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根据项目要求对测试仪系统参数进行针对性设计与分析。测试系统的主要技术指标为:谱段范围0.4~1.1 μm,焦距500 mm,入瞳口径150 mm,视场2ω=4°,成像质量RMS=λ/7 (λ=632.8 nm)。照明光源采用积分球,主要技术指标为:内径150 mm,出口Ф50 mm,出口处均匀性±2%。
采用Zemax对系统进行初步分析,一镜、两镜反射式系统视场不能满足要求,三反系统价格过高。透射式光学系统可满足使用要求,且成本合理,因此,选用了透射式光学系统进行实验验证。设计结果如图2所示,采用平行光管将靶标成像到无穷远的形式[17-18]。系统主要由前后两组镜头构成,口径Ф150 mm,焦距500 mm。前组包含一片单透镜与两片双胶合透镜,后组包含一片单透镜与一片双胶合透镜。系统中各视场、各波长的平均设计波像差为0.064λ(λ=0.6328 μm),均约合λ/15。下面对主要影响其检测精度和成像质量的因素进行分析。
(a)畸变与像差对质心位置的影响分析
主光线偏离理想像点产生的畸变与垂直于靶标线视场方向点扩散函数的非对称性都会造成配准误差。为精确地分析畸变、像差及靶标对中误差等因素对配准精度的影响,在Zemax中建立了如图3所示的模拟光学系统。系统出射平行光,照明焦距为2 210 mm的理想相机系统,在相机焦平面上采集点扩散函数,并将各视场点扩散函数错位叠加,仿真线光源成像。据此计算不同子午视场对应的光斑的弧矢位置,即配准误差,仿真结果如图4所示。
靶标对准误差为100 μm时,±2°子午视场角范围内光斑质心在x方向的偏移量为−0.5~0.1 μm,显示为桶形畸变,对配准精度影响不大。
(b)色差对质心位置的影响分析
测试仪采用同轴透射式光学系统,由于玻璃折射率影响,系统存在垂轴色差。图5为测试仪在2°有效视场范围内400 、700、1 100 nm谱段对应的垂轴色差。
如图6所示,当狭缝靶标穿过系统光轴,狭缝像质心没有偏移,垂轴色差对配准无影响。若狭缝偏离光轴D,线视场H处垂轴色差会导致质心产生偏移,偏移量P=A·D / H。视场角为2°时,H为17.46 mm,长短波垂轴色差A为4 μm,若狭缝偏移D为0.5 mm,计算得P为0.11 μm。实际测试仪狭缝靶标相对光轴的安装精度远优于0.5 mm,因此,色差对配准精度无影响。
(c)对温度影响的补偿
九谱段焦面配准测试系统要求在(20 ±2) ℃范围内工作。结构设计选择硬铝作为结构材料,线胀系数23.5×10−6/℃。温度的改变会引起结构的变化,从而改变系统的焦距等参数,造成像面光斑离焦及成像质量下降等问题。假定热胀冷缩过程中,透镜与外筒位置相对固定,则以前组透镜的最后一面顶点为坐标原点,分析温度变化时的最佳焦面位置,如表1所示。
表 1 温度变化对焦面位置的影响
Table 1. Focal plane position with temperature
Temperature 18 ℃ 20 ℃ 22 ℃ Focal plane position 428.946 429.012 429.077 ±2 ℃的温度变化,对应理想像面位置变化约为±65 μm。这里设计补偿杆来补偿焦面位置变化:采用尼龙材料制作补偿杆,其线胀系数为120×10−6/℃。补偿杆起始于测试仪镜筒前端面,后续串联到镜筒结构内,补偿杆与镜筒选择合适的长度比例,将像面位置变化减小到±3 μm以内,从而补偿温度引起的离焦,确保光学系统参数满足要求。
Development of focal plane registration instrument for a nine-spectral camera
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摘要:
多通道焦面遥感相机通道间的配准是相机总装定焦过程中的关键技术,通道间的配准精度决定了后续通道融合后的遥感图像质量。设计并完成了一套用于解决九谱段双通道焦面配准的测试仪。积分球照明一条狭缝,经平行光管成像在无穷远,照明九谱段相机,在相机焦面上得到一个竖直放大的狭缝像。四谱段、五谱段线阵探测器分别位于双通道焦平面上,分别调整双通道焦面的位置,计算四谱段、五谱段探测器上狭缝像的质心位置,通过质心位置来评价通道间像元的对准精度。该仪器解决了多通道焦面遥感相机通道间的配准问题,配准精度达亚像元量级,保证了通道融合后的图像质量。
Abstract:Multi-channel focal plane registration of remote-sensing camera is the key step in camera assembling process. Registration accuracy determines quality of the image after channel fusion. The development of focal plane registration instrument for a nine-spectral double-channel camera was proposed in this article. A slit was illuminated by integrating sphere and imaged at infinity with collimator. The infinity slit was recorded by the nine-spectral camera, and became an enlarged vertical slit on camera focal plane. The four-spectral detector and five-spectral detector were on two different focal plane channels. By approaching the slit image centroid position on two different detectors, the two focal plane channel were aligned together in high precision. This instrument solved the problem of high registration accuracy of multi-channel camera. Sub-pixel registration accuracy could be reached by the instrument, assuring the camera image composite.
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Key words:
- remote sensing camera /
- multi-channel focal plane /
- registration accuracy /
- multispectral
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表 1 温度变化对焦面位置的影响
Table 1. Focal plane position with temperature
Temperature 18 ℃ 20 ℃ 22 ℃ Focal plane position 428.946 429.012 429.077 -
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