The electron-optical system also includes deflectors and stigmators, usually made of small electromagnets. Unlike the lenses, the magnetic fields produced by the deflectors are oriented primarily to deflect the beam and not to focus it. The deflectors allow the position and angle of the beam at the sample position to be independently controlled (as is essential for STEM) and also ensure that the beams remain near the low-aberration centers of every lens in the lens stacks. The stigmators provide an auxiliary fine focusing, compensating for slight imperfections and aberrations that cause astigmatism—a lens having a different focal strength in different directions.