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Many people all over the world suf1er from eyediseases. According to the information of physicians there is I billion of' myopic sick people in the world, 69 million in Russia , 2 million 125 thousand in Moscow. Modern ophthalmology has a large arsenal o means or treating such patients. For using some methods o sight correction it is necessary to know the exact shape o the anterior surface or a cornea ( or the cor— neal topography ). For example, there exist microsurgical ope— rations o keratotomy, contact lens fitting etc. In order to use these methods successfully it is necessary to determine the optical power distribution and the radius o a cornea with the precision o 0.25 dioptre and 0.01 mm respectively. For this purpose at present special devices — kerato— scopes are usually used, for example, Topographical Modeling System (TMS ) [ 3 ] , Photokeratoscope PKS—1 000 [2 1 , Corneascope [4] , K—O1 [1 ] . All these instruments realize so called method of keratometry. The cornea to be examined is illuminated by the light from the mire, which looks like a series of the shining rings. The light is reflected by the cornea and a virtual image of the mire is created. This virtual image is projected by the lens to the image plane where the photo— or TV camera is placed. Usually the telecentric projected system is applied. The recorded image (keratogramm) is then processed by means of special algorithms. In all the above mentioned devices the shining rings must be located at a defined distance from the eye. If the longitudinal or lateral 'lisplacernents or the eye relative to the devioe take place the shape of the mire image at the keratogramrn will be distorted. This distortion leads to the errors oi the measurement results. There±ore, in these kerato— scopes the eye should be boated relative to the apparatus very accurately, with the error less than 0.25 mm. To provide such an aoouraoy o1 looation special arrangements are used, Lor example, a laser system or the eye alignment in Topogra— phical Modeling System,a changeable aperture stop in PKS—i000. An essetial disadvantage limiting the possibilities of the keratosoopes is the dependence of the measured oorneal topo— graphy precision results from the aoouraoy of the eye align— ment. This disadvantage deoreases the possibilities o the keratosoopes. For example, acoording to the results o the experimental research [6], TMS provides the necessary preci— sion for determining the optical power distribution of 0.25 dioptre 70% of the whole corneal area only. That is why the main problem o the keratosoopes optical system design is to eliminate the influence ot the eye align— ment on the measurement precision.
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