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An altazimuth mount points an optical telescope using altitude and azimuth rotations. A mount restricted to elevations no greater than the zenith must change its azimuth by approximately half a turn when a target crosses the zenith.

A zenith blind spot is a region in which an altazimuth mount cannot follow a target because the required axis motion exceeds a drive constraint. A finite slew across an exact zenith crossing gives a lost-track interval; a full forbidden footprint additionally depends on the target trajectory, acceleration and the control strategy. It need not be circular.