RU2125285C1 - Catadioptric telescope - Google Patents

Abstract

FIELD: astronomic instruments, in particular, for observation of Sun, Moon and planets. SUBSTANCE: optical system of device consists of main concave spherical mirror and correction element which are arranged in series. Correction element has two single lenses, first of which is designed as negative quasi-afocal meniscus which concave side is directed towards subject of observation. Second lens of correction element is negative and has mirror reflecting surface. Lenses of correction element are made from different glass types, which dispersion rates in visible spectrum are close. First lens is made from glass with large refraction index. EFFECT: increased light- gathering power, increased spectral range. 2 cl, 5 dwg, 2 tbl

Description

 The invention relates to the field of astronomical instruments and can be used in serial small-sized telescopes with a diameter of the active hole up to 750 mm, used to study the astroclimate, observations of the Sun, Moon and planets, as well as to perform astrophotographic, spectral, photometric and other works. The proposed optical system can also be used in serial telescopes produced for educational purposes, in particular for astronomy enthusiasts.

 Known catadioptric telescope system proposed by Argunov [1, 2], containing the main spherical mirror and a correction element consisting of lenses with spherical surfaces, one of which is a mirror located in a beam of rays converging from the main mirror, characterized in that the correction element is made of separated by an air gap of two or three lenses with a refractive index of at least 1.5, with the ratio of the equivalent focal length of the telescope to the focal length of the main mirror not exceeding 5.

 The main disadvantage of this system is unacceptably large residual chromatism, which reduces the image quality, narrows the spectral region of work and does not allow for a high aperture ratio of the telescope. To reduce it, the first corrector lens is made of glass with a refractive index of at least 1.7 (heavy flint), and the difference in the dispersion coefficients of the glasses is chosen as high as possible, and it is advisable to use at least one glass with a special dispersion course (special flint or fluorite ) However, despite this, it is not possible to sufficiently reduce the optical power of the corrector lenses, which is the reason for the large secondary spectrum and spherochromatic aberration and, in turn, leads to a deterioration in image quality, a narrowing of the spectral range of operation, and limitation of the relative aperture of this system to a value over 1:10. Three-lens variants of the correction element containing special technologically unfavorable grades of glass have a reduced secondary spectrum, however, in this case, the correction of residual axial aberrations covers only a relatively narrow visible region of the spectrum.

 The prototype of the invention is a catadioptric telescope system proposed by the author [3], containing a 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.

 A higher quality of aberration correction and an increase in aperture ratio in this system are achieved due to the fact that the second lens of the correction element is made in the form of a negative quasi-focal meniscus facing concavity to the object from a negative lens material with a mirror surface.

 The quasi-focal negative meniscus provides correction of spherical aberration and third-order coma and, in addition, significantly reduces position chromatism, and the second negative corrector lens along the beam made of the material of the first lens completely corrects the position chromatism, while the secondary spectrum has a value of two order less than in the Argunov system [1]. For a system with a K8 glass corrector [4] in the spectrum range from the F line (486.13 nm) to the C line (656.27 nm), a relative aperture of up to 1: 8 can be provided with an effective hole diameter of up to 750 mm.

 The expansion of the spectral region, necessary for reasons of universality, and an increase in luminosity to 1: 6.5-1: 7, which, with a reasonable exposure time (1-1.5 h), ensure the maximum penetrating ability of the telescope using modern photographic emulsions [5], in this It is difficult for the system to ensure, due to the large curvature of the meniscus surfaces and the associated presence of large (at a relative aperture of 1: 6.5-1: 7) residual aberrations, of which the residual spherical and spherochromatic are of primary importance Kaya aberration. In addition, in the near ultraviolet and infrared spectral regions, for which optical glasses are still sufficiently transparent (365-1500 nm), the secondary spectrum also requires reduction.

 The proposed catadioptric system of the telescope makes it possible to provide even higher technical characteristics: to improve the quality of the correction of axial aberrations, to expand the spectral range of work and increase the aperture of the system.

 The proposed system, like the prototype, contains the main concave spherical mirror installed along the beam and a correcting 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 .

Higher technical characteristics of the proposed system are provided by a new set of distinctive features
1. The lenses of the correction element are made of different grades of glass having quasi-close dispersion coefficients in the visible region of the spectrum, the first lens being made of glass with a high refractive index.

больше коэффициента дисперсии первой линзы

причем

Выполнение линз корректора из стекол квазиблизких по дисперсии, удовлетворяющих отличительным признакам п. 2, дает возможность еще в несколько раз уменьшить вторичный спектр системы для области от 365 до 1500 нм, а использование для первой линзы корректора стекла с большим показателем преломления, удовлетворяющим соотношению по 2, дает возможность увеличить радиусы кривизны этой линзы и уменьшить остаточную сферическую и сферохроматическую аберрации, что позволяет довести относительное отверстие предлагаемой системы до оптимальных значений 1:6,5-1:7.2. The refractive index of the first lens satisfies the ratio of 1.69 ≤ n ≤ 1.76, and the dispersion coefficient of the second lens

greater dispersion coefficient of the first lens

moreover

The implementation of the corrector lenses from glasses quasibi dispersion, satisfying the distinguishing features of clause 2, makes it possible to reduce the secondary spectrum of the system for the region from 365 to 1500 nm by several times, and the use of a glass corrector with a large refractive index for the first lens, satisfying the ratio of 2 , makes it possible to increase the radii of curvature of this lens and reduce the residual spherical and spherochromatic aberration, which allows you to bring the relative aperture of the proposed system to optimal values Niy 1: 6.5-1: 7.

 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:
FIG. 1 is an optical diagram of a catadioptric telescope; FIG. 2 - a secondary spectrum of options for the optical system of a catadioptric telescope; FIG. 3a - residual axial aberration of the prototype with a corrector made of glass grade STK12; FIG. 3b — residual axial aberrations of the proposed system with a corrector made of STK12 and KF6 glasses; FIG. 4a - residual axial aberrations of the proposed system with a corrector from glasses SSK11 and SK24; FIG. 4b — residual axial aberrations of the proposed system with a corrector made of STK10 and TK21 glasses; FIG. 5 - residual axial aberration of the fast version of the proposed system with a relative aperture of 1: 6.2 and a corrector made of STK10 and TK21 glasses.

 In FIG. 1 shows the proposed optical system of a catadioptric telescope. The system contains a main concave spherical mirror 1 installed along the beam and a correction element consisting of two single lenses 2 and 3, the first of which is made in the form of a negative quasi-focal meniscus facing concave 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.

 Rays of light, reflected from the main mirror 1, pass through the corrector lenses 2 and 3 and, reflected from the mirror surface of the lens 3, go back through the corrector, forming an image of the object of observation in the focal plane, which is located behind the main mirror 1.

фокусное расстояние и последний отрезок системы в линии спектра с длиной волны 546,07 нм (е); Δ - вынос плоскости изображения за лицевую поверхность главного зеркала: A - относительное отверстие;

показатели преломления, а

коэффициенты дисперсии линз 2 и 3 для линии спектра e; Δr_2,3/d₂- отношение разности радиусов мениска к его толщине, определяющее величину и знак оптической силы линзы 2. Все линейные параметры схемы приводятся в мм. Радиусы и осевые промежутки указаны для одного прохождения лучей через корректор и в обратном ходе луча не приводятся. В нижней части табл. 1 введена нумерация рассчитанных вариантов, а в верхней части обозначения марки стекла линз 2 и 3, использованные для расчета корректора [4]. Для сравнения, в табл. 1 включены аналогичные варианты 1 и 3 с корректором из одной марки стекла.We will justify the possibility of achieving the stated technical characteristics with specific examples of calculating six variants of the optical systems of a catadioptric telescope, the parameters of which are given in table. 1,
where r _1-5 are the radii of curvature of the surfaces; d _1-4 - the thickness of the lenses and air gaps, D - the diameter of the active holes of the system (diameter of the main mirror); sv⌀ ₂ - light diameter of the first surface of the corrector (determines the central screening of the pupil);

focal length and last segment of the system in the spectral line with a wavelength of 546.07 nm (e); Δ - the removal of the image plane beyond the front surface of the main mirror: A - relative hole;

refractive indices, and

lens dispersion coefficients 2 and 3 for the spectrum line e; Δr _2,3 / d ₂ - the ratio of the difference in the radius of the meniscus to its thickness, which determines the magnitude and sign of the optical power of the lens 2. All linear parameters of the circuit are given in mm. Radii and axial gaps are indicated for one passage of the rays through the corrector and are not given in the reverse direction of the beam. At the bottom of the table. 1, the numbering of the calculated options is introduced, and in the upper part the designations of the brand of glass lenses 2 and 3 used to calculate the corrector [4]. For comparison, in table. 1 similar options 1 and 3 are included with a corrector from the same glass brand.

 All systems tab. 1 are calculated with a visual type of aberration correction in the spectral region from F to C with a relative aperture of 1: 7. An exception is the particularly high-aperture version 6, where the relative aperture is 1: 6.2.

 Let us first consider the requirements for corrector lens glasses, which provide correction of the secondary spectrum in a wide range of wavelengths. Note that in the proposed system variants, the secondary spectrum was corrected in the range from the ultraviolet line i (365 nm) to the infrared region with a boundary wavelength of 1529.6 nm (option 2 of Table 1). Optical glasses are still sufficiently transparent for this spectral region. Other options table. 1 (4, 5, 6) are corrected in the spectral region from 436 to 852 nm. Three most suitable pairs of glasses were selected from the catalog [4]: STK12 / KF6; SSK11 / SK24 (manufactured in Germany); STK10 / TK21 (domestic equivalent of the previous pair), providing correction of the longitudinal secondary spectrum in this range.

Для наглядности, вторичный спектр некоторых систем табл. 2 (1, 2a и 4a) представлен в форме графиков на фиг. 2, где по оси ординат отложена длина волны λ (нм), а по оси абсцисс - величина продольного вторичного спектра

выраженная в долях фокусного расстояния системы.In the table. 2 shows the relative values of the longitudinal secondary spectrum in the proposed systems with these glasses (option 2a, 4a and 5) and in equivalent systems from the same glass grade (options 1 and 3). Non options table. 2 correspond to the numbers of the systems of the table. 1. In the column ΔP _λ tab. 2 shows the differences of the relative partial dispersions of glasses, expressed by the differences of the relative partial dispersions of glasses, expressed by the formula

For clarity, the secondary spectrum of some systems of the table. 2 (1, 2a and 4a) is presented in the form of graphs in FIG. 2, where the wavelength λ (nm) is plotted along the ordinate axis, and the longitudinal secondary spectrum is plotted along the abscissa axis

expressed in fractions of the focal length of the system.

(2)
Малая разность относительных частных дисперсий стекол, использованных для корректора, позволяет утверждать на основании зависимости (2), что и коэффициенты дисперсии стекол, составляющих в предлагаемой системе суперапохроматическую пару, должны быть близки по значению, что подтверждается данными табл. 1, откуда видно что

, а разность Δμ_e не превышает 7% от величины

.From the data of the column ΔP _λ tab. 2 it can be seen that in the glass vapor used for the corrector, the difference in the relative partial dispersions for the spectral region from F to C is a small positive value, not exceeding 0.001-0.0015. It is known that the relative partial dispersions of glasses (with the exception of special glasses and crystals) are linearly related to the dispersion coefficient [4]
P _{F, C} = a + bμ ₂ ,
Where

(2)
The small difference in the relative partial dispersions of the glasses used for the corrector allows us to assert on the basis of dependence (2) that the dispersion coefficients of the glasses that make up the superachromatic couple in the proposed system should be close in value, which is confirmed by the data in Table. 1, which shows that

, and the difference Δμ _e does not exceed 7% of the value

.

The foregoing becomes obvious, given the simple fact that in the system adopted as a prototype, with a corrector from one brand of glass, the secondary spectrum in the region from line i to line 1529.6 nm is about - 1 • 10 ^-4 f '. To compensate for this small, but still so noticeable effect on image quality, the magnitude of the secondary spectrum, of course, we need a relatively small difference in the relative partial dispersions of the glasses.

(с одинаковыми стеклами) и разности относительных частных дисперсий стекол ΔP_λ

В столбце 2б и 4б табл. 2 приведены значения вторичного спектра систем со стеклами СТК12/КФ6 и SSK11/SK24, рассчитанные по формуле (3). Видно, что имеется хорошее соответствие между данными, полученными точным лучевым расчетом вторичного спектра, и данными, полученными по эмпирической зависимости (3). В области спектра от F до C вторичный спектр еще в 1,5-2,4 раза меньше, чем в прототипе. В области спектра от линии g до линии c длиной волны 852,1 нм, вторичный спектр вариантов со стеклами SSK11/SK24 и соответствующей ей отечественной пары СТК10/ТК21 в 1,6 раза меньше, чем в прототипе. Совершенно уникальными свойствами обладает пара стекол СТК12/КФ6, у которой в ближней ультрафиолетовой области вторичный спектр уменьшен в 4,8, а в инфракрасной - в 3,2 раза.To prove the above, we consider the secondary spectrum of variants of equivalent systems 1, 2 and 3, 4 of the table. 2. We draw attention to the fact that the secondary spectrum of a system with different glasses can be represented as a linear function of the secondary spectrum of an equivalent system

(with identical glasses) and differences in the relative partial dispersions of glasses ΔP _λ

In column 2b and 4b of the table. Figure 2 shows the values of the secondary spectrum of systems with STK12 / KF6 and SSK11 / SK24 glasses, calculated by formula (3). It can be seen that there is a good agreement between the data obtained by accurate radiation calculation of the secondary spectrum and the data obtained from the empirical dependence (3). In the spectrum from F to C, the secondary spectrum is still 1.5-2.4 times less than in the prototype. In the spectrum from the g line to the line with a wavelength of 852.1 nm, the secondary spectrum of options with SSK11 / SK24 glasses and the corresponding domestic pair STK10 / TK21 is 1.6 times smaller than in the prototype. A pair of STK12 / KF6 glasses has completely unique properties, in which the secondary spectrum is reduced by 4.8 in the near ultraviolet region and 3.2 times in the infrared region.

, и, во-вторых, для уменьшения вторичного спектра по крайней мере в диапазоне от линии g по линии с длиной волны 852,1 нм необходимо соблюдение условия

.From the foregoing, we can conclude that suitable glass lenses for the corrector should have, firstly, close dispersion coefficients that differ by no more than 7%

and, secondly, to reduce the secondary spectrum, at least in the range from the g line along the line with a wavelength of 852.1 nm, the condition

.

 As for the correction of the secondary spectrum in a wider interval from line i to the line with a wavelength of 1529.6 nm, for this it is necessary to have a smoother function of the difference in the relative partial dispersions of the glasses, which is proportional at the edges of the compensated spectral range to the corresponding values of the secondary spectrum of the equivalent system. The proposed system with STK12 / KF6 glasses (option 2 of Table 1) best satisfies these conditions.

 The advantage of the proposed system over the prototype, which is provided when switching from a corrector from one glass brand to a corrector made up of different glass brands, is clearly seen when comparing residual axial aberrations in systems equivalent in diameter, relative aperture, corrector dimensions, meniscus thickness, and focal length planes behind the main mirror.

(мм). Соответствующие им значения волновой аберрации N_λ, выраженные в длинах волн своего цвета и рассчитанные в плоскости фокусировки, показанной на графике продольных аберраций штриховой линией, приведены справа. Графики аберраций приводятся для линий спектра i (365 нм); g (435,83 нм); F (486,13 нм); e (546,07 нм); C (656,27 нм) и длин волн 852,1 нм; 1128,6 нм и 1529,6 нм.In FIG. Figures 3, 4 and 5 show graphs of the residual axial aberrations: longitudinal and wave, for system variants from the table. 1. The ordinate of the entrance pupil y (mm) is plotted along the ordinate, and the longitudinal aberrations are plotted along the abscissa

(mm). The corresponding wave aberration values N _λ , expressed in wavelengths of a different color and calculated in the focusing plane shown in the graph of longitudinal aberrations by the dashed line, are shown on the right. Graphs of aberrations are given for the lines of the spectrum i (365 nm); g (435.83 nm); F (486.13 nm); e (546.07 nm); C (656.27 nm) and wavelengths of 852.1 nm; 1128.6 nm and 1529.6 nm.

 In FIG. 3a shows the axial aberration of the prototype with the glass mark of the corrector STK12 (option 1 of table 1), and in FIG. 3b, on a comparable scale, graphs of axial aberrations of the proposed equivalent system are presented, in which lens 2 is also made of STK12 glass and lens 3 is made of KF6 glass. A comparison of the course of the residual axial aberrations shows that in the proposed system in the spectrum range from line i to the line with a wavelength of 1529.6 nm, there is a significant decrease in residual spherical aberration and spherochromatism. If the graphs of the residual axial aberrations of the prototype have the characteristic shape of the curves determined by the sum of the aberrations of the third and fifth orders, and the spherochromatic aberration is corrected for the pupil area y = 0.7D / 2 and increases at the edge of the pupil, then in the proposed system the curves of residual aberrations have the form characteristic for systems with corrected fifth-order spherical aberration and corrected spherochromatism. From the graphs of FIG. 3b it can be seen that within the opening there are two nodes of intersection of the curves of longitudinal aberrations: on the edge of the pupil and on the area approximately corresponding to 0.55D / 2. Such correction of longitudinal axial aberrations leads to a decrease in the wave aberration interval in the spectrum range from F to C by about 2 times, and in the spectrum range from 365 to 1529.6 nm by more than 6 times, the latter is also associated with a decrease in the longitudinal secondary spectrum, what was said above.

 Thus, the use of glass with a refractive index of about 1.7 in the corrector for lens 2 and glass with a factor of about 1.5 for lens 3 (see Table 1, option 2) leads to a decrease in the residual spherical aberration and spherochromatism over a wide range of the spectrum.

 Since the proposed catadioptric system of the telescope, like the prototype, is not free of the chromaticity of the increase, depending on the glass corrector and free parameters of the system, which include the thickness of the quasi-focal meniscus 2, the corrector dimensions and the offset of the focus plane behind the main mirror, then use for lens 2 a group of relatively cheap glasses of the heavy flint type, having a refractive index of more than 1.7, is impossible, since the chromatism of the increase under the optimal conditions for correcting aberrations Thickness of the meniscus may be unacceptably increased. Since it is inversely proportional to the glass dispersion coefficient, the most suitable material for lens 2 should be considered glasses from the group of superheavy crowns (STK), in which the dispersion coefficients in the visible spectrum range from 45 to 57, and the refractive indices from 1.66 to 1, 79. Among these glasses, grades with a refractive index lower than 1.69 should not be selected because they correspond to glasses for lens 3 with a refractive index less than 1.5. There are no such glasses in the catalog [4], which are close in dispersion to STK type glasses, therefore, meniscus 2 glass selected from the STK group should have a refractive index of at least 1.69. True, in the catalog [4] there is a group of glasses represented so far by only one brand of TFK1 with a refractive index of 1.61 and a dispersion coefficient of about 65. Fused silica and glass LK1, having a refractive index of 1.46–, are suitable for this glass as an apochromatic pair. 1.44. Calculations, however, show that the longitudinal aberrations of the variants with these grades of correction glasses are much worse than those considered, and the expansion of the spectral region does not occur. On the other hand, an increase in the refractive index for glass lens 2 to 1.74-1.76 leads, as can be seen from the table. 1, to increase the refractive index of the glass of the lens 3 to a value of 1.66-1.67. The range of glass with such refractive indices is relatively small and it is difficult to select a pair that provides superchromatic correction in a wide spectral region. Options 4 and 5 of the table. 1 are, apparently, the best possible. An increase in the refractive index of the glass of lens 2 above 1.76 leads to even greater difficulties in choosing a pair and to technologically unfavorable grades of glass with low dispersion from the group of heavy barite flints. The most optimal range of changes in the refractive index of the glass of the lens 2 is, therefore, in the range from 1.69 to 1.76.

 Consider the axial aberrations of the proposed systems, which are at the upper limit of this range. In FIG. 4a shows graphs of longitudinal and wave aberrations of option 4 of the table. 1. The aberration plots of FIG. 4b correspond to option 5 of the same table. Both options are almost equivalent in aberrations, providing for a relative aperture of 1: 7 the spectral range of operation from the g line to the line with a wavelength of 1128.6 nm, and the wave aberrations in this spectral region do not go beyond the limits for option 4 - 0.17λ, and for option 5 - 0.26λ. By appropriate selection of the focusing plane, wave aberrations in the spectrum range from g to 852.1 nm can be significantly reduced. This suggests the possibility of an even greater increase in aperture ratio. In FIG. Figure 5 shows the graphs of axial aberrations of a particularly bright version of Table 6. 1. In the spectrum range from the g line to the line with a wavelength of 852.1 nm, wave aberrations do not go beyond 0.16λ. The relative aperture is 1: 6.2, which is significantly higher than in the prototype.

From the table. 1 shows that the proposed system, despite a different assortment of glass corrector lenses, retains all the features that combine it with the prototype and distinguish it from the analogue. Lens 3 is negative (see, r ₄ and r ₅ in Table 1), and the quasi-focal meniscus 2 is wrapped with concavity to the object of observation and has a weak negative optical power. In the table. 1 in the column Δr _2,3 / d ₂ at the top is the value of this ratio for the calculated options, and below for the afocal meniscus of the same material. It can be seen that the lower values in all cases are less than the upper ones; therefore, the meniscus optical power is negative. Thus, all the essential features that combine the proposed optical system of the telescope with the prototype are preserved.

As proved above, the proposed system improves the quality of correction of residual axial aberrations, broadens the spectral range of operation and increases the relative aperture to a value of 1: 6.2-1: 7, which is ensured by the presence of the following new distinguishing features:
1. The lenses of the correction element are made of different grades of glass having quasi-close dispersion coefficients in the visible region of the spectrum, the first lens being made of glass with a high refractive index.

больше коэффициента дисперсии первой линзы

причем

По отношению к прототипу предлагаемая система телескопа обладает двумя основными преимуществами:
лучшей коррекцией остаточных осевых аберраций, распространяющейся на значительно более широкую область спектра 365-1530 нм;
возможностью повышения светосилы до 1:6,5-1:7 за счет лучшего исправления остаточных аберраций на оси.2. The refractive index of the first lens satisfies the ratio: 1.69 ≤ n ≤ 1.76, and the dispersion coefficient of the second lens

greater dispersion coefficient of the first lens

moreover

In relation to the prototype, the proposed telescope system has two main advantages:
better correction of residual axial aberrations, extending to a much wider spectral region 365-1530 nm;
the ability to increase aperture ratio to 1: 6.5-1: 7 due to better correction of residual aberrations on the axis.

 Keeping all the structural advantages of the analogue and the prototype, such as the spherical shape of the optical surfaces and the relatively small size of the corrective lenses (about 1/3 of the diameter of the active hole), the proposed system due to the increase in aperture ratio has even greater compactness - the distance between the main mirror and the corrector only slightly exceeds diameter of the active hole (see table. 1).

 Correction of the residual axial aberrations of the system, achieved without the use of aspherical surfaces, allows one to develop its relative aperture to a value of 1: 7 and even large (1: 6.2), which allows reaching the penetrating limit at an acceptable exposure time (1-1.5 h) Telescope abilities on modern photographic emulsions.

The astigmatism and curvature of the image field, fundamentally incorrigible, as in the prototype, however, are quite small and even with a relative aperture value of 1: 7 make it possible to provide an image field with a diameter of 30 arc minutes. For a system with an active hole diameter of 200 mm, the astigmatism scattering spot on the surface of the best images does not exceed 2 '' arcs. The residual coma for a system of the same diameter and in the same field does not exceed 0.6 '' - 0.8 '', and distortion - a value of 0.006%. The chromaticity of increase for the lines of the spectrum from F to C, which are still well distinguished by the eye, is quite small and amounts to 0.08%. With a relative aperture of 1: 7, the correction of the residual axial aberrations of the best version of the system with corrector glasses STK12 / KF6 allows you to cover the spectral region 365-1530 nm with a telescope aperture diameter of 200-250 mm, in the spectral region from the h line to the line with a wavelength of 1530 nm the diameter of the active hole can be increased to 500-750 mm, while the wave aberrations on the axis along the edges of the spectral range do not go beyond 1 / 4λ.
The proposed system is significantly better than the prototype in relation to the level of spurious background. As you can easily see (see table. 1), the ratio of the radii of the negative lens 3 (r ₄ / r ₅ ) in the proposed system is much larger than in the prototype, where it ranged from 1.5 to 2. This leads to an even greater removal of the plane focusing of the glare that is formed when light is reflected from the fourth surface, from the image plane of the system. The enlightenment of the corrector lenses under such favorable conditions will eliminate the parasitic background almost completely, and therefore the proposed system can be confidently recommended for astrophotographic and photometric work.

 The best field of application of the proposed optical system is the production on its basis of serial, relatively cheap, small-sized (with a working hole diameter of 200-400 mm and a relative hole of 1: 7 - 1: 8) and universal telescopes 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 and Maksutov’s meniscus cassegrain, practically not inferior to them in image quality and allowing relatively simple means without aspheric surfaces and retouching, to ensure high aperture of the telescope and a sufficiently large field of good images of about 30 arc minutes.

Literature
1. Argunov P. P. "Catadioptric telescope", USSR Copyright Certificate N 158697, Bulletin N 22, 1963.

 2. Argunov P.P. "Catadioptric telescope." A New Technique in Astronomy.-M.: Nauka, 1965, no. 2, p. 8-16.

 3. Klevtsov Yu. A. "Catadioptric telescope", USSR Author's Certificate N 605189, Bulletin N 16, 1978.

 4. Optical glass of the USSR - GDR (joint catalog), V / O MASHPRIBORINTORG.

 5. Scheglov P. V. "Problems of optical astronomy" .- M .: Nauka, 1980, p. 272.

Claims (2)

 1. A catadioptric telescope 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 reflecting surface that differs the fact that the lenses of the correction element are made of different grades of glass having quasi-close dispersion coefficients in the visible region of the spectrum, the first lens being made ene of glass with a high refractive index.  2. The telescope according to claim 1, characterized in that the refractive index of the first lens satisfies a ratio of 1.69 ≤ n ≤ 1.76, and the dispersion coefficient of the second lens μe ″ is greater than the dispersion coefficient of the first lens μe ′, and (μe ″ -μe ′ ) /μe′≤7/100.m

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