Application of Refraction/diffraction Hybrid Optical System in Nano Star Sensor
Micro-nano satellite is a new kind of micro-satellite and is widely used.Nano sensor adapted to it should have the characteristics of small size,high precision and high detection sensitivity.Therefore,the optical system of nano star sensor should achieve large relative aperture and high imaging quality while miniaturization.Aiming at the requirement of nano star sensor optical system,the field of view and focal length of optical system are calculated by the system design accuracy index of star sensor.From the detection magnitude and SNR,the sensitivity model of star sensor and the diffraction efficiency formula are used to calculate the optical system's entry pupil aperture and detection band.A star sensor optical system with full field of view of 17°,focal length of 25 mm and relative aperture of 1:1.086 is designed.According to the negative dispersion characteristics of diffractive elements,the number of lenses is reduced by using a hybrid lens instead of a set of conventional material positive and negative lenses,and the total length of the optical system is shortened from 48 mm to 36.4 mm.In order to illustrate the advantages of this structure,various imaging indexes are compared.The results show that the addition of diffraction surface makes the point diffusion function tend to normal distribution,which is conducive to improving the accuracy of centroid extraction.The optical modulation transfer function at Nyquist frequency increases above 0.4.The maximum dispersion spot size of the optical system is 4.333 μm,and the value is 7.277 μm without addition.The energy concentration in 33 pixels is more than 90%;without joining,the value is 80%;the vertical color difference of the full field of view is less than 0.6 μm,while the value is more than 1.1 μm without addition,so the angle measurement error caused by the color deviation of the center of mass is controlled within 4.75".The diffraction efficiency of the diffraction element in the range of 520～780 nm is calculated by the scalar diffraction theory.The ghost image in the field of view of the system is analyzed by the stray light analysis software ASAP.The results show that,the ratio of luminous flux under the path of the image spot and the brightest ghost spot is greater than 3.94×104 in the primary diffraction,and the ratio of luminous flux under the path of the image spot and the brightest ghost spot is greater than 3.17×104 in the multi-level diffraction.There is no difference in the distribution position of the main ghost image in the two cases,only in the luminous flux value.According to the theoretical calculation of the maximum luminous flux of the system ghost image,when the limit magnitude is 6.5 and the signal-to-noise ratio is 8.1,the luminous flux ratio of the detection target under the image spot path and the luminous flux under the brightest ghost image spot path should be at least greater than 1.42×104.Therefore,the results of ASAP ghost tracking verify that the hybrid system meets the requirement of a signal-to-noise ratio greater than 8.1 when the limit magnitude is 6.5.The results show that when the deviation of aperture number is less than±2,curvature radius,thickness,eccentricity is less than±0.02 mm,inclination is less than±0.02°,refractive index deviation is less than±0.005,Abbe number deviation is less than±0.5,the dispersion spot radius has an 80%probability better than 6.98.It has a 90%probability better than 7.53 μm,which can meet the requirements of energy concentration greater than 90%in 3×3 pixels.
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