): the hole curvature centre may be located out of alignment with the secondary mirror optical axis and said axis may be unparallel to the normal to the flat annular mirror ("the collar”). This inevitably results in a telescope resolution degradation.
  • the closest to the technical spirit of the claimed invention is a device for aligning two-mirror centered optical systems [V.P. Ivanov, N.P. Larionov, A. V. Lukin, and A.A. Nyushkin, "Alignment of two- mirror centered optical systems using computer-generated hologram optical elements", Opticheskii Zhurnal, 20 fO, Vol. 77, No. 6, pp. 14- 18 (in Russia)].
  • This device contains a monochromatic light source and a collimator and a beamsplitter, which are tandem-mounted along a ray path, to form a reference branch and a working one, the latter having a focusing objective to form a point light source, a computer-generated hologram optical element, consisting of three adjusting coaxial computer-generated holograms, with two of them being reflective, one of them forming an autocollimating image of the point light source directly and the other one in conjunction with a respective zone of the reflecting surface of the primary mirror, a flat reference mirror mounted in the reference branch perpendicular to light beams, radiating from the beamsplitter, a second focusing objective and a position photodetector, connected to a data imaging and processing unit, which are mounted in the registering unit.
  • Disadvantages of the device are its insufficient reliability and efficiency of the alignment of two-mirror centered optical systems, consisting of a primary mirror and a secondary one, as well as a limited possibility of carrying out periodic alignment inspections of said systems being in the normal mode at observatories because, mainly, of considerable weight and overall dimensions of the substrate of the computer-generated hologram optical element.
  • the claimed engineering solution is aimed at enhancing reliability and efficiency of alignment inspections of two-mirror centered optical systems in the course of their assembly and alignment, as well as in the normal mode, in process of their operation at observatories, this solution has been found by aligning the optical axes of holograms with the axis of the secondary mirror, which is to be aligned, by a decrease in the number of elements in the optical design of the aligning device and a reduction in the number of segments, being under control, to a single one - a distance between the vertices of the reflecting surfaces of the primary and secondary mirrors.
  • the above problem is solved in the claimed device for aligning a two-mirror centered optical system, consisting of a primary mirror and a secondary one, containing a monochromatic light source and a collimator, a beamsplitter, which are tandem-mounted along a path of rays, to form a reference branch and a working one, the working branch having a focusing objective to form a point light source, two on-axis reflective adjusting computer-generated holograms, which are coaxial, with one of them forming an autocollimating image of the point light source directly and the other one in conjunction with the respective zone of the reflecting surface of the primary mirror, a flat reference mirror, mounted in the reference branch perpendicular to light beams, radiating from the beamsplitter, the registering unit containing a second focusing objective, a position photodetector, connected to a data imaging and processing unit, the both adjusting computer-generated holograms being made on the reflecting surface of the secondary mirror, mostly in its edge zone, the common
  • the beamsplitter is made as a prism-cube
  • the flat reference mirror is made as a prism-cube edge surface, containing a reflective coating on the reference branch side
  • D and /' are the entrance pupil diameter and the focal length of the focusing objective, respectively;
  • ⁇ is the radiation wavelength of the monochromatic light source
  • m is the annular zone number of the adjusting computer- generated hologram
  • A/i ;W is the path-length difference between the on-axis beam and the beam, corresponding to the m th zone middle of the hologram, forming an autocollimating image of the point light source directly;
  • ⁇ , ⁇ is the path-length difference between the on-axis beam and the beam, corresponding to the m' h zone middle of the hologram, forming an autocollimating image of the point light source in conjunction with the respective zone of the reflective surface of the primary mirror;
  • y ⁇ (x) and y ⁇ x - d) are the functions which determine the profile shape of the reflecting surfaces of the primary mirror and the secondary one, respectively, in the rectangular coordinate system Oxy, having the origin at the vertex 0 ⁇ of the primary mirror reflecting surface and the axis Ox, arranged in alignment with the optical axis of the two-mirror centered optical system;
  • d is the distance from the reflecting surface vertex of the secondary mirror to that of the primary one
  • ⁇ is the radiation wavelength of the monochromatic light source; and wherein each of the two adjusting computer-generated holograms is made as a couple of diametrically opposite portions of respective circular rings, with said couples being slewed around each other at a definite angle of say 90 degrees.
  • Fig.l shows a schematic optical diagram of the claimed device for aligning a two-mirror centered optical system.
  • Fig.2 shows an aperture of the secondary mirror, having apertures of on-axis reflective adjusting computer-generated holograms in its edge zone, with said apertures being made in the form of circular rings, with said holograms being coaxial between themselves and the secondary mirror reflecting surface.
  • Fig.3 shows an aperture of the secondary mirror, having apertures of reflective adjusting computer-generated holograms which are located in its edge zone, with said apertures being made in the form of a couple of diametrically opposite portions of respective circular rings of on-axis computer-generated holograms which are coaxial between themselves and the secondary mirror reflecting surface.
  • Fig.4 and Fig.5 show the results obtained during an experimental prototyping of the claimed device.
  • Fig.6 and Fig.7 show spatial frequency of adjusting holograms for the secondary mirror (Fig.6) and the primary mirror (Fig.7) of the T-170M telescope.
  • the claimed device for aligning a two-mirror centered optical system contains a monochromatic (laser) light source 1 , a collimator 2 and a beamsplitter 3 in the form of a prism-cube, having a semitransparent layer 4, which splits the incident beam into two parts, one of them (reflected from the semitransparent layer 4) propagates to the reference branch, and another one (having passed through the semitransparent layer 4) propagates to the working branch of the device.
  • the reference branch includes a reference flat mirror 5, which is located perpendicular to rays, emanating from the semitransparent layer 4 of the beamsplitter 3, said reference flat mirror being made as an edge surface of the prism-cube 3, having a reflective coating on the reference branch side.
  • the working branch includes along the path of beams a focusing objective 6 to form a point light source, a holed stop 7, whose plane 8 is in alignment with the back focal plane of the objective 6, a secondary mirror 9, which is to be aligned, of the two- mirror system, an edge zone of the reflecting surface * thereof containing adjusting on-axis reflective computer-generated holograms 10 and 11, which are mounted coaxially relative thereto, and a primary mirror 12, which is to be aligned, of the two-mirror system.
  • the stop 7 is mounted in the central hole of the primary mirror 12 so that the hole centre of the stop 7 is aligned with the vertex 0 ⁇ of the reflecting surface of the primary mirror 12 and the back focus of the focusing objective 6.
  • the registering unit contains a second focusing objective 13, a position photodetector 14 connected to a data imaging and processing unit 15. Such being the case, the back focal plane of the objective 13 is aligned with a photosensitive surface 16 of the position photodetector 14.
  • the hole of the stop 7 may be made round whose minimum diameter is determined by the condition
  • D and /' are the entrance pupil diameter and the focal length of the focusing objective 6, respectively;
  • ⁇ is the radiation wavelength of the monochromatic light source 1.
  • Each of the two adjusting computer-generated holograms 10 and 1 1 may be made as a couple of diametrically opposite portions of respective circular rings, with said couples being slewed around each other at a definite angle of say 90 degrees (Fig. 3).
  • the light beam from the monochromatic light source 1 propagates to the collimator 2 which transforms it into a parallel expanded light beam which falls on the' beamsplitting prism-cube 3, where some part of it passes through the semitransparent layer 4 and some of it is reflected thereby.
  • the reflected part of the light beam propagates to the reference branch, falls on the flat mirror surface 5 of the prism-cube 3, oriented perpendicular to the light rays of said beam, is reflected there from and under conditions of autocollimating path some part of it is reflected from the semitransparent layer 4 and some of it passes through it.
  • the reflected part of the reference light beam passes backward through the collimator 2 and falls into a hole of an output window of the light source 1 , and the part of the reference beam, having passed through the semitransparent layer 4, propagates to the registering unit and is focused by the second focusing objective 13 in the point A 0 , located on the photosensitive surface 16 of the position photodetector 14 due to an overlapping of this layer 16 with the back focal plane of the objective 13.
  • the image of this point is displayed on a screen of the data imaging and processing unit 15 (in the screen centre, Fig.l). It is used as a sighting point in the course of aligning the mirrors 9 and 12 of the two-mirror system.
  • the back focal plane of the objective * 6 is aligned with the working plane 8 of the stop 7, facing the secondary mirror 9.
  • the beam, formed by the hologram 10 passes backwards through the focusing objective 6, being transformed into a parallel beam, a part of it is reflected from the semitransparent layer 4 and focused by the second focusing objective 13 in the point A ⁇ 0 on the photosensitive surface 16 of the position photodetector 14.
  • the image of this point is displayed on the screen of the data imaging and processing unit 15. It is formed only by the secondary mirror 9.
  • the size of the point image on the screen of the unit 15 depends on the deviation of the estimated value of the distance from the back focus of the objective 6 to the vertex (3 ⁇ 4 of the reflecting surface of the secondary mirror 9, and its deviation from the image of the sighting point AQ in the transverse direction depends on the decentering of the secondary mirror 9.
  • the adjusting hologram 10 is made as a circular ring in the edge zone of the reflecting surface of the secondary mirror 9 (see Fig. 2, Reference Numeral 10).
  • the light beam, diffracted on the adjusting hologram 1 1, propagates to the primary mirror 12 of the system, which is to be aligned, forming a congruence of diffracted light beams, which * coincides with the estimated congruence of normals to the portion of the reflecting surface of the primary mirror 12, whereupon falls this light beam. That is why an autocollimated reflection of said light beam occurs upon this portion of the reflecting surface of the primary mirror 12, with said light beam falling backwards on the hologram 1 1 and being transformed again thereby (in the operating order of diffraction) into a congruence of diffracted beams, converging in this case in the hole centre of the stop 7.
  • this light beam propagates backwards through the objective 6, being transformed by the latter into a parallel beam, which is reflected from the semitransparent layer 4 and focused by the objective 13 in the point A u on the photosensitive surface 16 of the position photodetector 14.
  • the image of the point is displayed on the screen of the unit 15. Using the image size of the point A , one can determine the deviation from the estimated value of the distance between the vertices of the reflecting surfaces of the secondary mirror 9 and the primary mirror 12, and through the use of the transverse displacement relative to the image of the sighting point A 0 , one can determine the total value of the decentering of the secondary mirror 9 and the primary mirror 12.
  • the adjusting computer-generated hologram 1 when being illuminated by a homocentric light beam, forms (in the operating order of diffraction) a congruence of diffracted beams, which coincides with the congruence of normals to the portion of the reflecting surface of the primary mirror 12, where upon this light beam falls.
  • Its structure is made as a circular ring (circular annular aperture) in the edge zone of the reflecting surface of the secondary mirror 9 (see Fig. 2, Reference Numeral 1 1), coaxially with the structure of the hologram 10.
  • Structures of annular apertures of the on-axis holograms 10 and 1 1 consist of annular zones.
  • l jm is the path-length difference between the on-axis beam and the beam, corresponding to the m' h zone middle of the hologram 10;
  • l2,m is the path-length difference between the on-axis beam and the beam, corresponding to the m ,h zone middle of the hologram 1 1 ;
  • y ⁇ ⁇ x) and y 2 x - d) are the functions which determine the profile shape of the reflecting surfaces of the secondary mirror 9 and the primary mirror 12, respectively, in the rectangular coordinate system Oxy, having the origin at the vertex 0 ⁇ of the reflecting surface of the primary mirror 12, and the axis Ox, arranged in alignment with the optical axis of the two-mirror centered optical system;
  • d is the distance from the vertex 0 2 of the reflecting surface of the secondary mirror 9 to the vertex 0 of the reflecting surface of the primary mirror 12;
  • ⁇ is the radiation wavelength of the monochromatic light sour ce 1.
  • the prism-cube 3 is oriented so that the parallel beam, emerging from the collimator 2 and reflected by the semitransparent layer 4 of the prism-cube 3, falls perpendicular on the flat reference mirror 5 (the edge of the prism- cube 3). This is controlled by a trace of the light spot on the output window of the laser light source 1, formed by a light beam, reflected from the mirror 5.
  • the prism-cube 3 is rotated so that the beam is directed into the hole of the output window of the light source 1.
  • the photosensitive surface 16 of the position photodetector 14 is aligned with the back focal plane of the second focusing objective 13 to obtain a minimal circle of diffusion in order to control the image of the sighting point A 0 on the screen of the data imaging and processing unit 15.
  • the focusing objective 6 is selected to meet the condition
  • D and / are the entrance pupil diameter and the focal length of the focusing objective 6, respectively;
  • d is the distance from the vertex ⁇ 3 ⁇ 4 of the reflecting surface of the secondary mirror 9 to the vertex 0 ⁇ of the reflecting surface of the primary mirror 12;
  • ec m is the diameter of the secondary mirror 9.
  • the stop 7 is placed into the central hole of the primary mirror 12.
  • the operating plane 8 of the stop 7 will be tangent to the reflecting surface of the primary mirror 12 at its vertex 0 ⁇ .
  • the hole centre of the stop 7 is aligned with the vertex 0 ⁇ of the reflecting surface of the primary mirror 12.
  • the diameter of the round hole or the minimum linear size of the non-round shape hole of the stop 7 is virtually determined by the condition
  • 0 is the diameter of the round hole or the minimum linear size of the non-round shape hole of the stop 7;
  • D and / are the entrance pupil diameter and the focal length of the focusing objective 6, respectively;
  • ⁇ is the radiation wavelength of the monochromatic light source 1.
  • the unified unit consisting of the elements 1, 2, 3, 6, 13, 14 and 15, is oriented relative to the secondary mirror 9 of the system to be aligned.
  • the secondary mirror 9 of the system to be aligned.
  • it is mounted at the rear of the primary mirror 12, and with the help of longitudinal and transverse aligning displacements the back focus of the objective 6 is aligned with the hole centre in the stop 7 and an illumination of the secondary mirror 9 is controlled by a light beam diverging from the back focus of the objective 6.
  • By angular displacements of the unified unit one obtains a position wherein the aperture of the secondary mirror 9 is illuminated axially symmetrically to the light beam, falling on it.
  • longitudinal and transverse aligning displacements of the unified unit a fine alignment is made to obtain a fine alignment of the back focus of the objective 6 with the operating surface 8 of the stop 7 and its hole centre.
  • the secondary mirror 9 will be placed at the estimated distance d from the back focus of the objective 6 and, respectively, the vertex 0 of the reflecting surface of the primary mirror 12. In this case, the optical axes of the secondary mirror 9 and the primary mirror 12 will be aligned.
  • the adjusting holograms 10 and 11 In order to make the process of alignment easy and reduce the size of the "inoperative" area on the reflecting surface of the secondary mirror 9, which is occupied by the adjusting holograms 10 and 11, it is expedient to make them in the form of small-sized portions of diametrically opposite corresponding annular zones. This enables to make these portions (holograms aperture) within one annular zone, that allows to reduce considerably the area occupied by them on the reflecting surface of the secondary mirror 9 (see Fig. 3).
  • the apertures of the holograms 10 H 11 may be made of different shapes, for example, round or square. In such a case, they may be made round for one hologram and square - for another.
  • the optical axes of the holograms 10 and 1 1 will be aligned with the optical axis of the secondary mirror 9 with the least allowable error (practically not more than one micrometer). Meanwhile their centering with the secondary mirror 9 remains obviously invariable in future operation of the two-mirror system. This feature considerably enhances the reliability of alignment when using said device in a shop and in the normal mode of operation at observatories.
  • the portion of the reflecting surface of the secondary mirror 9 of the two-mirror system to be aligned which is used in the claimed device as a substrate for the adjusting holograms 10 and 11, provides a decrease in the number of elements in the optical design of the device and a reduction in the number of segments, being under control, to a single one, i.e. the distance between the vertices of the reflecting surfaces of the secondary mirror 9 and the primary mirror 12 of the two-mirror system.
  • it substantially enhances reliability and efficiency of the alignment of two-mirror systems as well as it reduces weight and overall dimensions of control instrumentation.
  • the adjusting holograms 10 and 1 1 may be synthesized for operations at a wavelength of ⁇ , which is less than the wavelength of the short-wavelength limit of the spectral operating range of the two-mirror system being aligned.
  • structures of the adjusting holograms 10 and 11 may be made in any light zone of the reflecting surface of the secondary mirror 9, because their negative effects at the operating wavelengths will be negligible.
  • the claimed device includes apertures of the computer-generated holograms 10 and 11, which may be made either annular or as two mutually perpendicular portions of these rings.
  • apertures of the computer-generated holograms 10 and 11 which may be made either annular or as two mutually perpendicular portions of these rings.
  • annular aperture there is a strong shielding of the central part of the entrance pupil of the optical system, that, as is known in the optical art, results in a substantial increase of secondary annular diffraction maxima and a narrowing of the central disk of the Airy ring.
  • each annular zone When a second variant of making the adjusting holograms 10 and 1 1 is used (shown in Fig. 3), the diametrically opposite portions of each annular zone are correspondent with two light-coloured spots, formed by two identical light beams at a definite convergence angle. They are overlapped in their focusing plane and undergo interference. Some various variants of their overlapping have been experimentally tested to simulate steps of the final alignment of the system.
  • Fig. 4 shows three images of Airy rings.
  • the Airy ring (Fig. 4b) corresponds to the case with no central shielding. It simulates an image of the circle of diffusion, corresponding to the sighting point AQ.
  • TWO other rings (Fig. 4a and 4c) relates to the case when a central shielding occurs. They simulate images of the circles of diffusion for the points A [0 and A N . Referring to Fig. 4a and Fig. 4c, the light- coloured zones in the centres of these images are smaller than the light-coloured central zone of the image (see Fig. 4b), which simulates an image of the circle of diffusion for the sighting point A 0 . It is obvious that it helps improve the accuracy of their overlapping.
  • the annular shape of apertures of the computer-generated holograms 10 and 1 1 helps improve the accuracy of the alignment process.
  • Fig. 5 shows images of the circles of diffusion in the focal plane of the focusing objective 13 of the registering unit, which correspond to three stages of their overlapping (mutual approach). The images are moved closer to each other by displacing longitudinally the element which simulates the secondary mirror 9 of the system to be aligned.
  • Full overlapping of the circles of diffusion (Fig. 5c) means that the two-mirror system has been aligned in the longitudinal direction.
  • the overlapped image is, in its turn, to be superimposed on the image of the sighting point AQ (not depicted in Fig. 5) by angular rotations of the secondary mirror 9.
  • Convergence and divergence of the examined circles of diffusion are very sensitive to a displacement of the simulator of the secondary mirror 9 along the optical axis, influencing positively the accuracy and, ultimately, reliability of the mirror alignment of the system being aligned.
  • the distance between the vertices of the reflecting surfaces of the primary and secondary mirrors is 3057.317 mm.
  • the permissible tilt for the secondary mirror of this telescope, while rotating about its centre, is 12 ⁇ , and the transverse displacement of the secondary mirror must not exceed 50 ⁇ . Because of these tolerances, the diameter of the round hole in the stop 7, mounted in the central hole of the primary mirror 12, must be not less than 0.4 mm.
  • Such holograms may be fabricated, for example, on circular ruling engine MDG-500, developed at OJSC "NPO "GIPO" and designed to manufacture on-axis computer-generated holograms up to 500 mm in diameter.

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  • Physics & Mathematics (AREA)
  • General Physics & Mathematics (AREA)
  • Chemical & Material Sciences (AREA)
  • Dispersion Chemistry (AREA)
  • Optics & Photonics (AREA)
  • Holo Graphy (AREA)
  • Diffracting Gratings Or Hologram Optical Elements (AREA)
  • Optical Recording Or Reproduction (AREA)

Abstract

A device may be used for aligning two-mirror telescopes in process of their assembly and operation at observatories, including spacebased observatories. The device contains a monochromatic light source, a collimator and a beamsplitter to form a reference branch and a working one. The working branch includes a focusing objective, two on-axis reflective adjusting computer-generated holograms, fabricated on the reflecting surface of a secondary mirror, mostly in its edge zone, arranged in alignment with its optical axis. Holograms apertures may be made either as circular rings or as couples of diametrically opposite portions of respective circular rings, which are slewed around each other at a definite angle. The central hole of the primary mirror contains a holed stop, whose centre is arranged in alignment with the reflecting surface vertex of said mirror and the focus of the focusing objective. The reference branch contains a flat reference mirror mounted perpendicular to light beams radiating from the beamsplitter, which may be made as a prism-cube, whose edge surface, containing a reflective coating on the reference branch side, is a flat reference mirror. A registering unit contains a second focusing objective, a position photodetector connected to a data imaging and processing unit. The technical result is a highly reliable and efficient alignment of two-mirror centered optical systems due to a high-precision unified optical element which is time invariant. Such being the case, it provides a de crease in the number of elements and a reduction in the number of segments, being under control, to a single one.

Description

Device for aligning a two-mirror centered optical system
The present invention relates to inspection technology and may be used for aligning two-mirror centered optical systems, including those of Cassegrain and Ritchey-Chretien telescopes, when assembling and aligning them in a workshop and in process of their routine operation at ground- and space-based observatories.
There has been provided a device for aligning optical systems of two-mirror telescopes, containing a point light source as a specular point, located on an optical axis of the system being aligned and coaxially mounted auxiliary optical elements [N.N. Mikhelson, "Reciprocal mirror alignment in two-mirror telescopes", Opticheskii Zhurnal, 1996, No. 3, pp. 66-68 (in Russia)], which may be used for aligning optical systems of Cassegrain and Ritchey-Chretien telescopes. The device contains auxiliary optical elements which are a spherical hole, made in the central zone of the secondary mirror reflecting surface, the curvature centre being in the equivalent focus eq' of the telescope, and a flat mirror with an annular aperture ("a collar"), surrounding the secondary mirror on its outer diameter side and rigidly fixecf to said mirror; with the flat mirror reflecting surface facing the telescope primary mirror reflecting surface. It is obvious that said flat annular mirror should be perpendicular to the secondary mirror optical axis and the hole curvature center should be located on said axis. Such being the case, when a point light source is placed in the equivalent focus Feq' of the telescope centered system, said spherical hole forms its autocollimating image. A second autocollimating image of the point light source is formed exactly the same way, its beams being reflected subsequently from the secondary mirror reflecting surface, a respective zone of the primary mirror reflecting surface, the flat annular mirror reflecting surface (from "the collar") and then in the back path of rays from the reflecting surfaces of the primary and secondary mirrors of the system. When its two autocollimating images coincide with the point light source, the telescope optical system is considered to be aligned.
A disadvantage of this device is its low alignment reliability due to inevitable errors in the course of manufacturing the secondary mirror having a spherical hole and a flat annular mirror ("a collar"): the hole curvature centre may be located out of alignment with the secondary mirror optical axis and said axis may be unparallel to the normal to the flat annular mirror ("the collar"). This inevitably results in a telescope resolution degradation.
The closest to the technical spirit of the claimed invention is a device for aligning two-mirror centered optical systems [V.P. Ivanov, N.P. Larionov, A. V. Lukin, and A.A. Nyushkin, "Alignment of two- mirror centered optical systems using computer-generated hologram optical elements", Opticheskii Zhurnal, 20 fO, Vol. 77, No. 6, pp. 14- 18 (in Russia)].
This device contains a monochromatic light source and a collimator and a beamsplitter, which are tandem-mounted along a ray path, to form a reference branch and a working one, the latter having a focusing objective to form a point light source, a computer-generated hologram optical element, consisting of three adjusting coaxial computer-generated holograms, with two of them being reflective, one of them forming an autocollimating image of the point light source directly and the other one in conjunction with a respective zone of the reflecting surface of the primary mirror, a flat reference mirror mounted in the reference branch perpendicular to light beams, radiating from the beamsplitter, a second focusing objective and a position photodetector, connected to a data imaging and processing unit, which are mounted in the registering unit.
Disadvantages of the device are its insufficient reliability and efficiency of the alignment of two-mirror centered optical systems, consisting of a primary mirror and a secondary one, as well as a limited possibility of carrying out periodic alignment inspections of said systems being in the normal mode at observatories because, mainly, of considerable weight and overall dimensions of the substrate of the computer-generated hologram optical element.
The claimed engineering solution is aimed at enhancing reliability and efficiency of alignment inspections of two-mirror centered optical systems in the course of their assembly and alignment, as well as in the normal mode, in process of their operation at observatories, this solution has been found by aligning the optical axes of holograms with the axis of the secondary mirror, which is to be aligned, by a decrease in the number of elements in the optical design of the aligning device and a reduction in the number of segments, being under control, to a single one - a distance between the vertices of the reflecting surfaces of the primary and secondary mirrors.
The above problem is solved in the claimed device for aligning a two-mirror centered optical system, consisting of a primary mirror and a secondary one, containing a monochromatic light source and a collimator, a beamsplitter, which are tandem-mounted along a path of rays, to form a reference branch and a working one, the working branch having a focusing objective to form a point light source, two on-axis reflective adjusting computer-generated holograms, which are coaxial, with one of them forming an autocollimating image of the point light source directly and the other one in conjunction with the respective zone of the reflecting surface of the primary mirror, a flat reference mirror, mounted in the reference branch perpendicular to light beams, radiating from the beamsplitter, the registering unit containing a second focusing objective, a position photodetector, connected to a data imaging and processing unit, the both adjusting computer-generated holograms being made on the reflecting surface of the secondary mirror, mostly in its edge zone, the common optical axis of on-axis adjusting computer-generated holograms being arranged in alignment with the optical axis of the secondary mirror, with a central hole of the primary mirror containing a holed stop, whose centre is brought into coincidence with the reflecting' surface vertex of the primary mirror and the back focus of the focusing objective;
and wherein the beamsplitter is made as a prism-cube, and the flat reference mirror is made as a prism-cube edge surface, containing a reflective coating on the reference branch side;
and wherein the stop hole is made round, whose minimum diameter is determined by the condition
0≥2Mf' \/D,
where 0 is the diameter of the round stop hole;
D and /' are the entrance pupil diameter and the focal length of the focusing objective, respectively;
λ is the radiation wavelength of the monochromatic light source; and wherein the distribution of radii p of the annular zone middles in the adjusting computer-generated holograms structures is determined by the conditions:
Al m[p(x - d), y2(x - d), d] = λ/w is for the hologram, forming an autocollimating image of the point light source directly,
l2,m[p(x - d),
Figure imgf000007_0001
yi(x - d), d] = λ/w is for the hologram, forming an autocollimating image of the point light source in conjunction with the respective zone of the reflecting surface of the primary mirror,
where m is the annular zone number of the adjusting computer- generated hologram;
A/i;W is the path-length difference between the on-axis beam and the beam, corresponding to the mth zone middle of the hologram, forming an autocollimating image of the point light source directly;
ΔΙ ,η is the path-length difference between the on-axis beam and the beam, corresponding to the m'h zone middle of the hologram, forming an autocollimating image of the point light source in conjunction with the respective zone of the reflective surface of the primary mirror;
y\(x) and y {x - d) are the functions which determine the profile shape of the reflecting surfaces of the primary mirror and the secondary one, respectively, in the rectangular coordinate system Oxy, having the origin at the vertex 0\ of the primary mirror reflecting surface and the axis Ox, arranged in alignment with the optical axis of the two-mirror centered optical system;
d is the distance from the reflecting surface vertex of the secondary mirror to that of the primary one;
λ is the radiation wavelength of the monochromatic light source; and wherein each of the two adjusting computer-generated holograms is made as a couple of diametrically opposite portions of respective circular rings, with said couples being slewed around each other at a definite angle of say 90 degrees.
Fig.l shows a schematic optical diagram of the claimed device for aligning a two-mirror centered optical system.
Fig.2 shows an aperture of the secondary mirror, having apertures of on-axis reflective adjusting computer-generated holograms in its edge zone, with said apertures being made in the form of circular rings, with said holograms being coaxial between themselves and the secondary mirror reflecting surface.
Fig.3 shows an aperture of the secondary mirror, having apertures of reflective adjusting computer-generated holograms which are located in its edge zone, with said apertures being made in the form of a couple of diametrically opposite portions of respective circular rings of on-axis computer-generated holograms which are coaxial between themselves and the secondary mirror reflecting surface.
Fig.4 and Fig.5 show the results obtained during an experimental prototyping of the claimed device.
Fig.6 and Fig.7 show spatial frequency of adjusting holograms for the secondary mirror (Fig.6) and the primary mirror (Fig.7) of the T-170M telescope.
The claimed device for aligning a two-mirror centered optical system contains a monochromatic (laser) light source 1 , a collimator 2 and a beamsplitter 3 in the form of a prism-cube, having a semitransparent layer 4, which splits the incident beam into two parts, one of them (reflected from the semitransparent layer 4) propagates to the reference branch, and another one (having passed through the semitransparent layer 4) propagates to the working branch of the device. The reference branch includes a reference flat mirror 5, which is located perpendicular to rays, emanating from the semitransparent layer 4 of the beamsplitter 3, said reference flat mirror being made as an edge surface of the prism-cube 3, having a reflective coating on the reference branch side. The working branch includes along the path of beams a focusing objective 6 to form a point light source, a holed stop 7, whose plane 8 is in alignment with the back focal plane of the objective 6, a secondary mirror 9, which is to be aligned, of the two- mirror system, an edge zone of the reflecting surface* thereof containing adjusting on-axis reflective computer-generated holograms 10 and 11, which are mounted coaxially relative thereto, and a primary mirror 12, which is to be aligned, of the two-mirror system. δ
In this case, the stop 7 is mounted in the central hole of the primary mirror 12 so that the hole centre of the stop 7 is aligned with the vertex 0\ of the reflecting surface of the primary mirror 12 and the back focus of the focusing objective 6. The registering unit contains a second focusing objective 13, a position photodetector 14 connected to a data imaging and processing unit 15. Such being the case, the back focal plane of the objective 13 is aligned with a photosensitive surface 16 of the position photodetector 14.
The hole of the stop 7 may be made round whose minimum diameter is determined by the condition
0≥2.44/' λ /£>,
where 0 is the diameter of the round stop hole;
D and /' are the entrance pupil diameter and the focal length of the focusing objective 6, respectively;
λ is the radiation wavelength of the monochromatic light source 1. Each of the two adjusting computer-generated holograms 10 and 1 1 may be made as a couple of diametrically opposite portions of respective circular rings, with said couples being slewed around each other at a definite angle of say 90 degrees (Fig. 3).
What happens is this.
The light beam from the monochromatic light source 1 propagates to the collimator 2 which transforms it into a parallel expanded light beam which falls on the' beamsplitting prism-cube 3, where some part of it passes through the semitransparent layer 4 and some of it is reflected thereby. The reflected part of the light beam propagates to the reference branch, falls on the flat mirror surface 5 of the prism-cube 3, oriented perpendicular to the light rays of said beam, is reflected there from and under conditions of autocollimating path some part of it is reflected from the semitransparent layer 4 and some of it passes through it. The reflected part of the reference light beam passes backward through the collimator 2 and falls into a hole of an output window of the light source 1 , and the part of the reference beam, having passed through the semitransparent layer 4, propagates to the registering unit and is focused by the second focusing objective 13 in the point A0, located on the photosensitive surface 16 of the position photodetector 14 due to an overlapping of this layer 16 with the back focal plane of the objective 13. The image of this point is displayed on a screen of the data imaging and processing unit 15 (in the screen centre, Fig.l). It is used as a sighting point in the course of aligning the mirrors 9 and 12 of the two-mirror system.
The part of the light beam that emerges from the collimator 2, having passed through the semitransparent layer 4, propagates to the working branch of said device, which then passes through the focusing objective 6, which transforms it into a convergent homocentric beam, centered in the back focus of this objective (a "specular" point), passes through the stop 7, whose hole centre is aligned with the back focus of the objective 6, and it falls as a divergent homocentric beam on the reflecting surface of the secondary mirror 9. Such bein^ the case, the back focal plane of the objective* 6 is aligned with the working plane 8 of the stop 7, facing the secondary mirror 9. The divergent homocentric beam diffracts on the on-axis computer-generated holograms 10 and 11. The light rays, diffracted on the on-axis computer-generated hologram 10 in the back direction in the operating order of diffraction, converge into a point in the hole centre of the stop 7, forming an autocollimating image of the "specular" point, formed by the focusing objective 6. Then, the beam, formed by the hologram 10, passes backwards through the focusing objective 6, being transformed into a parallel beam, a part of it is reflected from the semitransparent layer 4 and focused by the second focusing objective 13 in the point A \0 on the photosensitive surface 16 of the position photodetector 14. The image of this point is displayed on the screen of the data imaging and processing unit 15. It is formed only by the secondary mirror 9. The size of the point image on the screen of the unit 15 depends on the deviation of the estimated value of the distance from the back focus of the objective 6 to the vertex (¾ of the reflecting surface of the secondary mirror 9, and its deviation from the image of the sighting point AQ in the transverse direction depends on the decentering of the secondary mirror 9. The adjusting hologram 10 is made as a circular ring in the edge zone of the reflecting surface of the secondary mirror 9 (see Fig. 2, Reference Numeral 10).
The light beam, diffracted on the adjusting hologram 1 1, propagates to the primary mirror 12 of the system, which is to be aligned, forming a congruence of diffracted light beams, which * coincides with the estimated congruence of normals to the portion of the reflecting surface of the primary mirror 12, whereupon falls this light beam. That is why an autocollimated reflection of said light beam occurs upon this portion of the reflecting surface of the primary mirror 12, with said light beam falling backwards on the hologram 1 1 and being transformed again thereby (in the operating order of diffraction) into a congruence of diffracted beams, converging in this case in the hole centre of the stop 7. Then this light beam propagates backwards through the objective 6, being transformed by the latter into a parallel beam, which is reflected from the semitransparent layer 4 and focused by the objective 13 in the point Au on the photosensitive surface 16 of the position photodetector 14. The image of the point is displayed on the screen of the unit 15. Using the image size of the point A , one can determine the deviation from the estimated value of the distance between the vertices of the reflecting surfaces of the secondary mirror 9 and the primary mirror 12, and through the use of the transverse displacement relative to the image of the sighting point A0, one can determine the total value of the decentering of the secondary mirror 9 and the primary mirror 12. The adjusting computer-generated hologram 1 1, when being illuminated by a homocentric light beam, forms (in the operating order of diffraction) a congruence of diffracted beams, which coincides with the congruence of normals to the portion of the reflecting surface of the primary mirror 12, where upon this light beam falls. Its structure is made as a circular ring (circular annular aperture) in the edge zone of the reflecting surface of the secondary mirror 9 (see Fig. 2, Reference Numeral 1 1), coaxially with the structure of the hologram 10.
Structures of annular apertures of the on-axis holograms 10 and 1 1 consist of annular zones. The distribution of radii p of the annular zone middles in said structures is determined by the conditions: Al\ m[p(x - d), y2(x - d), d\ = λ/w is for the hologram 10,
h,m[p(x - d), yi(x), y2(x - d), d = λ/w is for the hologram 1 1 , where m is the annular zone number of the adjusting computer- generated hologram;
l jm is the path-length difference between the on-axis beam and the beam, corresponding to the m'h zone middle of the hologram 10;
l2,m is the path-length difference between the on-axis beam and the beam, corresponding to the m,h zone middle of the hologram 1 1 ;
y\{x) and y2 x - d) are the functions which determine the profile shape of the reflecting surfaces of the secondary mirror 9 and the primary mirror 12, respectively, in the rectangular coordinate system Oxy, having the origin at the vertex 0\ of the reflecting surface of the primary mirror 12, and the axis Ox, arranged in alignment with the optical axis of the two-mirror centered optical system;
d is the distance from the vertex 02 of the reflecting surface of the secondary mirror 9 to the vertex 0 of the reflecting surface of the primary mirror 12;
λ is the radiation wavelength of the monochromatic light sour ce 1.
In order to align the mirrors 9 and 12 of the two-mirror system, one has to carry out the following operations.
First, assemble and adjust a unified unit consisting of the elements 1, 2, 3, 6, 13, 14 and 15, wherein the prism-cube 3 is oriented so that the parallel beam, emerging from the collimator 2 and reflected by the semitransparent layer 4 of the prism-cube 3, falls perpendicular on the flat reference mirror 5 (the edge of the prism- cube 3). This is controlled by a trace of the light spot on the output window of the laser light source 1, formed by a light beam, reflected from the mirror 5. The prism-cube 3 is rotated so that the beam is directed into the hole of the output window of the light source 1. Then the photosensitive surface 16 of the position photodetector 14 is aligned with the back focal plane of the second focusing objective 13 to obtain a minimal circle of diffusion in order to control the image of the sighting point A0 on the screen of the data imaging and processing unit 15. The focusing objective 6 is selected to meet the condition
Figure imgf000015_0001
where D and / are the entrance pupil diameter and the focal length of the focusing objective 6, respectively;
d is the distance from the vertex <¾ of the reflecting surface of the secondary mirror 9 to the vertex 0\ of the reflecting surface of the primary mirror 12;
ec m is the diameter of the secondary mirror 9.
Then, the stop 7 is placed into the central hole of the primary mirror 12. The operating plane 8 of the stop 7 will be tangent to the reflecting surface of the primary mirror 12 at its vertex 0\. The hole centre of the stop 7 is aligned with the vertex 0\ of the reflecting surface of the primary mirror 12. The diameter of the round hole or the minimum linear size of the non-round shape hole of the stop 7 is virtually determined by the condition
0≥2 f' \ /D,
where 0 is the diameter of the round hole or the minimum linear size of the non-round shape hole of the stop 7;
D and / are the entrance pupil diameter and the focal length of the focusing objective 6, respectively;
λ is the radiation wavelength of the monochromatic light source 1.
Then, the unified unit, consisting of the elements 1, 2, 3, 6, 13, 14 and 15, is oriented relative to the secondary mirror 9 of the system to be aligned. For this purpose it is mounted at the rear of the primary mirror 12, and with the help of longitudinal and transverse aligning displacements the back focus of the objective 6 is aligned with the hole centre in the stop 7 and an illumination of the secondary mirror 9 is controlled by a light beam diverging from the back focus of the objective 6. By angular displacements of the unified unit one obtains a position wherein the aperture of the secondary mirror 9 is illuminated axially symmetrically to the light beam, falling on it. Then, if necessary, by longitudinal and transverse aligning displacements of the unified unit a fine alignment is made to obtain a fine alignment of the back focus of the objective 6 with the operating surface 8 of the stop 7 and its hole centre.
Having performed the above-mentioned operations of alignment and if one or two images of the autocoUimating points A\0 and An are missing, by transverse and angular displacements of the secondary mirror 9 the missing autocoUimating images are displayed on the screen of the unit 15, and by displacing the secondary mirror 9 along the optical axis one obtains the minimal circles of diffusion of autocoUimating images A 0 and An and superimposes them on the image of the sighting point A0 by displacing the secondary mirror 9. As a result, the secondary mirror 9 will be placed at the estimated distance d from the back focus of the objective 6 and, respectively, the vertex 0 of the reflecting surface of the primary mirror 12. In this case, the optical axes of the secondary mirror 9 and the primary mirror 12 will be aligned.
In order to make the process of alignment easy and reduce the size of the "inoperative" area on the reflecting surface of the secondary mirror 9, which is occupied by the adjusting holograms 10 and 11, it is expedient to make them in the form of small-sized portions of diametrically opposite corresponding annular zones. This enables to make these portions (holograms aperture) within one annular zone, that allows to reduce considerably the area occupied by them on the reflecting surface of the secondary mirror 9 (see Fig. 3). The apertures of the holograms 10 H 11 may be made of different shapes, for example, round or square. In such a case, they may be made round for one hologram and square - for another. This enables to determine rapidly and reliably to which of the two holograms belong the images on the screen of the unit 15 and define the direction and value of rotations and displacements of the secondary mirror 9 while aligning the two-mirror system as a whole. This feature considerably enhances reliability and efficiency of the alignment when using the claimed device.
Since while manufacturing the holograms 10 and 11, their centering is performed to a high degree of accuracy, for example, on the basis of the reference cylinder surface of the secondary minror 9, the optical axes of the holograms 10 and 1 1 will be aligned with the optical axis of the secondary mirror 9 with the least allowable error (practically not more than one micrometer). Meanwhile their centering with the secondary mirror 9 remains obviously invariable in future operation of the two-mirror system. This feature considerably enhances the reliability of alignment when using said device in a shop and in the normal mode of operation at observatories.
Moreover, the portion of the reflecting surface of the secondary mirror 9 of the two-mirror system to be aligned, which is used in the claimed device as a substrate for the adjusting holograms 10 and 11, provides a decrease in the number of elements in the optical design of the device and a reduction in the number of segments, being under control, to a single one, i.e. the distance between the vertices of the reflecting surfaces of the secondary mirror 9 and the primary mirror 12 of the two-mirror system. Thus, it substantially enhances reliability and efficiency of the alignment of two-mirror systems as well as it reduces weight and overall dimensions of control instrumentation.
It should be noted that the adjusting holograms 10 and 1 1 may be synthesized for operations at a wavelength of λ, which is less than the wavelength of the short-wavelength limit of the spectral operating range of the two-mirror system being aligned. Thus, in this case structures of the adjusting holograms 10 and 11 may be made in any light zone of the reflecting surface of the secondary mirror 9, because their negative effects at the operating wavelengths will be negligible.
Thus, it follows from the above substantiations that said device is really highly reliable and efficient.
The claimed device includes apertures of the computer-generated holograms 10 and 11, which may be made either annular or as two mutually perpendicular portions of these rings. In case of an annular aperture there is a strong shielding of the central part of the entrance pupil of the optical system, that, as is known in the optical art, results in a substantial increase of secondary annular diffraction maxima and a narrowing of the central disk of the Airy ring.
When a second variant of making the adjusting holograms 10 and 1 1 is used (shown in Fig. 3), the diametrically opposite portions of each annular zone are correspondent with two light-coloured spots, formed by two identical light beams at a definite convergence angle. They are overlapped in their focusing plane and undergo interference. Some various variants of their overlapping have been experimentally tested to simulate steps of the final alignment of the system.
Fig. 4 shows three images of Airy rings. The Airy ring (Fig. 4b) corresponds to the case with no central shielding. It simulates an image of the circle of diffusion, corresponding to the sighting point AQ. TWO other rings (Fig. 4a and 4c) relates to the case when a central shielding occurs. They simulate images of the circles of diffusion for the points A [0 and A N . Referring to Fig. 4a and Fig. 4c, the light- coloured zones in the centres of these images are smaller than the light-coloured central zone of the image (see Fig. 4b), which simulates an image of the circle of diffusion for the sighting point A0. It is obvious that it helps improve the accuracy of their overlapping. Thus, while aligning two-mirror optical systems, the annular shape of apertures of the computer-generated holograms 10 and 1 1 helps improve the accuracy of the alignment process.
Fig. 5 shows images of the circles of diffusion in the focal plane of the focusing objective 13 of the registering unit, which correspond to three stages of their overlapping (mutual approach). The images are moved closer to each other by displacing longitudinally the element which simulates the secondary mirror 9 of the system to be aligned. Full overlapping of the circles of diffusion (Fig. 5c) means that the two-mirror system has been aligned in the longitudinal direction. In order to complete the alignment, the overlapped image is, in its turn, to be superimposed on the image of the sighting point AQ (not depicted in Fig. 5) by angular rotations of the secondary mirror 9. Convergence and divergence of the examined circles of diffusion are very sensitive to a displacement of the simulator of the secondary mirror 9 along the optical axis, influencing positively the accuracy and, ultimately, reliability of the mirror alignment of the system being aligned.
In order to test whether the claimed device is suitable, we have calculated the spatial frequency of the adjusting holograms 10 and 1 1 for aligning mirrors designed for space telescope type T- 170M which is being developed in Russia in collaboration with foreign companies [A.A. Boyarchuk, N. V. Steshenko, and V. Yu. Terebizh, "Optical system of space telescope type T-170M", The Proceedings of the Crimean Astrophysical Observatory, 2008, Vol. 104, No. 1, pp.229-239].
Primary mirror parameters:
clear aperture 1700 ± 1 mm,
curvature radius at vertex 7820.0 ± 10 mm, eccentricity squared 1.029508 ± 0.0008, central hole diameter 440.0 mm,
central thickness (axially) 100.0 mm. Secondary mirror parameters:
clear aperture 399.07 + 1/- 0 mm, curvature radius at vertex 2214.761 ± 8 mm, eccentricity squared 2.848076 ± 0.005.
The distance between the vertices of the reflecting surfaces of the primary and secondary mirrors is 3057.317 mm.
The permissible tilt for the secondary mirror of this telescope, while rotating about its centre, is 12 μπι, and the transverse displacement of the secondary mirror must not exceed 50 μιη. Because of these tolerances, the diameter of the round hole in the stop 7, mounted in the central hole of the primary mirror 12, must be not less than 0.4 mm.
Calculations show that the maximum spatial frequency for the adjusting holograms 10 and 1 1 does not exceed 500 mm-1 (see Fig. 6 and Fig. 7). Such holograms may be fabricated, for example, on circular ruling engine MDG-500, developed at OJSC "NPO "GIPO" and designed to manufacture on-axis computer-generated holograms up to 500 mm in diameter.

Claims

We claim:
1. A device for aligning a two-mirror centered optical system consisting of a primary mirror and a secondary one, containing a monochromatic light source and a collimator and a beamsplitter, tandem-mounted along a ray path, to form a reference branch and a working one, the latter having a focusing objective to form a point light source, two on-axis reflective adjusting computer-generated holograms, which are coaxial, with one of them forming an autocollimating image of the point light source directly and the other one in conjunction with the respective zone of the reflective zone of the primary mirror, a flat reference mirror, mounted in the reference branch perpendicular to light beams, radiating from the beamsplitter, with a registering unit containing a second focusing objective, a position photodetector, connected to a data imaging and processing unit, wherein the both adjusting computer-generated holograms are made on the reflecting surface of the secondary mirror, mostly in its edge zone, the common optical axis of on-axis adjusting computer- generated holograms being arranged in alignment with the optical axis of the secondary mirror, with a central hole of the primary mirror containing a holed stop, whose centre is brought into coincidence with the reflecting surface vertex of the primary mirror and the back focus of the focusing objecti e.
2. A device for aligning a two-mirror centered optical system according to claim 1 , wherein the beamsplitter is made as a prism- cube, and the flat reference mirror is made as an edge surface of the prism-cube, containing a reflective coating on the reference branch side.
3. A device for aligning a two-mirror centered optical system according to claim 1 , wherein the stop hole is made round, whose minimum diameter is determined by the condition
0≥2Mf' \ /D,
where 0 is the diameter of the round stop hole;
D and /' are the entrance pupil diameter and the focal length of the focusing objective, respectively;
λ is the radiation wavelength of the monochromatic light source.
4. A device for aligning a two-mirror centered optical system according to claim 1 , wherein the distribution of radii p of the annular zones middles in the adjusting computer-generated holograms structures is determined by the conditions:
Al^m\p(x - d), )>2(x - d), d] = Xw is for the hologram, forming an autocollimating image of the point light source directly,
Al2,m[p(x - d),
Figure imgf000023_0001
= fon is for the hologram, forming an autocollimating image of the point light source in conjunction with the respective zone of the reflective surface of the primary mirror,
where m is the annular zone number of the adjusting computer- generated hologram;
A i m is the path-length difference between the on-axis beam and the beam, corresponding to the m,h zone middle of the hologram, forming an autocollimating image of the point light source directly;
Al2,m is the path-length difference between the on-axis beam and the beam, corresponding to the mth zone middle of the hologram, forming an autocollimating image of the point light source in conjunction with the respective zone of the reflective surface of the primary mirror;
y\{x) and y2(x - d) are the functions which determine the profile shape of the reflecting surfaces of the primary mirror and the secondary one, respectively, in the rectangular coordinate system Oxy, having the origin at the vertex 0 of the primary mirror reflecting surface and the axis Ox, arranged in alignment with the optical axis of the two-mirror centered optical system;
d is the distance from the reflecting surface vertex of the secondary mirror to that of the primary one;
λ is the radiation wavelength of the monochromatic light source.
5. A device for aligning a two-mirror centered optical system according to claim 1 , wherein each of the two adjusting computer- generated holograms is made as a couple of diametrically opposite portions of respective circular rings, with said couples being slewed around each other at a definite angle of say 90 degrees.
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