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Abstract
Telescopes play an essential important role in the fields of astronomical observation, emergency rescue, etc. The traditional telescopes achieve zoom function through the mechanical movement of the solid lenses, usually requiring refocusing after magnification adjustment. Therefore, the traditional telescopes lack adaptability, port-ability and real-time capability. In this paper, a continuous optical zoom telescopic system based on liquid lenses is proposed. The main components of the system consist of an objective lens, an eyepiece, and a zoom group composed of six pieces of liquid lenses. By adjusting the external voltages on the liquid lenses, the zoom telescopic system can achieve continuous optical zoom from ∼1.0× to ∼4.0× operating with an angular resolution from 28.648" to 19.098", and the magnification switching time is ∼50ms. The optical structure of the zoom telescopic system with excellent performance is given, and its feasibility is demonstrated by simulations and experiments. The proposed system with fast response, portability and high adaptability is expected to be applied to astronomical observation, emergency rescue and so on.
© 2024 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Telescopes are one of the most basic types of equipment in the fields of astronomical observation, emergency rescue, and other areas of science and technology [1–4]. Considering the observation requirements for targets at different distances, modern telescopes are usually expected to have a zoom function. The traditional telescopes achieve zoom function through the mechanical movement of the solid lenses, which inevitably increases the volume [5–7]. And it is usually necessary to refocus after magnification changes, which inevitably brings about the information loss of the observation target and lack of fast response time. It is a quite challenge to obtain a compact, lightweight, and high-quality telescope. In recent years, due to the compact structure, low power consumption, and fast tunability of the liquid lens [8–10], it has been widely used in optical imaging systems [11–31]. For zoom telescopes, the use of liquid lens can revolutionize the traditional zoom method, and achieve continuous optical zoom without any mechanical moving modules, which makes the telescopic system compact and lightweight [32–38]. While ensuring that the observation target always maintains clear in the field of view, the zoom speed can be effectively improved, and the imaging quality is enhanced.
In this paper, a continuous optical zoom telescopic system based on liquid lenses is proposed. The main part of the system consists of an objective lens, an eyepiece, and a zoom group for continuous optical zoom. The zoom effect of the proposed system is achieved by adjusting the focal length of the liquid lenses in the zoom group by external voltage. The key novelties of the proposed zoom telescopic system include: (1) A new method of zoom telescopic system design is revealed. The traditional zoom telescopes change magnification through distance compensation. The proposed system achieves continuous zoom function without mechanical movement only by changing the driving voltages of the liquid lenses, ensuring that the system can perform fast response time, strong adaptability, and stability. (2) An optical structure of zoom telescopic system with excellent performance is proposed. A zoom telescope system is designed, which consists of six liquid lenses, a prism, and several solid lenses. It can jointly correct aberrations during the zoom process and maintain low wavefront aberration. The prism compresses the optical path and makes the system present a positive image for users. (3) A complete prototype of the continuous optical zoom telescopic system is manufactured. high-resolution telescopic imaging with continuous magnification from ∼1.0× to ∼4.0× is realized. Our experiments demonstrate the feasibility of the proposed zoom telescopic system which can be applied in the fields of military reconnaissance, city surveillance and so on.
2. Structure and operating principle
Figure 1 shows the schematic structure of the proposed zoom telescopic system. For the visual telescopic system, the design principle is to optimize an objective lens, an eyepiece, and a liquid lens zoom group, separately. The objective lens consists of a Pechan prism and several solid lenses. For the visual optical system, the image of the object through the system is directly accepted by the human eye. Therefore, it is necessary to add an image rotation prism to make the system a upright image of the object. The Pechan prism consists of a Schmidt prism with a ridge surface and a Half-Penta prism, which can invert the image plane. Therefore, the Pechan prism is used as the image rotation prism. The eyepiece consisting of multiple solid lenses is connected to the entrance pupil of the human eye. The function of the solid lenses in eyepiece and objective lens is to correct aberrations and bear partial optical power. The zoom group is composed of electrowetting liquid lenses, which undertakes zoom and aberration correction functions.
Unlike the conventional mechanical zoom telescopic system, the proposed zoom telescopic system uses electrowetting liquid lenses to replace the relative displacement between solid lenses to achieve fast zoom function. The contact angle between the conductive liquid and insulating liquid is driven through the electrowetting effect to change the focal length. According to the Young-Lippmann equation, the relationship between the contact angle θ1 and the external voltage U can be described as follows [39]:
Different from the mechanical movement zoom mode of the traditional telescopes composed of solid lenses, the proposed zoom telescopic system achieves continuous optical zoom by applying an external voltage. Assuming that the proposed zoom telescopic system is the perfect image system, according to the Gaussian formula, the optical power Φ of the zoom telescopic system can be expressed as:
3. Simulation and experiment
3.1 Simulation
In order to verify the feasibility of the proposed system, the optical design software OpticStudio is used to simulate the zoom telescopic system. After evaluating and calculating the design requirements, it is eventually determined that the objective lens contains three pieces of double solid lenses and one piece of Pechan prism. Effective correction of spherical aberration and chromatic aberration can be achieved with the use of multiple double-glued lenses. In order to facilitate the optical design of the system, the Schmidt prism and the Half-Penta prism in the Pechan prism can be equivalent to two parallel plates. The relationship between the equivalent length L and the clear aperture D can be expressed by the following equations
As for the liquid lens zoom group, the liquid lenses we used are produced by Corning Varioptic, US, with the type of Arctic-58N0, whose effective aperture is 5.8 mm and the optical power can be changed from -13D(m−1) to 10D(m−1). Although the voltage of the electrowetting liquid lens can reach ∼70V in the driving process, the magnitude of the current is is below 100µA and is essentially negligible. Therefore, the power consumption of a single liquid lens is ∼20-40mW. Although liquid can adaptively adjust the focal length of the lens, the adjusting range is limited. To obtain a larger range of focal length adjustment, multiple liquid lenses need to be used in the system. However, increasing the number of liquid lenses will raise the manufacturing cost. After balancing the zoom range and manufacturing cost, it is finally determined that the system consists of six liquid lenses. Therefore, the power consumption of the whole system is ∼120mW-200mW, which can realize portable and long-endurance work.
The optimized objective lens, eyepiece, and zoom group are spliced into a complete zoom telescopic system. The last surface of the proposed system connected to the entrance pupil of the human eye is designed as an aperture stop. With the continuous improvement of magnification, the entrance pupil diameter of the proposed system accordingly self-adapting expands. The light transmission wavelengths of the system are set to 0.486µm, 0.587µm, and 0.656µm. Based on the adaptive adjustment of the focal length of the liquid lens, a new merit function is constructed to optimize the radii of the six liquid lenses, which purpose is to find the optimal solution of the system. The layout diagram of the proposed zoom telescopic system obtained by the final simulation is shown in Fig. 2. The simulation results show that the angular magnification of the zoom telescopic system can be tuned from ∼1.0× to ∼4.0×, and the zoom ratio is 4×.
For the visual imaging system, the wavefront map is selected as an evaluation index in priority. We simulated the wavefront map at the magnifications of 1.0×, 2.0×, 3.0× and 4.0×, respectively, as shown in Figs. 3(a)–3(d). When the magnification is 1.0×, 2.0× and 3.0×, the the peak-to-valley value (PTV) are less than 1/4 wavelength, which means that the aberrations are well corrected, and ideal imaging can be achieved. When the magnification is 4.0×, although PTV is larger than 1/4 wavelength, root mean square (RMS) of the wavefront is 0.2124λ, which can also demonstrate that the system has satisfactory imaging quality at this magnification. In order to give an intuitive aberration evaluation, we analyze spherical aberration (SPHA), coma aberration (COMA), astigmatic aberration (ASTI), field curvature (FCUR), distortion (DIST), longitudinal color (CLA), and transverse color (CTR) at different magnifications, as shown in Fig. 4. Almost all aberrations are controlled within ±0.001mm at any magnification. Although the spherical aberration is not well corrected at high magnification, it is also controlled within 0.006mm. This means that the imaging quality of the zoom telescopic system is satisfactory.
Fig. 3. Wavefront map of the zoom telescopic system at the magnifications of (a) 1.0×, (b) 2.0×, (c) 3.0×, and (d) 4.0×.
3.2 Experiment
The proposed zoom telescopic system is fabricated to evaluate the optical performance, as shown in Fig. 5. The aluminum alloy-6061 material is machined into the mechanical enclosure of the proposed zoom telescopic system by a turn-milling process. The solid lenses and the Pechan prism are fabricated by DaHeng New Epoch Technology Inc. Due to the existence of the Pechan prism, the objective lens part has a relative large volume, with a length of 70mm and a diameter of 55mm. The diameter and length of the eyepiece are 20mm and 28mm, respectively. The length of the zoom group is 50mm and has the same diameter as the eyepiece.
The focal length of the electrowetting liquid lens at different applied voltages are measured. The negative focal length range of the liquid lens is (-∞, -68mm), and the positive focal length range is (52 mm, +∞). According to the optimized solution of the liquid lens curvature in the optical design software and experimental data, we obtain driving voltage values of the six liquid lenses when the magnification varies from ∼1.0× to ∼4.0×. Figure 6 shows the driving voltage values of six liquid lenses under seven sets of different magnifications. To guarantee the continuous optical zoom of the proposed zoom telescopic system, 40 sets of voltage values corresponding to 40 sets of magnifications between 1.0× and 4.0× are preset actually. This means that the zoom telescopic system can achieve clear imaging at different magnification only by adjusting the voltages applied to the six liquid lenses without refocusing. Among them, liquid lens-2, liquid lens-5 and liquid lens-6 play the main role of magnification adjustment, while the other liquid lenses contribute to aberration correction and image plane compensation.
Since the proposed zoom telescopic system is a visual optical system, the intuitive feeling of the human eye cannot be directly recorded. Therefore, we select a photographic lens focusing to infinity to simulate the human eye for imaging experiments of the proposed zoom telescopic system. The camera has a 12 mm focal length and 2.8 F-number, connected with a CMOS whose target surface size is 7.4mm × 4.9mm and resolution is 3072 × 2048. The operating temperature range of the proposed zoom telescopic system is -20°C to 50°C, which is limited by the operating temperature of the liquid lenses. The imaging experiments were performed at a temperature of 20°C, which models most usage scenarios of the proposed system.
In the first experiment, a resolution test chart (ISO 12233) placed 6m away from the proposed zoom telescopic system is used to measure the angular resolution. The angular resolution of the system can be calculated by the ratio of the line-width that can be resolved to the distance value. During the continuous optical zoom process, we capture images at six magnifications of 1.0×, 2.0×, 3.0× and 4.0×, respectively, as shown in Figs. 7(a)-(d). At the magnification of 1.0×, the resolution target Group-6 can be clearly captured, indicating that the resolution can reach more than 0.83mm and the angular resolution can reach more than 28.648". With the increase in magnification, the resolution of the proposed microscope is improved continuously. When the magnification is increased to 4.0×, the resolution target Group-9 can be clearly captured, indicating that the resolution is increased to more than 0.55mm and the angular resolution can reach more than 19.098". Due to the fixed brightness of the environment and the exposure time of the camera doesn’t automatically adjust, the brightness and contrast of the image are affected to some extent. For the direct observation of the human eye which can automatically adjust the pupil, the effect will be better.
Fig. 7. Captured images of the resolution target. (a) Magnification 1.0×. (b) Magnification 2.0×. (c) Magnification 3.0×. (d) Magnification 4.0×. (e) The normalized intensity curves of the Group-6 at 1.0×. (f) The normalized intensity curves of the Group-7 at 2.0×. (g) The normalized intensity curves of the Group-8 at 3.0×. (h) The normalized intensity curves of the Group-9 at 4.0×.
In order to quantitatively analyze the resolution test results, we added normalized intensity curves to the acquired resolution target images. The normalized intensity curves represent the Group-6 at 1.0×, Group-7 at 2.0×, Group-8 at 3.0× and Group-9 at 4.0×, respectively, as shown in Fig. 7(e)-(h). The angular resolutions at 1×, 2×, 3× and 4× magnification are 28.648", 24.546", 21.451" and 19.098", respectively. It is further demonstrated that the resolution keeps improving as the magnification increases. The resolution test experiment proves that the proposed zoom telescopic system maintains excellent imaging quality throughout the continuous optical zoom process.
To further verify the practical imaging quality of the proposed zoom telescopic system, the common buildings in the city are used as the observation sample. By synergistically adjusting the voltage of the liquid lenses, the telescopic observation at different magnifications is obtained. The dynamic response video of the image capture process is included in Visualization 1. The response time of switching different magnifications is slightly different, which is proportional to the amount of magnification change. The response time for switching from 1× to 4× magnification is about ∼50ms, and no blurry imaging occurs. The proposed method is more real-time than the traditional zoom mode using mechanical gears that requires a response time of seconds. With the increase in magnification, the viewing field and the imaging brightness also reduce to some extent. Several groups of images taken under different magnifications are selected to intuitive evaluation, as shown in Figs. 8(a)-(f). Buildings A, B and C are 200m, 800m and 1800m away from the observation point, respectively. Three buildings can be imaged clearly at the same time, proving that the proposed zoom telescopic system has an extra-large depth of field. With the continuous improvement of magnification, the captured image is uniformly enlarged while maintaining high resolution.
Fig. 8. Captured images of the common buildings in the city. (a) Magnification 1.0×. (b) Magnification 1.5×. (c) Magnification 2.0×. (d) Magnification 3.0×. (e) Magnification 3.5×. (f) Magnification 4.0×. (g) Comparison of images captured with and without the proposed zoom telescopic system.
Remove the proposed zoom telescopic system and use the camera lens to capture the same scene directly, as shown in Fig. 8(g). It is obvious that the image quality can be significantly improved by using the proposed zoom telescopic system, which proves that the proposed zoom telescopic system contribute to improve the observation range and the observation of details of distant objects. However, it can be seen from the imaging experiments that the slight deficiency of the proposed zoom telescopic system includes: (1) due to the limited aperture of the liquid lens, the vignetting of the edge field of view occurs, which is more pronounced at high magnification; (2) the dispersion of the edge field of view caused by the inherent aberration of the liquid lens is relatively obvious. Precision control of surface shape, preparation of liquid materials that compensate for dispersion coefficient, and expansion of clear aperture can effectively solve the two problems.
The magnification range and zoom ratio are related to the number of liquid lenses, solid lenses and optical design. Among them, the biggest limitation is the focal length adjustment range of the liquid lenses. There is no doubt that the larger the number of liquid lenses, the larger the zoom ratio we can obtain. In our design, six liquid lenses are used to achieve a magnification ratio of 4×, which cannot be accomplished by reducing the number of liquid lenses. More liquid lenses can be used to break through the 4× zoom ratio. The reasons for not trying to further improve the zoom ratio are as follows: 1) The clear aperture of the liquid lens is limited. The increase of magnification will further enlarge the aperture of the incident beam, which will make the edge beam unable to pass through the system, seriously affecting the edge imaging quality. 2) Low transmittance of the liquid lens. The transmittance of electrowetting liquid lens is only 90%, far less than the transmittance of solid lens coated with antireflection film over 99%, which will inevitably bring scattering and refraction phenomena. Adding liquid lenses will increase the zoom range but cause serious scattering and refraction, which will affect the imaging quality. 3) High production cost. Most use cases do not require a larger zoom ratio, but the increase in the number of liquid lenses will lead to higher production costs.
4. Conclusion
In this paper, we design and fabricate a continuous optical zoom telescopic system based on liquid lenses. Without any mechanical movement, the proposed system controls the optical power of the liquid lenses by external voltage to realize continuous optical zoom. Simulation and experimental results show that the proposed system can achieve a continuous magnification change from ∼1.0× to ∼4.0× while operating with an angular resolution from 28.648" to 19.098". The zoom telescopic system has fast response time, strong adaptability, and stability. It can be applied in the fields of astronomical observation, city surveillance, and so on.
Funding
National Natural Science Foundation of China (U21B2034, U23A20368, 62175006); Science, Technology and Innovation Commission of Shenzhen Municipality (JCYJ20220818100413030).
Disclosures
The authors declare that there are no conflicts of interest related to this article.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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