RU2443005C2 - Catadioptric telescope - Google Patents

Abstract

FIELD: astronomy.

SUBSTANCE: telescope may be used for serial compact devices operated with charge-coupled devices for astrophotographic, spectral, photometric observations of celestial objects in the broad-spectrum, for astroclimate study as well as for serial photovisual telescopes with the effective focal aperture of 150-400 mm. The telescope contains the following components (following the ray path): main concave dish (1), the correction element comprising two negative single lenses (2 and 3) of different glass brands with the dispersion factor that is quasi-proximal in the visible spectrum, the first one being a quasi-afocal meniscus with its concavity overlooking the object, the second one with mirror surface, made of a material with lower refraction index, and a three-lens isoplanatic off-axis aberration compensator installed in front of the telescope's focal plane. The first lens following the ray path is a quasi-afocal negative meniscus (4) with its concavity overlooking the image plane, the second (5) and the third (6) compensator lenses are glued together. The first plane of the glued lens unit following the ray path is a flat one, the remaining planes have their concavities overlooking the object.

EFFECT: reduced stray light, enhanced the image quality on the periphery of the viewing field, increasing the compensator's flange focal distance.

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Description

The present invention relates to the field of astronomical instruments and can be used in serial small-sized telescopes for photographic purposes, working with charge communication devices (CCD cameras and digital cameras), which are used to conduct various observations of celestial objects in a wide spectral region, such as astrophotographic, spectral, photometric, astroclimate studies and the like. The proposed optical system can also be used in serial photovisual telescopes with a working hole of 150-400 mm.

Known catadioptric telescope proposed by the author, containing installed along the beam, the main concave spherical mirror 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. The corrector lenses are made of different grades of glass with quasi-close dispersion coefficients in the visible spectral range (Catadioptric telescope [Text]: Pat. 2125285 Russian Federation: IPC G02B 17/08, 23/02 / author Klevtsov Yu.A.; patent holder Design- Technological Institute of Applied Microelectronics SB RAS).

In the wavelength range of 436-700 nm, the working area of the spectrum of digital photographic equipment, such telescopes provide diffractive image quality on the axis with an active hole diameter of up to 800 mm and a relative hole of up to 1: 7. However, off-axis aberrations: astigmatism, field curvature, and magnification chromatism limit the field of view where high quality images can still be obtained.

The prototype of the invention is a catadioptric telescope, proposed by the author, containing a main concave spherical mirror mounted along the beam and a correction 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 material with lower refractive index. In front of the focal plane of the telescope, a three-lens isoplanatic compensator for off-axis aberrations is installed, 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. The first two compensator lenses along the beam can be glued together (Catadioptric telescope [Text]: Pat. 2248024 Russian Federation: IPC ⁷ G02V 23/06, 17/08 / author Klevtsov Yu.A .; patent holder of the State Unitary Enterprise “PO“ NPZ ”) .

The main disadvantages of the prototype are: the presence in the image plane of spurious light from the off-axis aberration compensator; relatively low, according to modern requirements, image quality of the point along the edges of the field of view (for systems with a working hole of 250 mm or more) a relatively small working distance of 45.5 mm (currently, working with modern photographic equipment requires a working distance of at least 55 mm).

The task of the invention is to lower the level of stray light background, reduce the effective scattering spot for the edge points of the field of view and increase the working scattering of the focus transducer to the currently accepted standards.

The task is achieved by the fact that in a catadioptric telescope 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 material with a lower refractive index a three-lens isoplanatic off-axis aberration compensator installed in front of the focal plane of the telescope, the first compensator lens in the direction of the beam is made in the form of a quasi-focal negative meniscus facing concavity to the image plane, the second and third compensator lenses are glued, and the first surface of the glued lens block is flat along the beam , and the remaining surfaces are turned concavity to the object of observation.

The optical surfaces of the quasiconcentric meniscus of the off-axis aberration compensator in the prototype are a source of second-order spurious reflections concentrated near the image plane. Replacing the quasiconcentric meniscus with a quasi-focal negative meniscus and moving it forward along the beam, as well as the greater curvature of this meniscus, help to remove the focus plane of spurious glare from the image plane. This reduces the spurious light background in the telescope. The gluing of the second and third lenses of the off-axis aberration compensator and the orientation of the surfaces of the glued block with concavity to the object of observation also contribute to the reduction of spurious light. The relatively large curvature and axial thickness of the first meniscus lens of the off-axis aberration compensator increases its working distance and compensates for the chromatic coma of the telescope, resulting in a decrease in the effective scattering spot at the edges of the field of view.

The applicant does not know catadioptric telescopes having features similar to those distinguishing the proposed catadioptric telescope from the prototype.

The invention is illustrated by the following graphic materials:

Figure 1 is an optical diagram of a catadioptric telescope;

Figure 2 - scatter diagrams for a catadiopric telescope with a working hole of 300 mm (relative hole - 1: 6);

The catadioptric telescope contains the main concave spherical mirror 1 installed along the beam and a correction element, including two single lenses 2 and 3 of different grades of glass with dispersion coefficients quasiblique in the visible region of the spectrum, the first of which 2 is made in the form of a quasi-focal negative meniscus, facing 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 (Figure 1). Near the focal plane of the telescope, a three-lens isoplanatic compensator for off-axis aberrations is installed, including a negative quasi-focal meniscus lens 4 and a glued block consisting of a positive lens 5 and a negative lens 6.

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 they are collected by lenses 4, 5, 6 of the off-axis aberration compensator, from which they exit approximately telecentric beams and are focused in the image plane of the telescope at a small distance behind the compensator.

Let us justify the possibility of achieving the claimed technical characteristics in the proposed catadioptric telescope.

The reasons for the appearance of spurious light in the prototype telescope, in the presence of a large brightness object in the field of view (bright stars and planets), are glare arising in the off-axis aberration compensator. The relatively low quantum efficiency of CCDs used in digital cameras leads to the fact that up to 30% of the light is reflected in the opposite direction. This light, in turn, is reflected from the optical surfaces of the off-axis aberration compensator back towards the image plane, where it forms rather intense glare, which, under certain conditions, can create a stray light background in the image plane. In addition, the cause of the appearance of stray light is also the so-called second-order glare, which is formed in the direct course of the rays when light is reflected from the surfaces of the off-axis aberration compensator. These glare are able to focus near the image plane and create bizarre halos around a bright object that interfere with the detection of faint celestial objects. One of the reasons for the appearance of back-reflection glare and second-order glare in the prototype telescope is the use of a quasi-concentric meniscus lens in the compensator for off-axis aberrations, which is turned concave to the image plane. The focus plane of the parasitic glare formed by such a lens is located near the image plane. This is precisely the reason for the formation of spurious light background.

The proposed catadioptric telescope uses a different principle for constructing a compensator for off-axis aberrations. The first lens 4 in the direction of the beam is made in the form of a quasi-focal negative meniscus facing concavity to the image plane. Since a positive glued lens 5, 6 is located between the lens 4 and the image plane, all this together allows you to remove the focus plane of the parasitic glare associated with the lens 4 from the image plane of the telescope. In addition, the large curvature and thickness of the lens 4 contributes to an increase in the working distance of the off-axis aberration compensator and compensation of the chromatic coma of the telescope, which, as a result, leads to a decrease in the effective scattering spot at the edges of the field of view. A glued block consisting of a positive lens 5 and a negative lens 6, which serves to increase the relative aperture of the telescope, also takes part in the formation and formation of spurious glare. Bonding of lenses 5 and 6 significantly reduces the intensity of spurious glare from the glued surface. The first flat surface of the glued block along the beam and the orientation of the remaining surfaces with concavity to the object of observation does not leave room for the concentration of spurious light near the image plane of the telescope.

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 a specific off-axis aberration compensator for the TAL-300K telescope (relative aperture 1: 8). The current opening of the telescope is 300 mm. The material of the lenses of the correction element is as follows: lens 2 - STK12; lens 3 - TK2. Lens material 4 - glass TK2; lenses 5 - glass BK8, and lenses 6 - glass OF4. The distance from the last surface of the lens 6 to the image plane is 76.2 mm, which makes it possible not only to meet the current standards of digital photographic equipment, but also to use the available margin of working distance for the installation of the side guide optical system compensator. The relative aperture of the telescope with an off-axis aberration compensator is 1: 6.

Figure 2 shows the scatter plots of the scattering spots of the TAL-300K telescope with the proposed compensator for off-axis aberrations in the spectrum range from 436 to 700 nm.

Scatter plots are calculated along six lines of the spectrum: 550; 436; 450; 600; 650 and 700 nm, taking into account the spectral sensitivity of the CCD matrix and the entire optical path of the device for five angles of field of view W _y = 0 °; 0.1675 °; 0.335 °; 0.5025 °; 0.67 ° (Isoplanatic focal length transducer for catadioptric telescope with meniscus corrector [Text] / Yu.A. Klevtsov. // Optical journal. - 2006 - T.73. - No. 8. - P.50-54).

Thus, the total angular field of the TAL-300K telescope with a compensator for off-axis aberrations of the proposed design is 2W _y = 1.34 °. The radius of the circumference of the restriction in the scatter plot of FIG. 2 is 15 μm.

From figure 2 it is seen that the diameter of the geometric scattering spot for the center of the field of view in the specified range of the spectrum and the selected focusing plane is less than 15 microns, and at the edge of the field in the direction of the meridional section (Y) is about 30 microns. The data (including light diffraction) for estimating the sizes of effective scattering spots (with an energy concentration of E = 80%), as well as an estimate of the resolution of a telescope with an off-axis aberration compensator in the spectrum range 436-700 nm, are shown in Table 1.

Table 1

2ρ (μm) at E = 80% for λ (nm) 436 ÷ 700 α "arcsec. at T = 20% for λ (nm) 436 ÷ 700 0,0 0,0 13.7 0.58 0.14 4.4 13.8 0.62 0.34 10.7 14.8 0.95 0.54 17.0 16,0 1,2 0.74 23,4 16,2 1,2 0.94 29.9 16.7 1,6 1.14 36,4 20.8 2.2 1.24 39.8 22.0 2.5 1.34 43.1 20,4 2.2

In table 1:

- угловое поле телескопа;

- the angular field of the telescope;

- линейное поле телескопа с преобразователем фокусного расстояния в плоскости изображения;

- a linear field of a telescope with a focal length converter in the image plane;

2ρ is the diameter of the effective scattering spot according to the energy concentration level E = 80%;

α "is the angular resolution in terms of contrast T = 20% for the worst direction of strokes of the worlds;

λ is the wavelength of light.

In the center of the field of view, the effective scattering spot does not exceed 13.7 μm, and over the field of view - 22 μm. In the prototype scheme with a 250 mm working hole with a relative hole with an off-axis aberration compensator of 1: 6.3, practically in the same spectral range and along the edge of the same angular field, an effective scattering spot with a diameter of 26 μm was obtained (Catadioptric telescope [Text]: pat 2248024 Russian Federation: IPC ⁷ G02V 23/06, 17/08 / author Klevtsov Yu.A .; patent holder of the State Unitary Enterprise “PO“ Oil Refinery ”).

A proportional recalculation shows that the gain in the size of the effective scattering spot, within the angular field of view of 1.3 °, which gives the proposed scheme of the catadiopric telescope, is at least 1.5.

Let us evaluate the stray light in the proposed scheme of a 300 mm catadiopric telescope with an off-axis aberration compensator.

Consider an enlightened and unenlightened compensator for off-axis aberrations. In the spectrum range 436–700 nm, we take the integral coefficient of light reflection from the antireflected refracting surface equal to 0.6%, and the integral mirror reflection coefficient — 96%. In the first case, the transmission of the entire telescope (excluding screening in the center of the pupil) will be τ ₀ = 0.87, in the second case, τ ₀ = 0.72. We will consider the quantum efficiency of the CCD matrix to be equal to 70% the intensity estimate for all off-axis aberrations, back-reflection flares, and second-order flares that appear in the compensator (the object of observation is in the center of the field of view). The calculation data are given in table 2.

table 2 i No. p / p

ζ _i τ _i Enlightened Unenlightened one 10 -99.4 -325.6 98.9 32,7 0,305 0.00150 0,00776 2 eleven -86.5 -388.0 70.5 23.8 0.223 0,00152 0.00860 3 12 -252.7 3212,0 23.8 8.4 -0.0787 0,00154 0.00879 four 13 -91.7 265.7 119.7 37,2 -0.345 0,00028 0,000205 5 fourteen -110.6 814.0 41,4 14,4 -0.136 0,00156 0.01308 6 10-14 -146.2 1900.7 22.5 8.1 -0.0769 0.0000304 0,00178 7 10-13 -71.7 -442.7 60.7 17.6 0.162 0.00000548 0,0000317 8 10-12 -105.0 394.6 84.7 28,4 -0.266 0.0000308 0.00150 9 10-11 -81.6 440.0 56.8 19.7 -0.186 0.0000312 0,00178 10 11-14 -109.7 790.5 41.7 14.7 -0.139 0.0000308 0,00197 eleven 12-14 -73.0 740.6 30,2 10.5 -0.0985 0.0000311 0,00202 12 13-14 -104.9 -659.8 48,4 16.9 0.159 0.00000561 0.0000470 13 11-13 -464.5 5073,0 21,4 9.5 -0.0916 0.00000554 0.0000351 fourteen 12-13 -90.2 263.4 118.9 36.9 -0.343 0.00000561 0.0000359 fifteen 11-12 -93.0 384.1 76.5 25.8 -0.242 0.0000312 0,00166

In table 2:

i - glare number (from 1 to 5 - glare of back reflection from the surfaces of the compensator, formed with the participation of the CCD matrix, from 6 to 15 - glare of the second order, formed when light is reflected from the surfaces of the compensator);

No. p / p - the number of the surface (surfaces) of the circuit involved in the formation of flare (the entrance pupil is surface No. 1). 10 - number of the first along the beam surface of the off-axis aberration compensator, 14 - number of its last surface;

- расстояние плоскости фокусировки блика от плоскости изображения;

- distance of the focus plane of the glare from the image plane;

- фокусное расстояние системы паразитного отражения;

- the focal length of the parasitic reflection system;

- внешний диаметр кольца блика на плоскости изображения (точка на оси);

- the outer diameter of the flare ring on the image plane (point on the axis);

- внутренний диаметр кольца блика на плоскости изображения (точка на оси);

- the inner diameter of the flare ring on the image plane (point on the axis);

(основной параметр блика);

(main flare parameter);

τ _i - the intensity of the flare (in fractions of the intensity of the object).

The calculation of the permissible brightness of an object m (in magnitude), which creates a stray light background on the image plane comparable to the natural background of the night sky (+22 magnitude per square second per Earth’s atmosphere), is calculated according to the dependence (New apochromatic mirror-lens systems with meniscus corrections [Text]: Research on geomagnetism, aeronomy and physics of the Sun / Yu.A. Klevtsov. // 1988. - No. 79. - S.178-185):

.

.

m is the brightness of the object (in magnitude);

τ ₀ - transmittance of the optical system;

∀ _e - relative focus (the reciprocal of the relative aperture of the optical system);

τ _i - flare intensity (in fractions of the object intensity);

ζ _i is the main flare parameter.

), паразитных бликов в катадиоприческом телескопе с предлагаемым компенсатором внеосевых аберраций, сфокусированных вблизи плоскости изображения, не образуется, чем он выгодно отличается от прототипа, где около яркого объекта всегда отчетливо видны два блика второго порядка, имеющие диаметр 1,5-2 мм.If we assume that in a certain area of the image field all the flares intersect, which is true when the object is outside the center of the field of view, then an analysis of the data in Table 2 gives m = -1.3 for the illuminated focus transducer, and for the unenlightened (or poor enlightened) focus transducer m = + 1.2. There are only 18 stars with a brightness greater than +1.2 in the northern sky. Thus, if there are relatively bright objects in the field of view (which happens quite rarely), even in the absence of optical illumination (or poor illumination), the intensity of stray light will be comparable to natural the background of the night sky and weak objects that are at the limit of visibility will not be lost. With good illumination of the optics, it is quite acceptable that the telescope also has planets in the field of view (with the exception of Venus, which has a brightness of 4.3). As can be seen from table 2 (see column

), parasitic glare in a catadioptic telescope with the proposed compensator for off-axis aberrations focused near the image plane does not form, which compares favorably with the prototype, where two second-order glares with a diameter of 1.5-2 mm are always clearly visible near a bright object.

Thus, it can be considered reasonable that the proposed catadioptric telescope with compensator for off-axis aberrations relative to the prototype has a lower level of spurious light background, better image quality of the edge points of the field of view and a greater working distance.

The proposed catadiopric telescope, 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 and compactness, has the following advantages with respect to the prototype:

1. Does not have spurious glare focused near the image plane;

2. The parasitic light background in a catadiopric telescope depends only on the quality of the antireflection coatings and, in the worst case, the complete absence of enlightenment, even if the brightest stars are in the field of view, it does not exceed the natural background of the night sky;

3. The proposed catadiopric telescope, ceteris paribus, provides about 1.5 times the best image quality in the field of view;

4. The off-axis aberration compensator of the proposed catadioptric telescope has a significantly greater working distance than in the prototype, which not only meets the current standards for photographic equipment, but also makes it possible to install some additional accessories behind the compensator, such as off-axis guide.

Due to these advantages, the proposed catadioptric telescope can be recommended for astrophotographic work with specialized CCD arrays and modern digital cameras designed for a narrow frame format of 24 × 36 mm (with a diagonal of 43.1 mm), as well as for photometric and spectral works in the wavelength range at least from 436 nm to 700 nm.

The best field of application of the proposed optical system of a cathodioptic telescope is the production on its basis of serial, relatively cheap, small-sized with an effective hole diameter of 150-400 mm with a relative aperture of 1: 5.5 to 1: 6 universal telescopes for educational purposes and astronomy enthusiasts . In this field of application, the proposed system of a catadiopric telescope, in view of its simplicity and relative cheapness, exceeds such well-known and widely used types of telescopes as the Ritchie-Chretien system, the meniscus Cassegrain of Maksutov and the Schmidt-Cassegrain system. The last two types of telescopes, the proposed system of a catadiopric telescope, in addition, still surpasses the image quality in a wide range of the spectrum, and the Schmidt-Cassegrain system - in terms of stray light. The proposed catadiopric telescope allows 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 1.34 °.

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 region of the spectrum, 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 a material with a lower refractive index, a three-lens isoplanatic comp an off-axis aberration ator installed in front of the focal plane of the telescope, characterized in that the first compensator lens along the beam is made in the form of a quasi-focal negative meniscus facing concavity to the image plane, the second and third compensator lenses are glued, the first surface of the glued lens block being flat along the beam , and the remaining surfaces are turned concavity to the object of observation.

Images