Disclosed is apparatus for forming topographs of large crystalline areas using X-ray diffraction microscopy techniques. The apparatus includes an X-ray source which directs an X-ray beam at a crystal which is mounted on a position control unit. The position control unit includes a scanning goniometer for translating the crystal and a low friction device for rotating the crystal both with respect to the incident X-ray beam. A beam detector is positioned to detect a diffracted beam as it leaves the crystal. The X-ray detector is connected to the position control unit through a feedback control unit. The feedback control unit operates by introducing a small input perturbation signal into the position control unit thereby causing the crystal to rotate back and forth and causing the diffracted X-ray beam to have an X-ray component which results from the input perturbation signal. The feedback control unit operates to monitor the intensity of the diffracted X-ray beam and the perturbation component thereof so as to adjust automatically the angular position of the position control unit toward an angle (the Bragg angle) which causes the X-ray detector to detect a maximum intensity.