| Focusing technology is one of the key technologies in the development ofspace optical remote sensing camera (referred hereinafter as a remote sensingcamera). As an important component of remote sensing camera, focusing system is akey part to ensure the imaging quality. Automatic focusing method based on digitalimages with simple structure high degree of intelligence has got more and moreapplications in a common digital camera. The key of auto-focusing based on imageprocessing is to establish the image quality assessment function (focus-checkingfunction). Under the guidance of real-time calculation with image quality evaluationfunction, the focusing action can be implemented correctly. Currently the imagequality evaluation function used in ordinary digital camera mostly relies on thecamera imaging the same scene several times. The specific imaging way of thepushbroom remote sensing camera leads to the result that images of a same scenecan not be obtained several times from one CCD. Therefore, the ordinaryfocus-checking function can not be directly applied to the pushbroom remotesensing camera. To solve this problem, this paper proposed one absolutefocus-checking function independant on repeatedly imaging of the same scene, andrealized the auto-focus by image in pushbroom remote sensing camera. On the otherside, after the research on pushbroom imaging principle, a method to get therepeated images of the same scene in the pushbroom remote sensing camera wasproposed, and the traditional focus-checking method was used. With corrected focusing method proposed the auto-focus by image in pushbroom remote sensingcamera was also realized. At last the experiment plate was set up, and the methodproposed in this paper was tested correspondedly. According to the above methodsthe specific study in this paper are as follow:1) With certain SNR, the MTF embodies the imaging system's transmissioncapacity of different frequencies, also reflects the imaging quality of the systemfrom the frequency domain. The inverse calculation of MTF from image does notdepend on the imaging of certain scene. The absolute focus-checking functionproposed in this paper, using the line spread function (LSF) witch is thecorrespondence of MTF in time domain, does not depend on the camera imagingone scene repeatedly. The focus-checking process based on LSF avoided the Fouriertransform, ensuring a high level of real-time in focus-checking process. This paperalso studied the method of picking up LSF from remote sensing images. Then withthe curve fitting the resolution of LSF is increased to0.1times the pixel size, whichensured a high accuracy.2) With the expanding coverage of remote sensing camera image, the size ofthe existing CCD device has been far from satisfying the requirements of remotesensing camera of large focal plane. The stitching technique of using multi-chipCCD is necessary to meet the image requirements. According to the pushbroomremote sensing camera imaging process, retaining a certain overlapping imagingarea in the splicing structure of the CCD, duplicate images based on the same scenefrom different CCDs can be got from these overlapping regions. From thisperspective, this paper proposed the method using the existing image focus-checkingfunction into the pushbroom remote sensing camera,which provides a strongfeasibility for the pushbroom remote sensing camera auto-focus.3) After analyzing the existing focus-methods, according to the focus-checkingmethod based on image proposed in this paper, the correct focus-method was set up.4) Aimed at the Auto-focusing method proposed in this paper, a validation andtesting board was built with corresponding software to implement the auto-focus.According to the results of the test, a detailed analysis of the accuracy and real-time was conducted. The results showed that the auto-focusing method proposed in thispaper for pushbroom remote sensing camera using image has a high feasibility, andprovids a solution for practical application.
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