Investigation Of Static MTF Measurement Technology Of Large Aperture Cameras Based On Slanted-edge Method

With the rapid development of optical technologies,the increasing requirements on camera’s resolution,precision,sensitivity,etc.,make camera’s aperture being larger and larger.In order to verify if camera’s image quality can reach design targets,the static MTF is measured in the stage of opto-electro abutting,presetting of vaccum focus plane,mechanical experiment,thermal experiment,etc.The information amount and precision of MTF measurement results directly determine the comprehensiveness and reliability of camera performance verifications.Currently,relevant domestic standards all recommend measuring camera MTF by the bar target method,which can get little information since it usually only measures MTF at the Nyquist frequency of the camera under test.The slanted-edge method(ISO 12233-2017),which has been recommended by the International Organization for Standardization,can obtain more information because it measures camera MTF over the whole frequency range.The slanted-edge method is mainly used to measure MTF of small aperture cameras and on-orbit MTF of space cameras.This dissertation applies the slanted-edge method to measure large aperture camera MTF,and introduces the principle of the slanted-edge method.To deal with the problems and challenges that are introduced to MTF measurement work by the increase of camera aperture,theoretical models are established to simulate the image chain of large aperture camera MTF measurement systems(CMMS).In the meantime,the influences of measurement equipment and eviromental factors on measuring MTF are investigated.Meanwhile,uncertainty analysis is carried on in this dissertation with the corresponding result manifesting that the expanded uncertainty of the slanted-edge method is 0.07.This dissertation mainly fulfills the following work:1.Mathematical models are established to simulate the image chain of the slanted-edge method based large aperture CMMS.The mathematical models consider the factors of diffraction,aberration,detector,air turbulence(AT),environment vibration(EV),temperature variation,gravity,etc.When the CMMS satisfies the spatially invariant imaging condition,the spatially invariant PSF imaging model and the spatially invariant MTF imaging model are established with the convolution imaging principle.While the CMMS is spatially variant,a computationally efficient spatially variant PSF imaging model is established by processing the CMMS’s known PSFs of finite fields with the principal component analysis PSF model to obtain basic functions.The coefficients of those basic functions are interpolated with two-dimentional interpolation methods.Thus,the linear superposition imaging process of spatially variant systems is simplified to the sum of the convolution of several basic functions of principal component analysis PSF model with the object.2.The impacts of measurement equipment on large aperture camera MTF measurement results are systematically studied.Firstly,the polychromatic imaging model of CMMS is established by superposing monochromatic PSFs of camera optical system with the weighting coefficients of the light source spectral characteristics and the CMMS spectral responsivity.Theoretical model of MTF measurement result is obtained with the CMMS’s eigenfunction to analyze the influence of light source spectral characteristics.Secondarily,mathematical models are derived to model obscurations induced by the secondary mirror(SM)in the CMMS and to model four common SM surporting structures.Obscurations in the CMMS are modeled with the former mathematical models in the virtual measurement system established by Zemax.The collimator obscuration induced error is corrected with the obscuration error correction function that is obtained by the ray trace method.Finally,the influence of the spatially variant aberrations of collimators is analyzed with the spatially variant PSF imaging model.The corresponding theories and technologies are all verified by simulation and experiment.3.A method is suggested to calibrate and correct the influence of environment factors on large aperture camera MTF measurement results.A modified knife-edge target is designed by adding a defocus star point and a focus star point to the dark aera of traditional knife-edge targets.Therefore,cameras can simultaneously capture images of the knife-edge target,the defocus star point,and the focus star point.When measuring MTF,the camera continuously captures sequential images of the modified knife-edge target.By analyzing the sequential defocus star point images with phase retrieval method,the wavefront error of the CMMS can be calculated to estimate the CMMS’s true wavefront error,which is used to calibrate AT and formulate error correction functions.In the meantime,centroids of the sequential focus star point images are extracted to calibrate the EV amount and formulate EV error correction functions.An experiment setup is established to verify the effectiveness of the proposed method.Experimental results demonstrate that the maximum range and the maximum standard error between the MTF curves has been reduced from 0.125 to0.085 and from 0.025 to 0.020 respectively over the whole frequency range,when AT induced errors are corrected.The MTF curve is augmented in some extent after EV induced errors being corrected.The former MTF curves are computed by camera’s consequently captured 100 knife-edge target images.Meanwhile,a method is discussed to calibrate and correct the influence of temperature variation and gravity on large aperture camera MTF measurement results.