In war, telescopic sight is a kind of essential equipment in light weapons. Up to now, most of the Chinese research on telescopic sight is about optical sight, night vision sight, and infrared thermal sight, while research on photoelectric sight with the function of working in both daylight and darkness is few. In this paper, the function is realized by switching the filter of the wide-band continuous zoom optical system that can realize the continuous change in the field of view. The design of a wide-band zoom optical system, which is mostly used in the field of security monitoring, rarely mentions the operating distance and considers the portability of the system. Therefore, after determining the focal length range of the objective optical system, this paper employs a triple-linkage zoom structure with four configurations, and the structure can fulfill the requirements of light weight and small size and realize the function of working in both daylight and darkness by focusing once at the front-fixed group. We hope that this design method can contribute to the light weight and small size of the photoelectric sight.

First, the focal length is determined by relevant parameters of the CCD detector, field of view, and operating distance. After that, the triple-linkage zoom structure with four configurations is selected for realizing light weight and small size after comparing the common structures used in the continuous zoom system. Then, the virtual surface with the focal power of 0 is added, and the structure is subjected to the Gaussian optical analysis by using the Gaussian bracket method. In the next step, the initial structure is substituted into ZEMAX to scale and adjust the lens and optimize the aberration. In this case, the front-fixed group is composed of three pieces of lenses, including one cemented doublet and two single lenses with high Abbe number, so as to keep the chromatic aberration as small as possible and reduce the lens aperture. In addition, aspheric surfaces are used to improve image quality and reduce the number of lenses. Moreover, by focusing once at the front-fixed group, imaging quality becomes excellent during the whole zoom process in the infrared band.

The optical system achieves a light weight and a small size when the zoom range is 25-250 mm. The total optical length of the objective lens is 199 mm, the maximal effective aperture is 58.1 mm, and the total weight of the lens is 203.6 g. The working waveband of the optical system is 0.48-0.68 μm and 0.80-0.90 μm, and the focusing distance is only 0.26 mm (Table 3). In the full field of view, the relative illumination is higher than 0.4 when the F-number is F_5.0-F_6.5 (Table 2). In the visible light waveband, the modulation transfer function (MTF) values of the central field of view of four configurations are all greater than 0.5 at 90 lp/mm (cut-off frequency), and the MTF curves of other fields of view are relatively smooth (Fig. 3). In the infrared waveband, the MTF values of the central field of view of four configurations are all greater than 0.3 at 90 lp/mm, and the MTF curves of other fields of view can meet the application requirements (Fig. 4). From the spot diagrams in the visible light waveband (Fig. 5) and the infrared waveband (Fig. 6), it can be seen that the root mean square (RMS) radii are below 8 μm, or in other words, the energy concentration of the system is fine. Overall, the imaging quality of this optical system is excellent. In addition, the motion curves of the zoom groups are smooth without inflection points during the zooming process, and there is a nonlinear constraint relationship between the compensation group and the zoom group, which can meet the requirements of engineering application (Fig. 7). After the tolerance allocation, it can be seen that the set tolerance meets the system image quality requirements, and the system has great achievability from the MTF value of each focal length at the cut-off frequency and the yield rate of the corresponding simulated product at this value (Table 4).

In this paper, the Gaussian bracket method is used to analyze the triple-linkage zoom structure with four configurations under the consideration of the operating distance, and the light and compact design of the zoom system is carried out. In addition, the defocus in the infrared band is improved by focusing once at the front-fixed group, so as to realize the wide-band design of the zoom system. In the design process, the contradiction between the aberration and the requirements of lightweight and small size is gradually solved, and finally the zoom objective optical system with a common aperture confocal plane is realized, of which the focal length is 25-250 mm in the visible light band and the infrared band. The system is lightweight and compact, and the imaging quality, illuminance, cam curve, and tolerance are all suitable for engineering applications, so it is expected that the system can be widely used in photoelectric sights with the function of working in both daylight and darkness.