RU2472190C1 - Catadioptric telescope - Google Patents

Abstract

FIELD: physics.

SUBSTANCE: telescope has, in the beam path, a main concave spherical mirror, a compensating element having two separate lenses made of different types of glass with quasi-near dispersion coefficients in the visible spectrum, the first of which is in form of a negative quasi-focal meniscus whose concave surface faces the observed object, and the second is negative, has a mirror reflecting surface and is made from material with a smaller refraction index, and an afocal field aberration corrector consisting of two lenses and is placed in front of the focal plane of the telescope. The negative lens of the corrector which is first on the beam path is made from flint glass in form of a quasi-focal meniscus whose concave surface faces the image plane, and the second positive lens of the corrector is plane-convex, is made from crown glass and its convex spherical surfaces faces the image plane.

EFFECT: wider angular field of view of high-quality images.

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Description

The present invention relates to the field of astronomical instruments and can be used in serial small-sized photovisual telescopes operating both in the visible region of the spectrum and with charge-coupled devices (CCD cameras and digital cameras) used for visual and astrophotographic observations of celestial objects .

Known catadioptric telescope system proposed by the author, containing the main concave spherical mirror and a corrective element of two single lenses, one of which is negative and has a mirror-reflecting surface mounted along the beam behind the main mirror. The first lens of the correction element along the beam is made in the form of a negative quasi-focal meniscus, facing concavity to the object of observation, from the material of a negative lens with a mirror surface [1].

The disadvantage of this system is the limited spectral range of the work.

The prototype of the invention is a catadioptric telescope system proposed by the author, containing the main concave spherical mirror mounted along the beam and a correction element consisting of two single lenses, the first of which is made in the form of a negative quasi-focal meniscus facing concavity to the object of observation, and the second negative and has a mirror reflective surface. The corrector lenses are made of different grades of glass having quasi-close dispersion coefficients in the visible region of the spectrum, the second lens being made of glass with a lower refractive index [2].

In the wavelength range of 400-700 nm, the working range of the spectrum of digital photographic equipment, such telescopes provide the diffraction quality of off-axis points of the image field with a diameter of not more than 10'-12 '. The ability to expand the angular field of view of high-quality images in the prototype is limited by off-axis aberrations: astigmatism, field curvature, magnification chromatism and chromatic coma, which is its main disadvantage.

The task of the invention is to expand the angular field of view of high quality images.

The problem is solved in that in a device containing a main concave spherical mirror mounted along the beam and a correcting element, including two single lenses from different grades of glass with dispersion coefficients quasi-close in the visible spectral region, the first of which is made in the form of a negative quasi-focal meniscus facing concavity to the object of observation, and the second negative, has a mirror reflective surface and is made of glass with a lower refractive index, in front of the focal plane The telescope has an afocal field aberration corrector consisting of two lenses, the first negative lens along the beam is made of flint glass in the form of a quasi-focal meniscus facing concavity to the image plane, the second positive lens is plano-convex made of crown type glass and facing a convex spherical surface to the image plane.

The installation of an afocal corrector of off-axis aberrations in front of the focal plane of the telescope, which consists of two lenses, makes it possible, without changing the focal length of the system, to influence field aberrations of the image: field curvature, astigmatism, magnification chromaticity, and chromatic coma.

The first flint corrector made of glass in the form of a negative quasi-focal meniscus makes it possible to correct field curvature and increase chromatism, as well as chromatic coma, however, the focal length of the system changes and the correction of residual aberrations on the axis is violated.

The second positive corrector lens made of glass of the “crown” type along the beam compensates for these defects. Its plano-convex shape is selected for reasons of manufacturability and minimization of stray light.

As a result of optimization of the design parameters of the afocal corrector lenses, the image quality on the system axis in the spectral region 400-700 nm remains practically unchanged, and the image quality of off-axis field points is significantly improved.

The applicant has not identified technical solutions having features that match the distinctive features of the invention. A catadioptric telescope with the claimed combination of essential features in known sources of information is also not found.

The proposed invention is illustrated by the following graphic materials:

Figure 1 - optical system catadioptric telescope;

Figure 2 - view of the scattering function of the point in the visible spectrum of 486-656 nm along the field of view of the telescope with an active hole of 300 mm (relative aperture - 1: 8);

Figure 3 - graphs of the concentration of energy in the image points, calculated for the spectral range of the digital camera company Canon (EOS 500D) 400-700 nm in the field of view of the telescope with a working hole of 300 mm (relative aperture - 1: 8).

Figure 1 shows the proposed optical system catadioptric telescope.

The system contains a main concave spherical mirror 1 installed along the beam and a correction element, including two single lenses 2 and 3 from different grades of glass with dispersion coefficients quasiblique in the visible spectral region, the first of which 2 is made in the form of a quasi-focal negative meniscus facing concavity to the object observation, and the second negative 3, has a mirror reflective surface and is made of material with a lower refractive index. Near the focal plane of the telescope, an afocal off-axis aberration corrector is installed, consisting of two lenses, including a negative quasi-focal meniscus 4 and a positive plane-convex lens 5.

The rays of light, reflected from the main mirror 1, pass through the lenses of the correction element 2 and 3 and, reflected from the mirror surface of the lens 3, go back through the corrector, the afocal correction element consisting of lenses 4, 5 and, without changing the convergence, are collected in the plane images of the telescope at a short distance behind the afocal corrector.

We will justify the possibility of achieving the claimed technical characteristics in the proposed scheme.

The reasons for the narrow angular field in the prototype telescope are the presence of field aberrations in its optical system: field curvature, astigmatism, magnification chromatism, and chromatic coma. To expand the field of view, it is necessary to compensate for these aberrations, while the focal length and the correction of residual aberrations on the axis of the telescope should not change.

A dual-lens afocal corrector installed in the proposed system of a catadioptric telescope near the image plane provides additional correction parameters for correcting off-axis aberrations. The first negative lens 4 of the afocal corrector along the beam is made of glass of the “flint” type in the form of a quasi-focal meniscus facing concavity to the image plane. The axial thickness of the meniscus serves as a correction parameter for influencing the field curvature and increase chromatism. It is known from the author’s work that menisci of this type, oriented by concavity to the image plane, introduce positive field curvature and increase chromatism into the system, capable of compensating for negative aberrations of these types of telescope at a certain surface curvature [3]. In addition, as it turned out during the aberration analysis of the scheme, such a meniscus of relatively large curvature is able to compensate for the chromatic coma of the telescope. However, it changes the focal length and violates the correction of the residual axial aberrations of the system. To compensate for these disorders, a positive plano-convex lens 5 is introduced into the afocal corrector, facing a convex spherical surface to the image plane. This shape of the lens is technologically advanced and does not give glare that can focus near the image plane. The introduction of a very insignificant spherical aberration into the image of the object of observation by the afocal corrector makes it possible to correct the astigmatism of the entire system, practically without violating the image quality of the axial point of the field of view.

As a specific example, confirming the validity of the connection of the considered distinguishing features with the achievement of the declared technical characteristics, we consider the optical system of the TAL-300K telescope with an afocal two-lens corrector of the proposed design. The effective opening of the telescope is 300 mm, the relative aperture is 1: 8. Lens material of the correction element: lens 2 - glass STK12; lens 3 - TK2 glass. Lens material of the afocal corrector: lens 4 - glass LF5; lens 5 - glass LK4. The distance from the last surface of the lens 5 to the image plane is 90.5 mm.

Figure 2 shows three-dimensional graphs of the polychromatic function of the scattering of a point in the visible range of the spectrum 486-656 nm for three angles of the field of view: 2y = 0 °; 2y = 0.5 ° and 2y = 0.75 °. As can be seen from these graphs, the Strehl number in the center of the field is 0.94, at the edge of the field 0.5 ° - 0.84 and at the edge of the field 0.75 ° - 0.8. As is known, the limiting value of this value in the absence of aberrations is 1, and the value acceptable from the point of view of image quality is 0.8 (a 20% decrease in illumination in the center of the diffraction disk is allowed - the so-called Strehl-Mareshal criterion). Thus, the telescope equipped with an afocal corrector of the proposed design provides an angular field of 2y = 0.75 ° with practically diffractive image quality in the visible region of the spectrum. Thus, the proposed system in the visible region of the spectrum has at least 4 times greater angular field with respect to the prototype.

Figure 3 shows graphs of the concentration of energy in the image of the point for the spectral region 400-700 nm and the same fields of view. The indicated spectral region corresponds to a Canon digital camera (EOS 500 D) with a CMOS sensor half the size of a narrow frame with a diagonal of 28 mm. Digital cameras of this brand are most often used by astronomy enthusiasts for shooting celestial objects. These graphs confirm, and accurate numerical estimates show, that at 80% light concentration the effective diameter of the scattering spot (2R) for the center of the field of view is 17.7 μm. For a system without an afocal corrector, the same value is 14.9 μm. For the field edge 2y = 0.5 °, the effective diameter of the scattering spot is 17.9 μm, and for the field edge 2y = 0.75 ° it is 19.1 μm. The same diameter of the effective scattering spot in the same region of the spectrum has at the edge of the field of view 16 'in the prototype system. Thus, the proposed system has at least three times the photographic angular field. The difference between the effective diameters of the spot scattering spot in the center of the field of view for a system with and without an afocal corrector is only 2.8 μm, that is, a value slightly larger than half the pixel of a CMOS sensor of a digital camera. However, it is precisely such a small value of the change in the spot scattering point that leads to the correction of astigmatism of the entire system. The difference in the effective diameters of the scattering spots of the proposed system within the field of view of 0.75 ° is small and does not exceed 1.4 microns. The diameter of the effective image of a star of 18-19 microns in the spectral range 400-700 nm is currently considered quite acceptable for photographic work. The linear field of view of the TAL-300K telescope for a field angle in the space of objects 2u = 0.75 ° is 32 mm, that is, a value even slightly larger than the diagonal of the frame of a digital camera is 28 mm.

Thus, it can be considered justified that the proposed telescope system with an afocal corrector for off-axis aberrations relative to the prototype has a 3-4 times larger angular field of high-quality images. This is ensured by the presence of a new set of distinctive features.

1. An afocal field aberration corrector consisting of two lenses is installed in front of the focal plane of the telescope.

2. The first negative lens in the direction of the beam is made of flint glass in the form of a quasi-focal meniscus facing concavity to the image plane.

3. The second positive lens is plano-convex, made of glass of the “crown” type and faces with a convex spherical surface to the image plane.

Thus, the proposed telescope system, while retaining all the structural advantages of the analogue and the prototype: the spherical shape of the optical surfaces, the small size of the corrective lenses and compactness, has in relation to the prototype 3-4 times a large angular field of view in the space of objects, which makes it possible to use it both for visual work requiring significant angular fields (for example, search for comets), and for astrophotographic work with digital cameras and specialized CCDs operating in a range of azone of the spectrum from 400 to 700 nm.

The best field of application of the proposed optical system is the production on its basis of serial, relatively cheap, small-sized universal telescopes with a relative aperture of at least 1: 8 for educational purposes and astronomy lovers. In this area of application, the proposed system, in view of its simplicity and relative cheapness, surpasses such well-known and widely used types of telescopes as the Ritchie-Chretien system, the meniscus Cassegrain of D. D. Maksutov and the Schmidt-Cassegrain system. The last two types of telescopes, the proposed system, in addition, still surpasses the image quality in a wide range of the spectrum. The proposed system allows relatively simple means without the use of aspherical surfaces and retouching to provide a sufficiently large field of good images up to 0.75 ° in size.

LITERATURE

1. Klevtsov Yu.A. Catadioptric telescope. // A.S. No. 605189. Bull. No. 16, 1978.

2. Klevtsov Yu.A. Catadioptric telescope. RF patent No. 2125285, bull. No. 2, 1999.

3. Klevtsov Yu.A. Isoplanatic focal length transducer for catadioptric telescope with meniscus corrector // Optical Journal. 2006. T.73, No. 8, p. 50-54.

Claims (1)

A catadioptric telescope containing the main concave spherical mirror mounted along the beam and a correction element, including two single lenses of different grades of glass with dispersion coefficients quasiblique in the visible spectral region, the first of which is made in the form of a negative quasi-focal meniscus that faces the observation object with concavity the second, negative, has a mirror reflective surface and is made of a material with a lower refractive index, characterized in that in front of the focal The telescope’s brightness set an afocal field aberration corrector consisting of two lenses, the first negative lens along the beam made of flint glass in the form of a quasi-focal meniscus facing concavity to the image plane, and the second positive lens flat-convex made of glass like kron ”and faces with a convex spherical surface to the image plane.

Images