Objective In fields such as laser detection and space observation, traditional long-focal-length optical systems often encounter problems of large volume, inconvenient operation and insufficient light transmission when aiming to enhance resolution and achieve ultra-long focal lengths. Moreover, existing telephoto objectives mostly have a relatively high telephoto ratio (0.5-0.8), and catadioptric structures are prone to introducing central obstruction, leading to imaging energy loss. Therefore, it is necessary to design a fully refractive large-aperture ultra-long focal length telephoto objective that can achieve a large aperture (Φ125 mm) and an ultra-long focal length (about 5000 mm), while reducing the telephoto ratio to ensure structural compactness, avoiding central obstruction and improving imaging quality, providing support for breaking through technical bottlenecks in the design of optical systems in related fields.

Methods The all-refractive compact telephoto system structure is adopted. The initial structural parameters of the front group (positive lens group) and the rear group (negative lens group) are calculated first through the focal length distribution formula. The basic parameters such as the front group focal length of 265 mm and the rear group focal length of −15.33 mm are determined by combining the proportion coefficient (a=0.053). Then, the evaluation function editor of the optical design software ZEMAX is used to limit the center and edge thicknesses of the air (MNCA, MXCA, etc. operands) and glass elements (MNCG,MXCG, etc. operands), and introduce a multi-lens optimized structure (1 piece in the front group and 3 pieces in the rear group) to perform iterative optimization on the lens group data. The optomechanical design adopts a multi-part tubular structure, and the flange connection is used to ensure the stability of the optical axis. The gasket and the index pin are set inside the lens chamber to correct the on-axis and off-axis aberrations. The Monte Carlo simulation method (ZEMAX, 200 iterations, cut-off frequency 26.34 lp/mm) is used to conduct tolerance analysis. The total length of the system is measured multiple times with a tape measure, and the actual focal length and telephoto ratio are obtained by converting the Airy disk images collected by the laser collimation at 532 nm and the camera.

Results and Discussions In terms of optical performance, the actual focal length of the system is 4982.74 mm (design value: 5000 mm, relative error: 0.3452%, Tab.7), the telephoto ratio is 0.106 (<0.2, Tab.8), the aperture is Φ125 mm, the F-number is 40, and the total length of the system is 530.22 mm (average of 5 measurements, Tab.6), which is significantly more compact than existing telephoto objectives. The imaging quality evaluation shows that the modulation transfer function (MTF) at 26.34 lp/mm is greater than 0.3 across the entire field of view (Fig.4), and the RMS radius of the spot across the entire field of view is less than 3 μm, which is much smaller than the Airy disk radius of the system (25.813 μm, Fig.5). The beam quality factor β is 1.005 (Tab.3), meeting the requirements for high-precision imaging. The tolerance analysis results indicate that 90% of the systems have an MTF greater than 0.31995059 at 26.34 lp/mm (Tab.5), with a relatively high design tolerance. The optomechanical structure is connected through flanges and corrected with shims and set screws (Fig.7, Fig.8), effectively ensuring the stability of the optical axis and the correction effect of aberrations. In actual measurements, the Airy disk boundary is clear (Fig.9), and all indicators meet the design expectations, successfully solving the problems of large volume and central obstruction in traditional systems.

Conclusions The designed all-refractive large-aperture super-long focal length telephoto objective lens has a telephoto ratio of 0.106, excellent imaging quality (all MTF, spot radius, and β factor meet the standards) and stable optical axis. The actual measurement results are highly consistent with the design expectations. It has broken through the technical limitations of existing technologies in both compactness and detection performance, providing a technical reference for the design of super-long focal length optical systems in related fields.