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为了验证无焦系统下延展理论的有效性,根据上述分析指导优化一个七倍望远无焦系统,校正632.8 nm和1064 nm波段的色差。七片无焦望远系统初始结构如图1所示。其对应的色差情况如图2所示,系统在0.707孔径处最大轴向色差约为3.446×10−3 D (屈光度),并且从焦移曲线图中得到信息, 632.8 nm和1064 nm两个波长的焦移相差0.00111 D,波段范围内最大焦移为0.0186 D,系统尚未能完全实现消色差设计。
Figure 2. Chromatic aberration diagram of initial structure of the unfocused system. (a) Longitudinal aberration; (b) Chromatic focal shift
为对系统色差进一步优化,应用无焦系统下的色散向量分析法指导系统中材料的选择与替换工作。对无焦系统初始结构进行色散向量分析,所选材料、相关光线追迹参数以及色散系数如表1所示。
Glass 1 2 3 4 5 6 7 SILICA CAF2 H-ZF71 H-ZPK7 H-K90GTI H-ZF73 H-ZLAF68C φi
yi
αi
η1
η20.00053
95.00
0.6277
−0.03602
−0.021310.00432
56.8459
1.8316
−0.02243
−0.00492−0.00485
49.1365
−1.5373
−0.07723
0.008290.00681
47.3583
2.0058
−0.02944
−0.00568−0.00944
38.6722
−1.8524
−0.03659
−0.017040.00910
26.5958
0.8449
−0.09580
0.01795−0.02417
17.0330
−0.9203
−0.04770
−0.00156Table 1. Initial structure ray tracing parameters and dispersion coefficients of the unfocused system
由表1中数据计算得到色散向量坐标,将系统的色散情况表示在如图3所示的二维色散向量空间下,所选材料在色散图中的位置分布如图3中黑点所示,实线表示正透镜组的色散向量之和,虚线表示负透镜组的色散向量之和,红色箭头代表组合向量
$ \overrightarrow {{G_0}} $ ,其模长经计算为0.02678。接着以色散向量分析法为指导,对系统中部分材料进行替换以减小
$ \overrightarrow {{G_0}} $ 的模长,从图3中可以看出,正透镜组表示的向量$\overrightarrow {{G_ + }} $ 的两个分量均为负值且其绝对值均小于负透镜组表示的向量$\overrightarrow {{G_ - }} $ 的两个正分量,因此,替换材料的思路仅从材料色散系数绝对值大小的角度来说,应该是适当增大正透镜组所选材料的色散系数而相应减小负透镜组的色散系数,使$\overrightarrow {{G_ + }} $ 和$\overrightarrow {{G_ - }} $ 的两个分量之间更加接近,从而实现减小和向量$ \overrightarrow {{G_0}} $ 的目标。以图3反映的正负透镜组材料色散系数的调整方向为主要原则,其中一阶色散系数起到更为主导的作用,同时对材料的获取难度以及价格综合考虑,对系统中第1、2、5片透镜的三组材料进行了相应替换,在表2中加粗示出。
Glass 1 2 3 4 5 6 7 H-K9L H-FK61 H-ZF71 H-ZPK7 H-QK3L H-ZF73 H-ZLAF68C φi
yi
αi
η1
η20.00054
95.00
0.5276
−0.03623
−0.017350.00460
55.7064
1.5488
−0.02691
−0.00845−0.00484
52.2936
−1.4353
−0.07723
0.008290.00717
48.6852
1.8429
−0.02944
−0.00568−0.00830
41.6452
−1.5618
−0.03436
−0.019720.00830
28.0373
0.7074
−0.09580
0.01795−0.02546
15.0993
−0.6296
−0.04770
−0.00156Table 2. Ray tracing parameters and dispersion coefficient after optimization of unfocused system
为确保系统的其他像差不会发生太大变化,在材料组合选择的基础上,对透镜的外形和间距进行了优化,表2给出了系统优化结果的光线追迹参数以及色散系数,其色散向量图如图4所示。
结合图4及数据反馈可知,替换材料后,正透镜组向量之和与负透镜组向量之和的偏差明显减小,和向量
$ \overrightarrow {{G_0}} $ 的模长显著下降到0.01174,同时材料选择方面也明显优于初始结构,均为生产频次较高、价格相对更低的常用玻璃。系统优化后色差校正结果如图5所示。从轴向色差曲线可以看出,在0.707孔径处曲线近似交汇在一点,色差值仅为2.705×10−5 D,从焦移曲线反馈来看,632.8 nm和1064 nm的焦移差减小到0.00021 D,波段内最大焦移仅为0.0047 D,系统色差得到有效校正。最终设计结果的MTF曲线如图6所示,替换材料优化后MTF曲线也完全达到了衍射极限,至此完成无焦系统的消色差设计。
Buchdahl model of achromatic method for unfocused systems
doi: 10.3788/IRLA20220374
- Received Date: 2022-05-31
- Rev Recd Date: 2022-08-27
- Publish Date: 2023-01-18
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Key words:
- unfocused system /
- Buchdahl model /
- dispersion vector
Abstract: The dispersion vector analysis method based on Buchdahl model can be used to guide material selection and replacement in optical design to obtain material combinations with good achromatic effect, but at present the theory is only applied to the design of focused systems due to the limitation of mathematical form, and there is no precedent for guiding the design of unfocused systems. To further investigate the application of this theory to the unfocused system, a deformed expression of the dispersion vector scale factor applicable to the unfocused system is proposed, and the material selection of a seven-fold telescopic unfocused system is carried out by this method. The optimized system has a maximum chromatic aberration of only 2.705×10−5 D (Diopter) at 0.707 aperture. The focal shift difference between 632.8 nm and 1 064 nm wavelengths is 0.000 21 D, and the maximum focal shift in the wavelength range is only 0.004 7 D. The chromatic aberration of the system is effectively corrected, and the MTF of each field of view completely reaches the diffraction limit. The research results enable the dispersion vector analysis method based on Buchdahl model to be applied to the design of unfocused systems as well, providing a new idea for the selection of achromatic material combinations for unfocused systems.