A heterodyne interferometer with spatially separated input beams is presented. The optical design realizes symmetric paths through glass and air, which results in a minimal dead path. An automatic correction of phase variations introduced by feeding the optics with fibers and by angle variations between the optics and the mirror is demonstrated. In addition to the resolution and linearity in the single digit picometer range, the long-term stability of the interferometer setup is investigated, involving the previous analysis of all different components of the setup. It is shown, that the long-term stability is still limited by the variations of the environmental conditions.
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