rus
Issue #4/2023
I. P. Shishkin, A. P. Shkadarevich
Erecting Sighting System with Variable Magnification
Erecting Sighting System with Variable Magnification
DOI: 10.22184/1993-7296.FRos.2023.17.4.318.324
The article describes the design of the pancratic erecting system that allows to expand the range of magnifications in the optical sight up to 5–8 times and obtain a high-quality reticle image when using the illumination.
The article describes the design of the pancratic erecting system that allows to expand the range of magnifications in the optical sight up to 5–8 times and obtain a high-quality reticle image when using the illumination.
Теги: pancratic erecting system reticle illumination variable magnification sight панкратическая оборачивающая система подсветка сетки прицел с переменным увеличением
Erecting Sighting System with Variable Magnification
I. P. Shishkin, A. P. Shkadarevich
Research and technical center “LEMT” of the belarus optical and mechanical association
Minsk, Republic of Belarus
The article describes the design of the pancratic erecting system that allows to expand the range of magnifications in the optical sight up to 5–8 times and obtain a high-quality reticle image when using the illumination.
Keywords: pancratic erecting system, variable magnification sight, reticle illumination
Article received on April 03, 2023
Article accepted on April 24, 2023
INTRODUCTION
In rece1nt years, the manufacturers of observation devices have been actively seeking to expand the range of sights by complementing it with new models with a large range of variable magnification.
Until quite recently, the 3–4x magnification range has been considered rather typical in the consumer market. At present, most of the world’s manufacturers have developed the sight models with a magnification range of 5–8x.
The undisputed leaders are the following optical sights: Steiner M5Xi 5–25×56; Schmidt & Bender 5–25×56; Swarovski Z8i 2.3–18×56; Zeiss 2.8–20×56 V8.
DESIGN
The optical system of a zoom telescopic sight in its classical form have an objective lens, a reticle, a two-lens erecting system and an eyepiece. The magnification range in the sights made according to such an optical layout [1–2] is limited by 3–4 times. Therefore, in order to expand the range, a radically new solution must be found.
One of the solutions may be use of a fixed collective lens arranged in front of the first moving component in the erecting system [3]. The lens is designed in such a way that the outermost beams of rays assuredly fall into the optical system throughout the entire motion range of the erecting system components. When a collective lens is installed in the focus of objective lens, it is possible to obtain the minimum clear apertures of the erecting system lenses and the widest field of view of the telescopic sight.
The relative position and diameters of the erecting system lenses must ensure the transmission of light beams with optimal vignetting to obtain the rated field of view and remove the exit pupil of the sight in the entire range of magnifications. In this case, the rotary mechanism angle when changing the magnification should be minimal (within 180°).
The geometry and length of the guide grooves, along which the lenses are moving, depend on the optical design. The shorter the length movement, the easier it is technologically to obtain the movement accuracy and minimal image defocusing in the erecting system.
Vignetting
In order to maintain acceptable image quality over the entire range of magnifications, vignetting of the exit pupil is applied deliberately, for example, by reducing the outer diameter of the first movable lens of the erecting system. This approach makes it possible to noticeably reduce the spherical aberration of the beams, especially at low magnifications. Moreover, the outer diameter of the lens is designed in such a way that when changing magnifications, the field of view is not cut off.
Reticle
The sight reticle includes a crosshair and sighting elements that allow to quickly estimate the distance to the target and make the necessary adjustments during the shooting process, depending on the wind direction, the target type and its movement. The reticle can be mounted at the back focus of the objective lens (1st focal plane) or at the object-side focus of the eyepiece (2nd focal plane).
Illumination
Most modern sights are equipped with the LED reticle illumination. If the reticle is located in the 1st focal plane, and the reticle is illuminated through its side face, then the LED beam pattern must be consistent with the numerical aperture of the sight’s erecting system. In the sights with a magnification range greater than 5x, the numerical aperture of the erecting system must be large enough to provide the required luminosity (exit pupil diameter) and field of view.
Table 1 shows the parameters of the erecting system with a 5-time zoom range for extreme values.
An increase in the numerical aperture in the erecting system leads to the enlarged aberrations occurred, for example, when the illumination is turned on (doubling of the reticle image) at maximum magnification. The aperture limitation by setting a real diaphragm near the first lens of the erecting system can lead to occurrence of a spurious exit pupil in a plane that does not coincide with the main exit pupil (image of the entrance pupil of the lens). All these factors are must be considered in the optical design.
Five-Time Magnification
Figure 1 shows two options of the erecting system with a 5‑time zoom magnification at the maximum linear magnification β = –5x and a numerical aperture Na = 0.25.
The left illustration demonstrates an original 3‑component erecting system, an optimized erecting system is presented on the right. The reticle 1 is installed in the 1st focal plane, the first lens is a fixed collective lens 2, and two components (3–4) are movable. These options have the same travel path of components (the air gap adjustment law), length and focal distances, but different configurations. Table 2 shows the aberration calculation results over 8 lens surfaces (Seidel sums) for both options of the erecting system. The calculation demonstrates that the first Seidel sum value that determines the spherical aberration in the original version, when the lenses face each other, is much higher than for the optimized option.
To assess the image quality in both options of the erecting systems, we use a scatter (spot) diagram. The diagram shows the image for three field spots (0°, 2° and 3°). When comparing the diagrams, it is possible to note that the spot size in the optimized option is at least two times smaller than in the original one.
Achromatization
Achromatization is an effective method to adjust aberrations in any optical system. If the erecting system is achromatized by selecting a combination of glasses in one of the joints, it is possible to significantly improve the image quality.
Figure 3 demonstrates a plot of the chromatic focus shift within the working spectrum (486–656 nm). Thus, in the achromatized system, the spot size will be approximately 3 times smaller than in the original option.
TEST
The tests of the telescopic sight prototype model have confirmed the erecting system simulation results. Figure 4 shows an image of the reticle obtained on the stand during the prototype control of the original (left) and optimized (right) erecting systems.
Boresight offset
The boresight offset when changing magnifications is an important parameter of an optical sight. In the high-quality sights, this value does not exceed 0.7–1 cm per 100 m. The offset value is subject to exposure by the focal distance of the lens, the focal distances of the pancratic system components, and the linear magnification. All of the above parameters determine the requirements for centering and tilting of individual lenses of the erecting system.
Table 3 demonstrates the value and direction of the boresight offset in the focal plane of the lens, depending on the decentering and inclination of lenses of a 3‑component erecting system with a 5‑time zoom magnification.
The values given in the table show that decentering and inclination of the collective lens (magnification 5–25x), decentering and tilting of the 1st lens (magnification 25x), decentering and tilting of the 2nd lens (magnification 5x) make the greatest contribution to the offset level.
To ensure that the boresight is offsetted by no more than 1 cm per 100 m within the entire magnification range, the total value of ΣΔ should be as follows:
ΣΔ = 0,0001 · f’об.
With an objective lens focus of 250 mm, ΣΔ should not exceed 0.025 mm
Eight-Time magnification
Figure 5 demonstrates the erecting system with an 8‑time zoom magnification, which design is supplemented by a fixed plano-concave lens 5 on the eyepiece side, and the back focus 1 of the objective lens coincides with the plane of the collective lens 2.
CONCLUSION
The proposed design of the erecting system expands the magnification range of pancratic telescopic sights and provides a high-quality image of the illuminated reticle.
I. P. Shishkin, A. P. Shkadarevich
Research and technical center “LEMT” of the belarus optical and mechanical association
Minsk, Republic of Belarus
The article describes the design of the pancratic erecting system that allows to expand the range of magnifications in the optical sight up to 5–8 times and obtain a high-quality reticle image when using the illumination.
Keywords: pancratic erecting system, variable magnification sight, reticle illumination
Article received on April 03, 2023
Article accepted on April 24, 2023
INTRODUCTION
In rece1nt years, the manufacturers of observation devices have been actively seeking to expand the range of sights by complementing it with new models with a large range of variable magnification.
Until quite recently, the 3–4x magnification range has been considered rather typical in the consumer market. At present, most of the world’s manufacturers have developed the sight models with a magnification range of 5–8x.
The undisputed leaders are the following optical sights: Steiner M5Xi 5–25×56; Schmidt & Bender 5–25×56; Swarovski Z8i 2.3–18×56; Zeiss 2.8–20×56 V8.
DESIGN
The optical system of a zoom telescopic sight in its classical form have an objective lens, a reticle, a two-lens erecting system and an eyepiece. The magnification range in the sights made according to such an optical layout [1–2] is limited by 3–4 times. Therefore, in order to expand the range, a radically new solution must be found.
One of the solutions may be use of a fixed collective lens arranged in front of the first moving component in the erecting system [3]. The lens is designed in such a way that the outermost beams of rays assuredly fall into the optical system throughout the entire motion range of the erecting system components. When a collective lens is installed in the focus of objective lens, it is possible to obtain the minimum clear apertures of the erecting system lenses and the widest field of view of the telescopic sight.
The relative position and diameters of the erecting system lenses must ensure the transmission of light beams with optimal vignetting to obtain the rated field of view and remove the exit pupil of the sight in the entire range of magnifications. In this case, the rotary mechanism angle when changing the magnification should be minimal (within 180°).
The geometry and length of the guide grooves, along which the lenses are moving, depend on the optical design. The shorter the length movement, the easier it is technologically to obtain the movement accuracy and minimal image defocusing in the erecting system.
Vignetting
In order to maintain acceptable image quality over the entire range of magnifications, vignetting of the exit pupil is applied deliberately, for example, by reducing the outer diameter of the first movable lens of the erecting system. This approach makes it possible to noticeably reduce the spherical aberration of the beams, especially at low magnifications. Moreover, the outer diameter of the lens is designed in such a way that when changing magnifications, the field of view is not cut off.
Reticle
The sight reticle includes a crosshair and sighting elements that allow to quickly estimate the distance to the target and make the necessary adjustments during the shooting process, depending on the wind direction, the target type and its movement. The reticle can be mounted at the back focus of the objective lens (1st focal plane) or at the object-side focus of the eyepiece (2nd focal plane).
Illumination
Most modern sights are equipped with the LED reticle illumination. If the reticle is located in the 1st focal plane, and the reticle is illuminated through its side face, then the LED beam pattern must be consistent with the numerical aperture of the sight’s erecting system. In the sights with a magnification range greater than 5x, the numerical aperture of the erecting system must be large enough to provide the required luminosity (exit pupil diameter) and field of view.
Table 1 shows the parameters of the erecting system with a 5-time zoom range for extreme values.
An increase in the numerical aperture in the erecting system leads to the enlarged aberrations occurred, for example, when the illumination is turned on (doubling of the reticle image) at maximum magnification. The aperture limitation by setting a real diaphragm near the first lens of the erecting system can lead to occurrence of a spurious exit pupil in a plane that does not coincide with the main exit pupil (image of the entrance pupil of the lens). All these factors are must be considered in the optical design.
Five-Time Magnification
Figure 1 shows two options of the erecting system with a 5‑time zoom magnification at the maximum linear magnification β = –5x and a numerical aperture Na = 0.25.
The left illustration demonstrates an original 3‑component erecting system, an optimized erecting system is presented on the right. The reticle 1 is installed in the 1st focal plane, the first lens is a fixed collective lens 2, and two components (3–4) are movable. These options have the same travel path of components (the air gap adjustment law), length and focal distances, but different configurations. Table 2 shows the aberration calculation results over 8 lens surfaces (Seidel sums) for both options of the erecting system. The calculation demonstrates that the first Seidel sum value that determines the spherical aberration in the original version, when the lenses face each other, is much higher than for the optimized option.
To assess the image quality in both options of the erecting systems, we use a scatter (spot) diagram. The diagram shows the image for three field spots (0°, 2° and 3°). When comparing the diagrams, it is possible to note that the spot size in the optimized option is at least two times smaller than in the original one.
Achromatization
Achromatization is an effective method to adjust aberrations in any optical system. If the erecting system is achromatized by selecting a combination of glasses in one of the joints, it is possible to significantly improve the image quality.
Figure 3 demonstrates a plot of the chromatic focus shift within the working spectrum (486–656 nm). Thus, in the achromatized system, the spot size will be approximately 3 times smaller than in the original option.
TEST
The tests of the telescopic sight prototype model have confirmed the erecting system simulation results. Figure 4 shows an image of the reticle obtained on the stand during the prototype control of the original (left) and optimized (right) erecting systems.
Boresight offset
The boresight offset when changing magnifications is an important parameter of an optical sight. In the high-quality sights, this value does not exceed 0.7–1 cm per 100 m. The offset value is subject to exposure by the focal distance of the lens, the focal distances of the pancratic system components, and the linear magnification. All of the above parameters determine the requirements for centering and tilting of individual lenses of the erecting system.
Table 3 demonstrates the value and direction of the boresight offset in the focal plane of the lens, depending on the decentering and inclination of lenses of a 3‑component erecting system with a 5‑time zoom magnification.
The values given in the table show that decentering and inclination of the collective lens (magnification 5–25x), decentering and tilting of the 1st lens (magnification 25x), decentering and tilting of the 2nd lens (magnification 5x) make the greatest contribution to the offset level.
To ensure that the boresight is offsetted by no more than 1 cm per 100 m within the entire magnification range, the total value of ΣΔ should be as follows:
ΣΔ = 0,0001 · f’об.
With an objective lens focus of 250 mm, ΣΔ should not exceed 0.025 mm
Eight-Time magnification
Figure 5 demonstrates the erecting system with an 8‑time zoom magnification, which design is supplemented by a fixed plano-concave lens 5 on the eyepiece side, and the back focus 1 of the objective lens coincides with the plane of the collective lens 2.
CONCLUSION
The proposed design of the erecting system expands the magnification range of pancratic telescopic sights and provides a high-quality image of the illuminated reticle.
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