Symmetrical astigmatic focus sensing system

Claim 1

In an optical playback apparatus in which a multi-turn information storage track of a video record is supported in a reading plane for scansion by a reading beam derived from a source of coherent light to retrieve information stored in said track, an improved focus error correction system comprising: means for directing said reading beam along a first optical path extending between said source of coherent light and said storage track; means disposed in said first optical path for converging said reading beam to an intermediate light spot; an objective lens, included in said first optical path, for focusing the light energy emanating from said intermediate spot into a reading spot on said storage track, registration of said reading spot on said track corresponding to optimum focusing condition when said track is positioned in said reading plane; a beam-splitter, also included in said first optical path, for establishing a second optical path for light reflected from said track and for relaying a reflected image of said reading spot into said second path; a pair of spaced apart orthogonally disposed cylinder lenses of substantially identical focal lengths positioned along said second path and comprising, in conjunction with said light converging means, first and second telescopes for forming respective first and second astigmatic images of said reading spot along said second path, said images being spaced apart a distance substantially equal to the separation of said cylinder lenses, and, the spatial positions of said astigmatic images being effectively displaceable along said second path distances proportional to a misregistration between said track and said reading plane; photoreceptor means effectively interposed in said second path approximately midway between said first and second astigmatic images and responsive to relative variations in the light energy of said astigmatic images, attributable to displacement of said images due to a misregistration between said storage track and said reading plane, for developing an error signal indicative of the sense and extent of such a departure from said optimum focusing condition; and means, responsive to said error signal, for effecting a relative displacement between said objective lens and said track to re-establish said optimum focusing condition.