Before starting to scan it is very important to correctly align the camera with the microscope so that the scanning lines are exactly horizontal or vertical. Go the upper glass edge of your slide. See how in our case the upper line, which is the camera view frame, is not parallel to the lower line, which is the glass slide.
To align the camera with the microscope, loosen the thumb screw and rotate the camera adapter. When both lines are parallel tighten the thumb screw on the camera adapter. Move the slide from one side to the other to make sure that the alignment is correct.