RU2248024C2 - Katadioptrical telescope - Google Patents

Abstract

FIELD: optics.

SUBSTANCE: device has, mounted along beam path, main concave spherical mirror, correcting element, including two singular lenses of different grade glass with quasi-close in visible spectrum range dispersion coefficients, first of them is made in form of negative quasi-focal meniscus, directed by concave portion to observation object, and second one, negative, has mirror reflecting surface and is made of material with lesser refraction coefficient and three-lens isoplanatic compensator of non-axial aberrations, the last along beam path component of which is made in form of quasi-concentric meniscus, directed by concave portion to image plane.

EFFECT: better image quality, higher efficiency.

3 cl, 3 dwg

Description

The present invention relates to the field of astronomical instruments and can be used in serial small-sized telescopes used to study the astroclimate, observations with a charging communication device (LAN cameras) of various celestial objects, as well as for conducting astrophotographic, spectral, photometric and other works in a wide range spectrum. The proposed optical system can also be used in serial amateur telescopes with a working hole diameter of 150-400 mm.

Known catadioptric telescope system proposed by the author [1], containing the main concave spherical mirror and a correction 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 corrective element lens in this system in the system is made in the form of a negative quasi-focal meniscus, turned concave to the object of observation, from a material of a negative lens with a mirror surface. This significantly reduces the secondary spectrum, making it imperceptibly small for the spectral range of 486-656 nm, in which the human eye has the greatest sensitivity. The main disadvantage of this system is off-axis aberration. Astigmatism and field curvature, which are fundamentally unavoidable in this system, increase with an increase in the extension of the focus plane beyond the main mirror. This leads to the fact that in serial telescopes constructed according to this scheme, having a focus plane extending beyond the main mirror of the order of magnitude of the active hole, it is necessary to limit the equivalent relative aperture to 1:10, and the field of view to 30-40 angular minutes, which obviously not enough, for example, for photographic work using the most common photo equipment with a frame size of 24 × 36 mm.

In addition, when CCD arrays are increasingly used in observational astronomy at the present stage of development of light-receiving equipment, and photographic emulsions that are sensitive to a wide spectral range are widely used, the spectral range of 486-656 nm even seems to be insufficient for an amateur telescope and a work area is needed spectrum 400-900 nm with a relative aperture of the Cassegrain system up to 1: 6-1: 7 and an angular field of view of up to 1.5 degrees.

The prototype of the invention is a catadioptric telescope system proposed by the author [2], 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 is 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 spectrum.

The spectral range of this system is limited only by the short-wavelength transmission limit of the glass and the spectral range of the antireflection coating of the corrector lenses. In the operating spectral range of 400–900 nm, such telescopes provide diffractive image quality on the axis with an active hole diameter of 150–400 mm.

However, in the prototype [2], the astigmatism and the curvature of the field cannot be corrected; therefore, when the relative aperture is increased, for example, to a value of 1: 7, the field of view, where it is still possible to obtain a high-quality image, depending on the diameter of the active aperture of the system, cannot be more than 30 arc minutes.

The proposed system of a catadioptric telescope allows to provide higher technical characteristics, namely: to increase the relative aperture and increase the field of view of the telescope.

The proposed system, like the prototype, contains the main concave spherical mirror installed along the beam and a correcting 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 facing concavity to object of observation, and the second negative, has a mirror reflective surface and is made of material with a lower refractive index.

Higher technical characteristics of the proposed system are provided by a new set of distinctive features:

1. A three-lens isoplanatic compensator for off-axis aberrations is installed in front of the focal plane of the telescope, and the last component of the compensator along the beam is made in the form of a quasiconcentric meniscus facing concavity to the image plane.

2. The first two off-axis lens compensators of off-axis aberrations can be glued.

3. The refractive index of the first correcting element along the lens beam for the spectral line 589.3 nm does not exceed 1.66, and the dispersion coefficient is greater than the dispersion coefficient of the second lens by an amount not exceeding 3%.

The installation of an off-axis aberration compensator with positive optical power near the focal plane of the telescope of the lens isoplanatic (with corrected coma and slight spherical aberration) makes it possible to reduce the equivalent focal length of the telescope and, consequently, increase its equivalent relative aperture and at the same time introduce very insignificant spherical aberration on the axis correct astigmatism of the entire system.

The special design of the off-axis aberration compensator, which satisfies the features of Claims 1 and 2, makes it possible to expand the field of view by correcting astigmatism, field curvature, and increase chromatism.

The implementation of the corrector lenses from glasses that meet the characteristics of paragraph 3, makes it possible to build a corrector from glasses of increased uniformity and light transmission.

The author does not know the optical systems of telescopes that have features similar to those that distinguish the proposed system from the prototype, therefore this optical system has significant differences.

The proposed invention is illustrated by the following graphic materials:

Figure 1 is an optical diagram of a catadioptric telescope with an isoplanatic compensator for off-axis aberrations;

figure 2 - scatter diagrams for a telescope with a working hole of 200 mm (relative hole - 1: 6.5);

figure 3 - scatter diagrams for a telescope with a working hole of 250 mm (relative hole - 1: 6.3).

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 negative quasi-focal meniscus reversed by concavity to the object of 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, a three-lens isoplanatic off-axis aberration compensator is installed, including single lenses 4, 5 and 6, and the last component of the compensator 6 along the beam is made in the form of a quasi-concentric meniscus facing concavity to the image plane. In the proposed version of the telescope, the single lenses of the compensator 4 and 5 can be glued with their internal surfaces.

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, leaving which are collected by the lenses 4, 5, 6 of the off-axis aberration compensator in the image plane of the telescope on a short distance behind the compensator.

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

The expansion of the field of view in a catadioptric telescope prototype [2] is hindered by a number of factors: firstly, off-axis aberrations that are fundamentally unrecoverable in this scheme — astigmatism and field curvature, which also increase with increasing focal plane offset beyond the main mirror, and secondly, the presence of in this chromaticity system, an increase that is small only for small angular fields and the spectral region to which the human eye is most sensitive, but for the spectral region 400-900 nm in the angular field of the system up to 1.5 degrees, can be well, a very substantial - 2-3 times larger magnitude of the photographic scattering of modern films (20-30 microns). The relative aperture in the prototype telescope with the focus removed beyond the main mirror is of the order of magnitude of the active hole, namely, with this extension, serial Cassegrain telescopes are built for ease of placement of light-receiving devices, it does not exceed 1: 8 1: 8.5. A number of astrophotographic works require a relative hole, a higher one. In the prototype telescope’s serial circuit, increasing the relative aperture further means not only degrading the image quality on the system axis, but also narrowing the tolerances on its parameters and alignment so that the implementation of such a scheme in serial production becomes unrealistic, in addition, as mentioned above, such the path does not expand the field of view.

In order to circumvent these difficulties, an additional optical element, a compensator for off-axis aberrations, is introduced into the diagram of Fig. 1. The compensator has a positive optical power, and therefore reduces the focal length of the telescope and increases its equivalent relative aperture. The special isoplanatic design of the off-axis aberration compensator corrects astigmatism, field curvature and magnification chromatism. The compensator contains three single lenses 4, 5 and 6 (figure 1). Lens 6 is a quasi-focal negative meniscus, very close to concentric, facing concavity to the image plane. This lens is used to correct field curvature and magnification chromatism. The equivalent optical power of the lens unit 4 and 5 is positive, due to which the compensator increases the relative aperture of the telescope. The lens block 4 and 5 is independently corrected for position chromatism and coma, but it has a relatively small negative spherical aberration, which is introduced in order to compensate for the fundamentally incorrigible negative astigmatism of the system: the main mirror 1 is a correction element (lenses 2, 3). This type of correction of the optical system in optics is called isoplanatic.

The longitudinal spherical aberration on the axis of the telescope is negative and depends on the position of the compensator for off-axis aberrations. The smaller the back segment of the system (the distance between the last surface of the lens 6 (Fig. 1) and the image plane), the lower the spherical aberration and the better the image quality. Therefore, the back section of the system should be selected only slightly larger than the working section of the camera. For a camera with a 24 × 36 mm frame format, this segment is 45.5 mm and this distance is enough to ensure the image quality in the proposed telescope system that is close to diffraction in the center of the field of view and satisfies the requirements for the size of the effective scattering spot (20 -30 μm) for modern light-receiving devices in its peripheral part.

In a wide spectral region of 400–900 nm in the field of view of up to 1.5 degrees, correction of the increase chromatism is of great importance. The quasiconcentric meniscus 6 gives the opposite in sign chromatism of the increase relative to the system: the main mirror 1 is a correction element (lenses 2 and 3). You can choose the thickness of this meniscus in such a way as to completely correct this aberration, but in a wide spectral region, providing along the edge of the field of view correction of chromaticity of magnification for the extreme lines, for example 400 and 900 nm, it is impossible to obtain simultaneous correction of chromaticity of magnification for the rays of the visible region from for the presence in the system of secondary chromatism increase. This aberration is associated with the choice of grades of glass lenses 4 and 5 of the compensator for off-axis aberrations. To provide a spectral range of 436–768 nm, glasses with the usual dispersion path are sufficient, and the lens block 4 and 5 of the off-axis aberration compensator can have the usual achromatic (for two spectral lines) correction of position chromatism. In this case, the choice of glass grades for lenses 4 and 5 is relatively free and makes it possible to glue them with internal surfaces into a single unit, which simplifies the design, reduces stray light and increases the light transmission of the system. But, in order to expand the spectral range of the telescope to 400–900 nm, it is necessary to completely correct the secondary chromatism of magnification. To do this, it is enough to make one of the lenses 4 or 5 out of glass with a special dispersion course. The lens block 4 and 5 of the compensator will have an apochromatic (for three spectral lines) correction of the position chromatism. From the point of view of minimizing chromatic aberrations, it is best to use a positive lens 5 from special glass, and use fluorite or a glass replacing it from a series of special crowns as a material for this lens (glass OK4 GOST 3514-94). Since the linear expansion coefficient of these materials differs significantly from ordinary glass, which is used in lens 4 to provide apochromatic correction of the power component, it is not possible to glue lenses 4 and 5 into a single assembly and the compensator with correction of chromatic aberrations in a wide spectral region of 400–900 nm will contain three individual lenses. The alternation of the optical powers of lenses 4 and 5 in the compensator can be arbitrary. Both possible designs do not have significant differences in the sense of correction of residual aberrations.

The range of the glass range of the correction element in the proposed system of the catadioptric telescope is significantly different from that for the prototype system [2]. In particular, for the meniscus of the prototype system, glass from a series of superheavy crowns was used with a refractive index for the line 589.3 nm from 1.69 to 1.76, the dispersion coefficient of which in the visible region of the spectrum was less than the dispersion coefficient of the negative lens 3.

In the indicated region of refractive index variation, optical glass plants do not produce superheavy crowns with increased uniformity and light transmission suitable for use in astronomical instruments. This creates great difficulties in the serial selection of such glass for the meniscus in the prototype system [2].

However, for meniscus 2 (Fig. 1) of the proposed system, glasses with a refractive index of 1.61-1.66 can be used, which also corresponds to the region of heavy crowns, which are better mastered in production, have high transparency and uniformity. Moreover, in contrast to the prototype, the dispersion coefficient of meniscus 2 in the visible region of the spectrum should be not less, but more than that of negative lens 3, by an amount not exceeding 3%.

As a specific example, confirming the validity of the relationship of the considered distinguishing features with the achievement of the claimed technical characteristics, we present the scatter diagrams for two systems calculated according to the proposed optical design.

A system with an active hole diameter of 200 mm has a relative aperture of 1: 8.75 without an off-axis aberration compensator. The glass grades for the lenses of the correction element used in this scheme are TK14 and KF4 (according to GOST 3514-94). The off-axis aberration compensator increases the equivalent relative aperture to a value of 1: 6.5. The brands of compensator glasses: for lenses 4 and 6 - TF4, for lens 5 BF12. Lenses 4 and 5 are glued together. The angular field of view is 1.3 degrees. The rear section of the system is 53 mm. Scatter plots in the spectral range 436-768 nm and five angles of the field of view of 0, 0.325, 0.65, 0.975 and 1.3 degrees are shown in figure 2.

The circuit with a diameter of the active hole of 250 mm has a relative aperture of 1: 8.5 without an off-axis aberration compensator. The glass grades for the lenses of the correction element used in this scheme are STKZ and BK10. The off-axis aberration compensator increases the equivalent relative aperture to a value of 1: 6.3. The brands of compensator glasses: for lens 4 - KF4, for lens 5 - fluorite and for lens 6 - TF4. Lenses 4 and 5 are not glued. The angular field of view is 1.3 degrees. The back section of the system is 55 mm. Scatter plots in the spectral range of 400-900 nm and five angles of field of view of 0, 0.325, 0.65, 0.975 and 1.3 degrees are shown in Fig.3.

The radius of the circumference of the restriction in the scatter diagrams of FIGS. 2 and 3 is 15 μm.

From figure 2 and 3 it is seen that the transverse aberrations of the proposed system with a compensator for off-axis aberrations in the specified range of the spectrum and the selected focusing plane fit for an angular field of up to 1/3 degree in the amount of 15-20 microns, and at the edge of the field are about 30 microns . An accurate calculation of the diameter of the effective scattering spot, which contains 80% of the light energy reaching the focusing plane in a given spectrum range, taking into account the diffraction and spectral sensitivity of a thin multilayer CCD matrix [3], allows us to state that both systems in the indicated spectral ranges are in an angular field up to 1/3 of a degree give an image of a point object (star) with an effective diameter of 17-18 microns, and within the field of view up to 1.3 degrees - 24-26 microns, which meets the most stringent requirements for the image of a point test object that for astro and substantially fits in to the image quality requirements for astronomical systems operating with CCDs for which the effective size of the photosensitive spots currently represent 7-9 microns, and therefore, the star image may have a size of 14-18 microns.

All brands of optical glass TK14, KF4, STKZ, BK10 used in the corrector according to GOST 3514-94 have the highest category of optical uniformity and satisfy claim 3 of the patent formula.

As proved above, the proposed system provides high quality optical images in a wide range of the spectrum, while its relative aperture and angular field can be increased. This is ensured by the presence of a new set of distinctive features:

1. A three-lens isoplanatic compensator for off-axis aberrations is installed in front of the focal plane of the telescope, and the last component of the compensator along the beam is made in the form of a quasiconcentric meniscus facing concavity to the image plane.

2. The first two off-axis lens compensators of off-axis aberrations can be glued.

3. The refractive index of the first correcting element along the lens beam for the spectral line 589.3 nm does not exceed 1.66, and the dispersion coefficient is greater than the dispersion coefficient of the second lens by an amount not exceeding 3%.

The proposed system, while retaining all the structural advantages of the analogue and prototype: the spherical shape of the optical surfaces, the small size of the corrective lenses, compactness and the absence of stray light, has the following main advantages with respect to the prototype:

1. The possibility of increasing the relative aperture without loss of image quality in a wide spectral region and narrowing the tolerances on the system parameters and its centering to a value of 1: 6.3.

2. The possibility of increasing the angular field of view to a value of 1.3 degrees due to the introduction of an off-axis aberration compensator of a special design in the optical system of the telescope that corrects astigmatism, field curvature and increase chromatism.

3. The ability to improve image quality by expanding the acceptable range of corrector glasses and the resulting opportunity to use glass grades with high transmittance and uniformity in the corrector.

Due to these advantages, the proposed telescope system can be recommended for astrophotographic work and observations with CCD arrays, performed in a wide spectral range of 400-900 nm.

The best field of application of the proposed optical system is the production on its basis of serial, relatively cheap, small-sized with an effective hole diameter of 150 - 400 mm and a relative aperture of 1: 6.3 universal telescopes for educational purposes and astronomy lovers. In this area of application, the proposed system, due to its simplicity and relative cheapness, surpasses such well-known and widely used types of telescopes as the Ritchie-Chretien system, the “meniscus Cassegrain” of Maksutov and the Schmidt-Cassegrain system, which, in addition, it also surpasses in image quality in a wide range of the spectrum and allows using relatively simple means without the use of aspherical surfaces and retouching to provide a high aperture of the telescope and a sufficiently large field of good images with a size of 1.3 grams baleen.

LITERATURE

1. Klevtsov Yu.A. "Catadioptric telescope." USSR copyright certificate No. 605189, Bulletin No. 16, 1978.

2. Klevtsov Yu.A. "Catadioptric telescope." Patent for the invention of the Russian Federation No. 2125285, Bulletin No. 2, 1999.

3. G. Walker "Astronomical observations", translation from English. under the editorship of P.V. Shcheglova M .: Mir, 1990, p. 309.

Claims (3)

1. A catadioptric telescope containing a main concave spherical mirror mounted along the beam and a correcting element, including two single lenses of different grades of glass with dispersion coefficients quasi-close in the visible region of the spectrum, the first of which is made in the form of a negative quasi-focal meniscus turned concave to the object of observation and the second, negative, has a specular reflective surface and is made of a material with a lower refractive index, characterized in that in front of the focal plane three-lens telescope mounted isoplanatic compensator off-axis aberrations, the latter being downstream of the compensator component beam is formed as a meniscus kvazikontsentricheskogo facing concave to the image plane. 2. The telescope according to claim 1, characterized in that the first two off-axis lenses of the off-axis aberration compensator can be glued. 3. The telescope according to claim 1 or 2, characterized in that the refractive index of the first correction element along the lens beam for a spectral line with a wavelength of 589.3 nm does not exceed 1.66, and the dispersion coefficient is greater than the dispersion coefficient of the second lens by, not exceeding 3%.

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